- #596
Kea
- 859
- 0
marcus said:
Fighting words indeed. Check out the table on page 6!
Loop-and-allied QG bibliography
- Thread starter marcus
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In summary, Rovelli's program for loop gravity involves coupling the standard model to quantized QG loops, allowing for interactions between eigenvalues of length and momentum. This approach allows for non-perturbative calculations without infinity problems and does not require a continuum limit. The main difference in loop gravity is that the excitations of space are represented by polymers, or ball-and-stick models, that can be labeled with numbers to determine the volume and area of any region or surface. This allows for a more intuitive understanding of the geometry of the universe.
Fighting words indeed. Check out the table on page 6!
- #597
- 24,775
- 792
http://arxiv.org/abs/0705.2656
Inflationary universe in loop quantum cosmology
Xin Zhang, Yi Ling
25 pages, 5 figures
"Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed." http://arxiv.org/abs/0705.2629
Dual Computations of Non-abelian Yang-Mills on the Lattice
J. Wade Cherrington, Dan Christensen, Igor Khavkine
18 pages, 7 figures
"In the past several decades there have been a number of proposals for computing with dual forms of non-abelian Yang-Mills theories on the lattice. Motivated by the gauge-invariant, geometric picture offered by dual models and successful applications of duality in the U(1) case, we revisit the question of whether it is practical to perform numerical computation using non-abelian dual models. Specifically, we consider three-dimensional SU(2) pure Yang-Mills as an accessible yet non-trivial case in which the gauge group is non-abelian. Using methods developed recently in the context of spin foam quantum gravity, we describe a Metropolis algorithm for sampling the dual ensemble and efficiently computing the dual amplitude. We relate our algorithms to prior work in non-abelian dual computations of Hari Dass and his collaborators, addressing several problems that have (to the best our knowledge) been left open. We report results of spin expectation value computations over a range of lattice sizes and couplings that are in agreement with our conventional lattice computations. We conclude with an outlook on further development of dual methods and their application to problems of current interest."
brief mention:
http://arxiv.org/abs/0705.2643
Symmetries, Singularities and the De-Emergence of Space
Thibault Damour, Hermann Nicolai
10 pages
though in a very different formal context, this paper reminded me of the recent work of Kirill Krasnov and Yuri Shtanov----where the metric disappears near the singularity, but spacetime evolution continues because the theory of gravity is non-metric. you could say that in Krasnov-Shtanov non-metric gravity the conventional idea of space also "de-emerges" near a classical singularity---while the classical singularity is removed. For the Krasnov-Shtanov paper see post #591 or http://arxiv.org/abs/0705.2047
EDITED IN to reply to francesca's next post:
Dear francesca, thanks for helping by supplying the papers of Corichi et al and Padmanabhan. However look back at post #595
https://www.physicsforums.com/showpost.php?p=1334313&postcount=595
from four days ago. they are the first two papers mentioned in that post, IIRC. :-)
Inflationary universe in loop quantum cosmology
Xin Zhang, Yi Ling
25 pages, 5 figures
"Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed." http://arxiv.org/abs/0705.2629
Dual Computations of Non-abelian Yang-Mills on the Lattice
J. Wade Cherrington, Dan Christensen, Igor Khavkine
18 pages, 7 figures
"In the past several decades there have been a number of proposals for computing with dual forms of non-abelian Yang-Mills theories on the lattice. Motivated by the gauge-invariant, geometric picture offered by dual models and successful applications of duality in the U(1) case, we revisit the question of whether it is practical to perform numerical computation using non-abelian dual models. Specifically, we consider three-dimensional SU(2) pure Yang-Mills as an accessible yet non-trivial case in which the gauge group is non-abelian. Using methods developed recently in the context of spin foam quantum gravity, we describe a Metropolis algorithm for sampling the dual ensemble and efficiently computing the dual amplitude. We relate our algorithms to prior work in non-abelian dual computations of Hari Dass and his collaborators, addressing several problems that have (to the best our knowledge) been left open. We report results of spin expectation value computations over a range of lattice sizes and couplings that are in agreement with our conventional lattice computations. We conclude with an outlook on further development of dual methods and their application to problems of current interest."
brief mention:
http://arxiv.org/abs/0705.2643
Symmetries, Singularities and the De-Emergence of Space
Thibault Damour, Hermann Nicolai
10 pages
though in a very different formal context, this paper reminded me of the recent work of Kirill Krasnov and Yuri Shtanov----where the metric disappears near the singularity, but spacetime evolution continues because the theory of gravity is non-metric. you could say that in Krasnov-Shtanov non-metric gravity the conventional idea of space also "de-emerges" near a classical singularity---while the classical singularity is removed. For the Krasnov-Shtanov paper see post #591 or http://arxiv.org/abs/0705.2047
EDITED IN to reply to francesca's next post:
Dear francesca, thanks for helping by supplying the papers of Corichi et al and Padmanabhan. However look back at post #595
https://www.physicsforums.com/showpost.php?p=1334313&postcount=595
from four days ago. they are the first two papers mentioned in that post, IIRC. :-)
Last edited:
- #598
francesca
- 147
- 45
http://arxiv.org/abs/0705.2440"
Quantum Structure of Geometry: Loopy and fuzzy?
Authors: Alejandro Corichi, Jose A. Zapata
Abstract: In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy.
http://arxiv.org/abs/0705.2533"
Dark Energy and Gravity
Authors: T. Padmanabhan
Abstract: I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).
Quantum Structure of Geometry: Loopy and fuzzy?
Authors: Alejandro Corichi, Jose A. Zapata
Abstract: In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy.
http://arxiv.org/abs/0705.2533"
Dark Energy and Gravity
Authors: T. Padmanabhan
Abstract: I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).
Last edited by a moderator:
- #599
- 24,775
- 792
http://arxiv.org/abs/0705.3024
Symmetries, Horizons, and Black Hole Entropy
S. Carlip
6 pages; first prize essay, 2007 Gravity Research Foundation essay contest
"Black holes behave as thermodynamic systems, and a central task of any quantum theory of gravity is to explain these thermal properties. A statistical mechanical description of black hole entropy once seemed remote, but today we suffer an embarrassment of riches: despite counting very different states, many inequivalent approaches to quantum gravity obtain identical results. Such 'universality' may reflect an underlying two-dimensional conformal symmetry near the horizon, which can be powerful enough to control the thermal characteristics independent of other details of the theory. This picture suggests an elegant description of the relevant degrees of freedom as Goldstone-boson-like excitations arising from symmetry breaking by the conformal anomaly."
Symmetries, Horizons, and Black Hole Entropy
S. Carlip
6 pages; first prize essay, 2007 Gravity Research Foundation essay contest
"Black holes behave as thermodynamic systems, and a central task of any quantum theory of gravity is to explain these thermal properties. A statistical mechanical description of black hole entropy once seemed remote, but today we suffer an embarrassment of riches: despite counting very different states, many inequivalent approaches to quantum gravity obtain identical results. Such 'universality' may reflect an underlying two-dimensional conformal symmetry near the horizon, which can be powerful enough to control the thermal characteristics independent of other details of the theory. This picture suggests an elegant description of the relevant degrees of freedom as Goldstone-boson-like excitations arising from symmetry breaking by the conformal anomaly."
- #600
ensabah6
- 695
- 0
Hey Marcus, I do not know how to search this thread but did you ever post this paper?
http://arxiv.org/abs/gr-qc/0612134
From Quantum Hydrodynamics to Quantum Gravity
G.E. Volovik
(Submitted on 21 Dec 2006 (v1), last revised 17 Jan 2007 (this version, v5))
We discuss some lessons from quantum hydrodynamics to quantum gravity.
Comments: 20 pages, 1 figure, rapporteur article for Proceedings of MG11, session `Analog Models of and for General Relativity', references added
Subjects: General Relativity and Quantum Cosmology (gr-qc); Soft Condensed Matter (cond-mat.soft); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:gr-qc/0612134v5
http://arxiv.org/abs/gr-qc/0612134
From Quantum Hydrodynamics to Quantum Gravity
G.E. Volovik
(Submitted on 21 Dec 2006 (v1), last revised 17 Jan 2007 (this version, v5))
We discuss some lessons from quantum hydrodynamics to quantum gravity.
Comments: 20 pages, 1 figure, rapporteur article for Proceedings of MG11, session `Analog Models of and for General Relativity', references added
Subjects: General Relativity and Quantum Cosmology (gr-qc); Soft Condensed Matter (cond-mat.soft); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:gr-qc/0612134v5
- #601
- 24,775
- 792
http://arxiv.org/abs/0705.3892
Spin foam model from canonical quantization
Sergei Alexandrov
24 pages
(Submitted on 26 May 2007)
"We suggest a modification of the Barrett-Crane spin foam model of 4-dimensional Lorentzian general relativity motivated by the canonical quantization. The starting point is Lorentz covariant loop quantum gravity. Its kinematical Hilbert space is found as a space of the so-called projected spin networks. These spin networks are identified with the boundary states of a spin foam model and provide a generalization of the unique Barrette-Crane intertwiner. We propose a way to modify the Barrett-Crane quantization procedure to arrive at this generalization: the B field (bi-vectors) should be promoted not to generators of the gauge algebra, but to their certain projection. The modification is also justified by the canonical analysis of Plebanski formulation. Finally, we compare our construction with other proposals to modify the Barret-Crane model."
Spin foam model from canonical quantization
Sergei Alexandrov
24 pages
(Submitted on 26 May 2007)
"We suggest a modification of the Barrett-Crane spin foam model of 4-dimensional Lorentzian general relativity motivated by the canonical quantization. The starting point is Lorentz covariant loop quantum gravity. Its kinematical Hilbert space is found as a space of the so-called projected spin networks. These spin networks are identified with the boundary states of a spin foam model and provide a generalization of the unique Barrette-Crane intertwiner. We propose a way to modify the Barrett-Crane quantization procedure to arrive at this generalization: the B field (bi-vectors) should be promoted not to generators of the gauge algebra, but to their certain projection. The modification is also justified by the canonical analysis of Plebanski formulation. Finally, we compare our construction with other proposals to modify the Barret-Crane model."
- #602
francesca
- 147
- 45
cosmology
http://arxiv.org/abs/0705.4398"
The Dark Side of a Patchwork Universe
Authors: Martin Bojowald
Abstract: While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often http://arxiv.org/abs/associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
I always like Bojowald jobs, and this one seems very readable at the first sight...
Loop Quantum Cosmology could be the main issue of the month
(see below how many papers this week )
and the conference on phenomenology is coming, we have the program now:
http://www.sissa.it/app/QGconference/program.htm"
http://arxiv.org/abs/0705.4449"
Title: Relic gravitons as the observable for Loop Quantum Cosmology
Authors: Jakub Mielczarek, Marek Szydlowski
http://arxiv.org/abs/0705.3741"
Title: Black hole state counting in loop quantum gravity
Authors: P. Mitra
http://arxiv.org/abs/0705.3375"
Title: Unsuccessful cosmology with Modified Gravity Models
Authors: Antonio De Felice, Mark Hindmarsh
http://arxiv.org/abs/0705.4002"
Title: Dynamics of Quintom and Hessence Energies in Loop Quantum Cosmology
Authors: Hao Wei, Shuang Nan Zhang
http://arxiv.org/abs/0705.4398"
The Dark Side of a Patchwork Universe
Authors: Martin Bojowald
Abstract: While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often http://arxiv.org/abs/associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
I always like Bojowald jobs, and this one seems very readable at the first sight...
Loop Quantum Cosmology could be the main issue of the month
(see below how many papers this week
) and the conference on phenomenology is coming, we have the program now:
http://www.sissa.it/app/QGconference/program.htm"
http://arxiv.org/abs/0705.4449"
Title: Relic gravitons as the observable for Loop Quantum Cosmology
Authors: Jakub Mielczarek, Marek Szydlowski
http://arxiv.org/abs/0705.3741"
Title: Black hole state counting in loop quantum gravity
Authors: P. Mitra
http://arxiv.org/abs/0705.3375"
Title: Unsuccessful cosmology with Modified Gravity Models
Authors: Antonio De Felice, Mark Hindmarsh
http://arxiv.org/abs/0705.4002"
Title: Dynamics of Quintom and Hessence Energies in Loop Quantum Cosmology
Authors: Hao Wei, Shuang Nan Zhang
Last edited by a moderator:
- #603
- 24,775
- 792
thanks for spotting these, francesca, especially the Bojowald.
Here are some more that appeared in the past week:
http://arxiv.org/abs/0706.0471http://arxiv.org/abs/0706.0469
Properties of the Volume Operator in Loop Quantum Gravity I: Results
Johannes Brunnemann, David Rideout
37 pages, 7 figures
(Submitted on 4 Jun 2007)
"We analyze the spectral properties of the volume operator of Ashtekar and Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the classical volume expression for regions in three dimensional Riemannian space. Our analysis considers for the first time generic graph vertices of valence greater than four. Here we find that the geometry of the underlying vertex characterizes the spectral properties of the volume operator, in particular the presence of a `volume gap' (a smallest non-zero eigenvalue in the spectrum) is found to depend on the vertex embedding. We compute the set of all non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of valence 5--7, and argue that these sets can be used to label spatial diffeomorphism invariant states. We observe how gauge invariance connects vertex geometry and representation properties of the underlying gauge group in a natural way. Analytical results on the spectrum on 4-valent vertices are included, for which the presence of a volume gap is proved. This paper presents our main results; details are provided by a companion paper arXiv:0706.0382v1."
http://arxiv.org/abs/0706.0382
Properties of the Volume Operator in Loop Quantum Gravity II: Detailed Presentation
Authors: Johannes Brunneman, David Rideout
95 pages, 65 figures
(Submitted on 4 Jun 2007)
"The properties of the Volume operator in Loop Quantum Gravity, as constructed by Ashtekar and Lewandowski, are analyzed for the first time at generic vertices of valence greater than four. The present analysis benefits from the general simplified formula for matrix elements of the Volume operator derived in gr-qc/0405060, making it feasible to implement it on a computer as a matrix which is then diagonalized numerically. The resulting eigenvalues serve as a database to investigate the spectral properties of the volume operator. Analytical results on the spectrum at 4-valent vertices are included. This is a companion paper to arXiv:0706.0469, providing details of the analysis presented there."
See also companion paper arXiv:0706.0469
http://arxiv.org/abs/0706.0283
Cosmography in testing loop quantum gravity
Marek Szydlowski, Wlodzimierz Godlowski, Tomasz Stachowiak
19 pages, 1 figure
(Submitted on 2 Jun 2007)
"It was recently suggested by Martin Bojowald that quantum gravity effects give rise to new, potentially observable effects. We check whether this is the case for astronomical tests by trying to constrain the density parameters of the Friedmann equation with a $(-)(1+z)^6$ type of contribution. We describe different interpretations of such an additional term: geometric effects of Loop Quantum Cosmology, effects of braneworld cosmological models, non-standard cosmological models in metric-affine gravity, and models with spinning fluid. Kinematical (or geometrical) tests based on null geodesics are insufficient to separate individual matter components when they behave like perfect fluid and scale in the same way. Still, it is possible to measure their overall effect. We use recent measurements of the coordinate distances from Fanaroff-Riley type IIb (FRIIb) radio galaxy (RG) data, supernovae type Ia (SNIa) data, baryon oscillation peak and cosmic microwave background radiation (CMBR) observations to obtain stronger bounds for the contribution of the considered type. We demonstrate that, while rho^2 corrections are very small, they can be tested by astronomical observations -- at least in principle. Bayesian criteria of model selection (Bayesian factor, AIC, and BIC) are used to check if additional parameters are detectable in the present epoch. As it turns out, the LambdaCDM model is favoured over the bouncing model driven by loop quantum effects. Or, in other words, the bounds obtained from cosmography are very weak, and from the point of view of the present data this model is indistinguishable from the LambdaCDM one."
http://arxiv.org/abs/0706.0174
Entropy signature of the running cosmological constant
Authors: Alfio Bonanno, Martin Reuter
57 pages, 7 figures
(Submitted on 1 Jun 2007 (v1), last revised 3 Jun 2007 (this version, v2))
"Renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values are investigated using a system of cosmological evolution equations which allows for an unrestricted energy exchange between the vacuum and the matter sector. It is demonstrated that the scale dependence of the gravitational parameters, the cosmological constant in particular, leads to an entropy production in the matter system. The picture emerges that the Universe started out from a state of vanishing entropy, and that the radiation entropy observed today is essentially due to the coarse graining (RG flow) in the quantum gravity sector which is related to the expansion of the Universe. Furthermore, the RG improved field equations are shown to possesses solutions with an epoch of power law inflation immediately after the initial singularity. The inflation is driven by the cosmological constant and ends automatically once the RG running has reduced the vacuum energy to the level of the matter energy density."
http://arxiv.org/abs/0706.0179
Lattice Refining Loop Quantum Cosmology and Inflation
William Nelson, Mairi Sakellariadou (King's College, London)
12 pages
(Submitted on 1 Jun 2007)
"We study the importance of lattice refinement in achieving a successful inflationary era. We solve, in the continuum limit, the second order difference equation governing the quantum evolution in loop quantun cosmology, assuming both a fixed and a dynamically varying lattice in a suitable refinement model. We thus impose a constraint on the potential of a scalar field, so that the continuum approximation is not broken. Considering that such a scalar field could play the role of the inflaton, we obtain a second constraint on the inflationary potential so that there is consistency with the CMB data on large angular scales. For a $m^2\phi^2/2$ inflationary model, we combine the two constraints on the inflaton potential to impose an upper limit on $m$, which is severely fine-tuned in the case of a fixed lattice. We thus conclude that lattice refinement is necessary to achieve a natural inflationary model."http://arxiv.org/abs/0706.0142
Quantum gravity phenomenology via Lorentz violations
Stephano Liberati
21 pages, 1 figure
(Submitted on 1 Jun 2007)
"The search for a quantum theory of gravity has been one of the main aims of theoretical physics for many years by now. However the efforts in this direction have been often hampered by the lack of experimental/observational tests able to select among, or at least constrain, the numerous quantum gravity models proposed so far. This situation has changed in the last decade thanks to the realization that some QG inspired violations of Lorentz symmetry could be constrained using current experiments and observations. This study it is not only allowing us to test at higher and higher energies a fundamental symmetry of spacetime but it is also providing us with hints and perspectives about the fundamental nature of gravity."
Here are some more that appeared in the past week:
http://arxiv.org/abs/0706.0471http://arxiv.org/abs/0706.0469
Properties of the Volume Operator in Loop Quantum Gravity I: Results
Johannes Brunnemann, David Rideout
37 pages, 7 figures
(Submitted on 4 Jun 2007)
"We analyze the spectral properties of the volume operator of Ashtekar and Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the classical volume expression for regions in three dimensional Riemannian space. Our analysis considers for the first time generic graph vertices of valence greater than four. Here we find that the geometry of the underlying vertex characterizes the spectral properties of the volume operator, in particular the presence of a `volume gap' (a smallest non-zero eigenvalue in the spectrum) is found to depend on the vertex embedding. We compute the set of all non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of valence 5--7, and argue that these sets can be used to label spatial diffeomorphism invariant states. We observe how gauge invariance connects vertex geometry and representation properties of the underlying gauge group in a natural way. Analytical results on the spectrum on 4-valent vertices are included, for which the presence of a volume gap is proved. This paper presents our main results; details are provided by a companion paper arXiv:0706.0382v1."
http://arxiv.org/abs/0706.0382
Properties of the Volume Operator in Loop Quantum Gravity II: Detailed Presentation
Authors: Johannes Brunneman, David Rideout
95 pages, 65 figures
(Submitted on 4 Jun 2007)
"The properties of the Volume operator in Loop Quantum Gravity, as constructed by Ashtekar and Lewandowski, are analyzed for the first time at generic vertices of valence greater than four. The present analysis benefits from the general simplified formula for matrix elements of the Volume operator derived in gr-qc/0405060, making it feasible to implement it on a computer as a matrix which is then diagonalized numerically. The resulting eigenvalues serve as a database to investigate the spectral properties of the volume operator. Analytical results on the spectrum at 4-valent vertices are included. This is a companion paper to arXiv:0706.0469, providing details of the analysis presented there."
See also companion paper arXiv:0706.0469
http://arxiv.org/abs/0706.0283
Cosmography in testing loop quantum gravity
Marek Szydlowski, Wlodzimierz Godlowski, Tomasz Stachowiak
19 pages, 1 figure
(Submitted on 2 Jun 2007)
"It was recently suggested by Martin Bojowald that quantum gravity effects give rise to new, potentially observable effects. We check whether this is the case for astronomical tests by trying to constrain the density parameters of the Friedmann equation with a $(-)(1+z)^6$ type of contribution. We describe different interpretations of such an additional term: geometric effects of Loop Quantum Cosmology, effects of braneworld cosmological models, non-standard cosmological models in metric-affine gravity, and models with spinning fluid. Kinematical (or geometrical) tests based on null geodesics are insufficient to separate individual matter components when they behave like perfect fluid and scale in the same way. Still, it is possible to measure their overall effect. We use recent measurements of the coordinate distances from Fanaroff-Riley type IIb (FRIIb) radio galaxy (RG) data, supernovae type Ia (SNIa) data, baryon oscillation peak and cosmic microwave background radiation (CMBR) observations to obtain stronger bounds for the contribution of the considered type. We demonstrate that, while rho^2 corrections are very small, they can be tested by astronomical observations -- at least in principle. Bayesian criteria of model selection (Bayesian factor, AIC, and BIC) are used to check if additional parameters are detectable in the present epoch. As it turns out, the LambdaCDM model is favoured over the bouncing model driven by loop quantum effects. Or, in other words, the bounds obtained from cosmography are very weak, and from the point of view of the present data this model is indistinguishable from the LambdaCDM one."
http://arxiv.org/abs/0706.0174
Entropy signature of the running cosmological constant
Authors: Alfio Bonanno, Martin Reuter
57 pages, 7 figures
(Submitted on 1 Jun 2007 (v1), last revised 3 Jun 2007 (this version, v2))
"Renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values are investigated using a system of cosmological evolution equations which allows for an unrestricted energy exchange between the vacuum and the matter sector. It is demonstrated that the scale dependence of the gravitational parameters, the cosmological constant in particular, leads to an entropy production in the matter system. The picture emerges that the Universe started out from a state of vanishing entropy, and that the radiation entropy observed today is essentially due to the coarse graining (RG flow) in the quantum gravity sector which is related to the expansion of the Universe. Furthermore, the RG improved field equations are shown to possesses solutions with an epoch of power law inflation immediately after the initial singularity. The inflation is driven by the cosmological constant and ends automatically once the RG running has reduced the vacuum energy to the level of the matter energy density."
http://arxiv.org/abs/0706.0179
Lattice Refining Loop Quantum Cosmology and Inflation
William Nelson, Mairi Sakellariadou (King's College, London)
12 pages
(Submitted on 1 Jun 2007)
"We study the importance of lattice refinement in achieving a successful inflationary era. We solve, in the continuum limit, the second order difference equation governing the quantum evolution in loop quantun cosmology, assuming both a fixed and a dynamically varying lattice in a suitable refinement model. We thus impose a constraint on the potential of a scalar field, so that the continuum approximation is not broken. Considering that such a scalar field could play the role of the inflaton, we obtain a second constraint on the inflationary potential so that there is consistency with the CMB data on large angular scales. For a $m^2\phi^2/2$ inflationary model, we combine the two constraints on the inflaton potential to impose an upper limit on $m$, which is severely fine-tuned in the case of a fixed lattice. We thus conclude that lattice refinement is necessary to achieve a natural inflationary model."http://arxiv.org/abs/0706.0142
Quantum gravity phenomenology via Lorentz violations
Stephano Liberati
21 pages, 1 figure
(Submitted on 1 Jun 2007)
"The search for a quantum theory of gravity has been one of the main aims of theoretical physics for many years by now. However the efforts in this direction have been often hampered by the lack of experimental/observational tests able to select among, or at least constrain, the numerous quantum gravity models proposed so far. This situation has changed in the last decade thanks to the realization that some QG inspired violations of Lorentz symmetry could be constrained using current experiments and observations. This study it is not only allowing us to test at higher and higher energies a fundamental symmetry of spacetime but it is also providing us with hints and perspectives about the fundamental nature of gravity."
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http://arxiv.org/abs/0706.1057
Effective equations for isotropic quantum cosmology including matter
Martin Bojowald, Hector Hernandez, Aureliano Skirzewski
42 pages
(Submitted on 7 Jun 2007)
"Effective equations often provide powerful tools to develop a systematic understanding of detailed properties of a quantum system. This is especially helpful in quantum cosmology where several conceptual and technical difficulties associated with the full quantum equations can be avoided in this way. Here, effective equations for Wheeler-DeWitt and loop quantizations of spatially flat, isotropic cosmological models sourced by a massive or interacting scalar are derived and studied. The resulting systems are remarkably different from that given for a free, massless scalar. This has implications for the coherence of evolving states and the realization of a bounce in loop quantum cosmology. http://arxiv.org/abs/0706.0985
Bulk Entropy in Loop Quantum Gravity
Etera R. Livine, Daniel R. Terno
13 pages
(Submitted on 7 Jun 2007)
"In the framework of loop quantum gravity (LQG), having quantum black holes in mind, we generalize the previous boundary state counting (gr-qc/0508085) to a full bulk state counting. After a suitable gauge fixing we are able to compute the bulk entropy of a bounded region (the "black hole") with fixed boundary. This allows us to study the relationship between the entropy and the boundary area in details and we identify a holographic regime of LQG where the leading order of the entropy scales with the area. We show that in this regime we can fine tune the factor between entropy and area without changing the Immirzi parameter."
Effective equations for isotropic quantum cosmology including matter
Martin Bojowald, Hector Hernandez, Aureliano Skirzewski
42 pages
(Submitted on 7 Jun 2007)
"Effective equations often provide powerful tools to develop a systematic understanding of detailed properties of a quantum system. This is especially helpful in quantum cosmology where several conceptual and technical difficulties associated with the full quantum equations can be avoided in this way. Here, effective equations for Wheeler-DeWitt and loop quantizations of spatially flat, isotropic cosmological models sourced by a massive or interacting scalar are derived and studied. The resulting systems are remarkably different from that given for a free, massless scalar. This has implications for the coherence of evolving states and the realization of a bounce in loop quantum cosmology. http://arxiv.org/abs/0706.0985
Bulk Entropy in Loop Quantum Gravity
Etera R. Livine, Daniel R. Terno
13 pages
(Submitted on 7 Jun 2007)
"In the framework of loop quantum gravity (LQG), having quantum black holes in mind, we generalize the previous boundary state counting (gr-qc/0508085) to a full bulk state counting. After a suitable gauge fixing we are able to compute the bulk entropy of a bounded region (the "black hole") with fixed boundary. This allows us to study the relationship between the entropy and the boundary area in details and we identify a holographic regime of LQG where the leading order of the entropy scales with the area. We show that in this regime we can fine tune the factor between entropy and area without changing the Immirzi parameter."
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http://arxiv.org/abs/0706.1534
Coupling gauge theory to spinfoam 3d quantum gravity
Simone Speziale
18 pages
(Submitted on 11 Jun 2007)
"We construct a spinfoam model for Yang-Mills theory coupled to quantum gravity in three dimensional riemannian spacetime. We define the partition function of the coupled system as a power series in g_0^2 G that can be evaluated order by order using grasping rules and the recoupling theory. With respect to previous attempts in the literature, this model assigns the dynamical variables of gravity and Yang-Mills theory to the same simplices of the spinfoam, and it thus provides transition amplitudes for the spin network states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly that the partition function has a semiclassical limit given by the Regge discretization of the classical Yang-Mills action."
http://arxiv.org/abs/0706.1279
The wedding ring of MOND and non-exotic dark matter
B. Famaey, G.W. Angus, G. Gentile, H.S. Zhao
2 pages, research note submitted to A&A
(Submitted on 9 Jun 2007)
"The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND."
http://arxiv.org/abs/0706.1654
Gravity as an emergent phenomenon: A conceptual description
T. Padmanabhan
11 pages; Summary of several plenary talks in the conferences including XXIII Texas Symposium in Relativistic Astrophysics (Melbourne, Dec, 06); IAGRG Meeting (Delhi, Feb, 07); International workshop on theoretical high energy physics (Roorkee, Mar, 2007); to appear in the proceedings of IWTHEP
(Submitted on 12 Jun 2007)
"I describe several broad features of a programme to understand gravity as an emergent, long wavelength, phenomenon (like elasticity) and discuss one concrete framework for realizing this paradigm in the backdrop of several recent results."
Coupling gauge theory to spinfoam 3d quantum gravity
Simone Speziale
18 pages
(Submitted on 11 Jun 2007)
"We construct a spinfoam model for Yang-Mills theory coupled to quantum gravity in three dimensional riemannian spacetime. We define the partition function of the coupled system as a power series in g_0^2 G that can be evaluated order by order using grasping rules and the recoupling theory. With respect to previous attempts in the literature, this model assigns the dynamical variables of gravity and Yang-Mills theory to the same simplices of the spinfoam, and it thus provides transition amplitudes for the spin network states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly that the partition function has a semiclassical limit given by the Regge discretization of the classical Yang-Mills action."
http://arxiv.org/abs/0706.1279
The wedding ring of MOND and non-exotic dark matter
B. Famaey, G.W. Angus, G. Gentile, H.S. Zhao
2 pages, research note submitted to A&A
(Submitted on 9 Jun 2007)
"The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND."
http://arxiv.org/abs/0706.1654
Gravity as an emergent phenomenon: A conceptual description
T. Padmanabhan
11 pages; Summary of several plenary talks in the conferences including XXIII Texas Symposium in Relativistic Astrophysics (Melbourne, Dec, 06); IAGRG Meeting (Delhi, Feb, 07); International workshop on theoretical high energy physics (Roorkee, Mar, 2007); to appear in the proceedings of IWTHEP
(Submitted on 12 Jun 2007)
"I describe several broad features of a programme to understand gravity as an emergent, long wavelength, phenomenon (like elasticity) and discuss one concrete framework for realizing this paradigm in the backdrop of several recent results."
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http://arxiv.org/abs/0706.1899
Spacetime Lagrangian Formulation of Barbero-Immirzi Gravity
L. Fatibene, M. Francaviglia, C. Rovelli
12 pages
(Submitted on 13 Jun 2007)
"We shall here discuss a new spacetime gauge-covariant Lagrangian formulation of General Relativity by means of the Barbero-Immirzi SU(2)-connection on spacetime. To the best of our knowledge the Lagrangian based on SU(2) spacetime fields seems to appear here for the first time."
the above is a continuation of work posted earlier this year by the same authors:
http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages
(Submitted on 26 Feb 2007)
"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."
http://arxiv.org/abs/0706.1979
Note on black hole radiation spectrum in Loop Quantum Gravity
Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 2 figures
(Submitted on 14 Jun 2007)
"Recent detailed analysis within the Loop Quantum Gravity calculation of black hole entropy show a stair-like structure in the behavior of entropy as a function of horizon area. The non-trivial distribution of the degeneracy of the black hole horizon area eigenstates is at the origin of this behavior. This degeneracy distribution is analyzed and a phenomenological model is put forward to study the possible implications of this distribution in the black hole radiation spectrum."
Spacetime Lagrangian Formulation of Barbero-Immirzi Gravity
L. Fatibene, M. Francaviglia, C. Rovelli
12 pages
(Submitted on 13 Jun 2007)
"We shall here discuss a new spacetime gauge-covariant Lagrangian formulation of General Relativity by means of the Barbero-Immirzi SU(2)-connection on spacetime. To the best of our knowledge the Lagrangian based on SU(2) spacetime fields seems to appear here for the first time."
the above is a continuation of work posted earlier this year by the same authors:
http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages
(Submitted on 26 Feb 2007)
"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."
http://arxiv.org/abs/0706.1979
Note on black hole radiation spectrum in Loop Quantum Gravity
Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 2 figures
(Submitted on 14 Jun 2007)
"Recent detailed analysis within the Loop Quantum Gravity calculation of black hole entropy show a stair-like structure in the behavior of entropy as a function of horizon area. The non-trivial distribution of the degeneracy of the black hole horizon area eigenstates is at the origin of this behavior. This degeneracy distribution is analyzed and a phenomenological model is put forward to study the possible implications of this distribution in the black hole radiation spectrum."
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Accidentally duplicated a post, so I'll edit to avoid repeating content. An important aspect of the papers being posted on arxiv is their rate of publication in peer-review journals. Prime examples of good places to publish Loop cosmology and gravity research are Physical Review D and Physical Review Letters. As a sample publication history: in 2006 Martin Bojowald posted 12 papers on arxiv of which 4 were published in Physical Review D.
For completeness, at the end of the list I'll tally the publication results.
7. arXiv:astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
Phys.Rev.Lett. 98 (2007) 031301
8. arXiv:gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
Phys.Rev. D75 (2007) 064022
9. arXiv:gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
Phys.Rev. D74 (2006) 123512
10. arXiv:gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
Gen.Rel.Grav. 38 (2006) 1771-1795
11. arXiv:gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
5 pages, 1 figure
Phys. Rev. D 75 (2007) 081301(R)
12. arXiv:gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at 'Einstein's Legacy in the New Millenium,' Puri, India, December 2005
Int.J.Mod.Phys. D15 (2006) 1545-1559
13. arXiv:hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics ``Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006
Int.J.Geom.Meth.Mod.Phys. 4 (2007) 25-52
14. arXiv:gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
Phys.Rev. D74 (2006) 044033
15. arXiv:gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
Class.Quant.Grav. 23 (2006) 4983-4990
16. arXiv:gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153
17. arXiv:gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)
18. arXiv:gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at this http URL
Living Rev.Rel. 8 (2005) 11
Of the dozen 2006 papers, those published in Physical Review D (PRD) were 4
in Physical Review Letters (PRL) 1
in Classical and Quantum Gravity (CQG) 1
in General Relativity and Gravitation (GRG) 1
in International Journal of Modern Physics 1
in International Journal of Geometrical Methods in Modern Physics 1
in Living Reviews in Relativity 1
chapters in books 2
For completeness, at the end of the list I'll tally the publication results.
7. arXiv:astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
Phys.Rev.Lett. 98 (2007) 031301
8. arXiv:gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
Phys.Rev. D75 (2007) 064022
9. arXiv:gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
Phys.Rev. D74 (2006) 123512
10. arXiv:gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
Gen.Rel.Grav. 38 (2006) 1771-1795
11. arXiv:gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
5 pages, 1 figure
Phys. Rev. D 75 (2007) 081301(R)
12. arXiv:gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at 'Einstein's Legacy in the New Millenium,' Puri, India, December 2005
Int.J.Mod.Phys. D15 (2006) 1545-1559
13. arXiv:hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics ``Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006
Int.J.Geom.Meth.Mod.Phys. 4 (2007) 25-52
14. arXiv:gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
Phys.Rev. D74 (2006) 044033
15. arXiv:gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
Class.Quant.Grav. 23 (2006) 4983-4990
16. arXiv:gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153
17. arXiv:gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)
18. arXiv:gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at this http URL
Living Rev.Rel. 8 (2005) 11
Of the dozen 2006 papers, those published in Physical Review D (PRD) were 4
in Physical Review Letters (PRL) 1
in Classical and Quantum Gravity (CQG) 1
in General Relativity and Gravitation (GRG) 1
in International Journal of Modern Physics 1
in International Journal of Geometrical Methods in Modern Physics 1
in Living Reviews in Relativity 1
chapters in books 2
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http://arxiv.org/abs/0706.2342
Gravitational Wilson Loop and Large Scale Curvature
Herbert W. Hamber, Ruth M. Williams
22 pages, 6 figures
(Submitted on 15 Jun 2007)
"In a quantum theory of gravity the gravitational Wilson loop, defined as a suitable quantum average of a parallel transport operator around a large near-planar loop, provides important information about the large-scale curvature properties of the geometry. Here we shows that such properties can be systematically computed in the strong coupling limit of lattice regularized quantum gravity, by performing a local average over rotations, using an assumed near-uniform measure in group space. We then relate the resulting quantum averages to an expected semi-classical form valid for macroscopic observers, which leads to an identification of the gravitational correlation length appearing in the Wilson loop with an observed large-scale curvature. Our results suggest that strongly coupled gravity leads to a positively curved (De Sitter-like) quantum ground state, implying a positive effective cosmological constant at large distances."
Bojowald also recently showed that a positive effective cosmological constant may not need to be put in "by hand" but could be *derived* from quantum gravity. Is there some connection with Hamber and Williams result? Francesca reported this Bojowald paper a few posts back in this thread, and Jal in a separate thread, but I will repeat it since it seems especially important:
http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
(Submitted on 30 May 2007)
While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
Meanwhile Michael Turner, apparently unaware of Bojowald's result, or the Hamber Williams paper, surveys the problem.
http://arxiv.org/abs/0706.2186
Cosmic Acceleration, Dark Energy and Fundamental Physics
Michael S. Turner, Dragan Huterer (KICP, University of Chicago)
10 pages, 8 figures, invited review for Journal of the Physical Society of Japan, in press
(Submitted on 14 Jun 2007)
"A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (p\sim -rho), very smooth form of energy called 'dark energy' accounting for about 75% of the Universe. The 'simplest' explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science."
brief mention:
Masters Thesis in LQG by Muxin Han, a student of Jorge Pullin at LSU
http://arxiv.org/abs/0706.2623
Quantum Dyanmics of Loop Quantum Gravity
95 pages
http://arxiv.org/abs/0706.2445
GRB Cosmology
Volker Bromm, Abraham Loeb
24 pages, 9 figures, review to appear in "Gamma-ray Bursts" (CUP)
(Submitted on 18 Jun 2007)
"Current observations are about to open up a direct window into the final frontier of cosmology: the first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium (IGM) up to redshifts of several tens, when the first (Population III) stars had formed. The Swift mission, or future satellites such as EXIST, might be the first observatories to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up observations of the GRB afterglow emission would allow to probe the ionization state and metal enrichment of the IGM as a function of redshift."
Gravitational Wilson Loop and Large Scale Curvature
Herbert W. Hamber, Ruth M. Williams
22 pages, 6 figures
(Submitted on 15 Jun 2007)
"In a quantum theory of gravity the gravitational Wilson loop, defined as a suitable quantum average of a parallel transport operator around a large near-planar loop, provides important information about the large-scale curvature properties of the geometry. Here we shows that such properties can be systematically computed in the strong coupling limit of lattice regularized quantum gravity, by performing a local average over rotations, using an assumed near-uniform measure in group space. We then relate the resulting quantum averages to an expected semi-classical form valid for macroscopic observers, which leads to an identification of the gravitational correlation length appearing in the Wilson loop with an observed large-scale curvature. Our results suggest that strongly coupled gravity leads to a positively curved (De Sitter-like) quantum ground state, implying a positive effective cosmological constant at large distances."
Bojowald also recently showed that a positive effective cosmological constant may not need to be put in "by hand" but could be *derived* from quantum gravity. Is there some connection with Hamber and Williams result? Francesca reported this Bojowald paper a few posts back in this thread, and Jal in a separate thread, but I will repeat it since it seems especially important:
http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
(Submitted on 30 May 2007)
While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
Meanwhile Michael Turner, apparently unaware of Bojowald's result, or the Hamber Williams paper, surveys the problem.
http://arxiv.org/abs/0706.2186
Cosmic Acceleration, Dark Energy and Fundamental Physics
Michael S. Turner, Dragan Huterer (KICP, University of Chicago)
10 pages, 8 figures, invited review for Journal of the Physical Society of Japan, in press
(Submitted on 14 Jun 2007)
"A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (p\sim -rho), very smooth form of energy called 'dark energy' accounting for about 75% of the Universe. The 'simplest' explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science."
brief mention:
Masters Thesis in LQG by Muxin Han, a student of Jorge Pullin at LSU
http://arxiv.org/abs/0706.2623
Quantum Dyanmics of Loop Quantum Gravity
95 pages
http://arxiv.org/abs/0706.2445
GRB Cosmology
Volker Bromm, Abraham Loeb
24 pages, 9 figures, review to appear in "Gamma-ray Bursts" (CUP)
(Submitted on 18 Jun 2007)
"Current observations are about to open up a direct window into the final frontier of cosmology: the first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium (IGM) up to redshifts of several tens, when the first (Population III) stars had formed. The Swift mission, or future satellites such as EXIST, might be the first observatories to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up observations of the GRB afterglow emission would allow to probe the ionization state and metal enrichment of the IGM as a function of redshift."
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http://arxiv.org/abs/0706.3658
Field theory on kappa--Minkowski space revisited: Noether charges and breaking of Lorentz symmetry
L. Freidel, J. Kowalski-Glikman, S. Nowak
22 pages, 1 figure
(Submitted on 25 Jun 2007)
"This paper is devoted to detailed investigations of free scalar field theory on $\kappa$-Minkowski space. After reviewing necessary mathematical tools we discuss in depth the Lagrangian and solutions of field equations. We analyze the spacetime symmetries of the model and construct the conserved charges associated with translational and Lorentz symmetry. We show that the version of the theory usually studied breaks Lorentz invariance in a subtle way: There is an additional trans-Planckian mode present, and an associated conserved charge (the number of such modes) is not a Lorentz scalar."
http://arxiv.org/abs/0706.3688
Why the Standard Model
Ali H. Chamseddine, Alain Connes
13 pages
(Submitted on 25 Jun 2007)
"The Standard Model is based on the gauge invariance principle with gauge group U(1)xSU(2)xSU(3) and suitable representations for fermions and bosons, which are begging for a conceptual understanding. We propose a purely gravitational explanation: space-time has a fine structure given as a product of a four dimensional continuum by a finite noncommutative geometry F. The raison d'etre for F is to correct the K-theoretic dimension from four to ten (modulo eight). We classify the irreducible finite noncommutative geometries of K-theoretic dimension six and show that the dimension (per generation) is a square of an integer k. Under an additional hypothesis of quaternion linearity, the geometry which reproduces the Standard Model is singled out (and one gets k=4)with the correct quantum numbers for all fields. The spectral action applied to the product MxF delivers the full Standard Model,with neutrino mixing, coupled to gravity, and makes predictions(the number of generations is still an input)."
http://arxiv.org/abs/0706.3690
A Dress for SM the Beggar
Ali H. Chamseddine
4 pages
(Submitted on 25 Jun 2007)
"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states." http://arxiv.org/abs/0706.3586
Graceful exit via polymerization of pre-big bang cosmology
Giuseppe De Risi, Roy Maartens, Parampreet Singh
(Submitted on 25 Jun 2007)
"We consider a phenomenological modification of the Pre Big Bang scenario using ideas from the resolution of curvature singularities in Loop Quantum Cosmology. We show that non-perturbative Loop modifications to the dynamics, arising from the underlying polymer representation, can resolve the graceful exit problem. The curvature and the dilaton energy stay finite at all times, in both the string and Einstein frames. In the string frame, the dilaton tends to a constant value at late times after the bounce."
Subjects: High Energy Physics - Theory (hep-th)
Report number: IGPG-07/6-9http://arxiv.org/abs/0706.3239
Black hole entropy, curved space and monsters
Stephen D.H. Hsu, David Reeb
4 pages
(Submitted on 21 Jun 2007)
"We investigate the microscopic origin of black hole entropy, in particular the gap between the maximum entropy of ordinary matter and that of black holes. Using curved space, we construct configurations with entropy greater than their area in Planck units. These configurations have pathological properties and we refer to them as monsters. When monsters are excluded we recover the entropy bound on ordinary matter S < A3/4. This bound implies that essentially all of the microstates of a semiclassical black hole are associated with the growth of a slightly smaller black hole which absorbs some additional energy. Our results suggest that the area entropy of black holes is the logarithm of the number of distinct ways in which one can form the black hole from ordinary matter and smaller black holes, but only after the exclusion of monster states."
Steve Hsu is a proven researcher with a good publication record. Although this paper sounds very strange, I'll gamble on it being of interest.
Field theory on kappa--Minkowski space revisited: Noether charges and breaking of Lorentz symmetry
L. Freidel, J. Kowalski-Glikman, S. Nowak
22 pages, 1 figure
(Submitted on 25 Jun 2007)
"This paper is devoted to detailed investigations of free scalar field theory on $\kappa$-Minkowski space. After reviewing necessary mathematical tools we discuss in depth the Lagrangian and solutions of field equations. We analyze the spacetime symmetries of the model and construct the conserved charges associated with translational and Lorentz symmetry. We show that the version of the theory usually studied breaks Lorentz invariance in a subtle way: There is an additional trans-Planckian mode present, and an associated conserved charge (the number of such modes) is not a Lorentz scalar."
http://arxiv.org/abs/0706.3688
Why the Standard Model
Ali H. Chamseddine, Alain Connes
13 pages
(Submitted on 25 Jun 2007)
"The Standard Model is based on the gauge invariance principle with gauge group U(1)xSU(2)xSU(3) and suitable representations for fermions and bosons, which are begging for a conceptual understanding. We propose a purely gravitational explanation: space-time has a fine structure given as a product of a four dimensional continuum by a finite noncommutative geometry F. The raison d'etre for F is to correct the K-theoretic dimension from four to ten (modulo eight). We classify the irreducible finite noncommutative geometries of K-theoretic dimension six and show that the dimension (per generation) is a square of an integer k. Under an additional hypothesis of quaternion linearity, the geometry which reproduces the Standard Model is singled out (and one gets k=4)with the correct quantum numbers for all fields. The spectral action applied to the product MxF delivers the full Standard Model,with neutrino mixing, coupled to gravity, and makes predictions(the number of generations is still an input)."
http://arxiv.org/abs/0706.3690
A Dress for SM the Beggar
Ali H. Chamseddine
4 pages
(Submitted on 25 Jun 2007)
"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states." http://arxiv.org/abs/0706.3586
Graceful exit via polymerization of pre-big bang cosmology
Giuseppe De Risi, Roy Maartens, Parampreet Singh
(Submitted on 25 Jun 2007)
"We consider a phenomenological modification of the Pre Big Bang scenario using ideas from the resolution of curvature singularities in Loop Quantum Cosmology. We show that non-perturbative Loop modifications to the dynamics, arising from the underlying polymer representation, can resolve the graceful exit problem. The curvature and the dilaton energy stay finite at all times, in both the string and Einstein frames. In the string frame, the dilaton tends to a constant value at late times after the bounce."
Subjects: High Energy Physics - Theory (hep-th)
Report number: IGPG-07/6-9http://arxiv.org/abs/0706.3239
Black hole entropy, curved space and monsters
Stephen D.H. Hsu, David Reeb
4 pages
(Submitted on 21 Jun 2007)
"We investigate the microscopic origin of black hole entropy, in particular the gap between the maximum entropy of ordinary matter and that of black holes. Using curved space, we construct configurations with entropy greater than their area in Planck units. These configurations have pathological properties and we refer to them as monsters. When monsters are excluded we recover the entropy bound on ordinary matter S < A3/4. This bound implies that essentially all of the microstates of a semiclassical black hole are associated with the growth of a slightly smaller black hole which absorbs some additional energy. Our results suggest that the area entropy of black holes is the logarithm of the number of distinct ways in which one can form the black hole from ordinary matter and smaller black holes, but only after the exclusion of monster states."
Steve Hsu is a proven researcher with a good publication record. Although this paper sounds very strange, I'll gamble on it being of interest.
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http://arxiv.org/abs/0706.4431
The Einstein static universe in Loop Quantum Cosmology
Luca Parisi, Marco Bruni, Roy Maartens, Kevin Vandersloot
6 pages, 7 figures
(Submitted on 29 Jun 2007)
"Loop Quantum Cosmology strongly modifies the high-energy dynamics of Friedman-Robertson-Walker models and removes the big-bang singularity. We investigate how LQC corrections affect the stability properties of the Einstein static universe. In General Relativity, the Einstein static model with positive cosmological constant Lambda is unstable to homogeneous perturbations. We show that LQC modifications can lead to a centre of stability for a large enough positive value of Lambda."
http://arxiv.org/abs/0706.4452
What is the Mathematical Structure of Quantum Spacetime?
Louis Crane
25 pages
(Submitted on 29 Jun 2007)
"We survey indications from different branches of Physics that the fine scale structure of spacetime is not adequately described by a manifold. Based on the hints we accumulate, we propose a new structure, which we call a quantum topos. In the process of constructing a quantum topos for quantum gravity, we propose a new, operational approach to the problem of the obervables in quantum gravity, which leads to a new mathematical point of view on the state sum models."
http://arxiv.org/abs/0706.4481
Isogravity: Toward an Electroweak and Gravitational Unification
Stephon H.S. Alexander
(Submitted on 29 Jun 2007)
"We present a model that unites the electroweak interaction with general relativity without specifying a space-time metric. This is made possible by embedding the kinetic terms for gravity and electroweak theory using one SL connection variable. The gauge theory is specified without relying on a space-time metric. We show that once a symmetry breaking mechanism is implemented that selects a global time-like direction, the electroweak theory and general relativity emerges with their associated massless degrees of freedom; the spin 1 vector boson and the spin 2 graviton."
The Einstein static universe in Loop Quantum Cosmology
Luca Parisi, Marco Bruni, Roy Maartens, Kevin Vandersloot
6 pages, 7 figures
(Submitted on 29 Jun 2007)
"Loop Quantum Cosmology strongly modifies the high-energy dynamics of Friedman-Robertson-Walker models and removes the big-bang singularity. We investigate how LQC corrections affect the stability properties of the Einstein static universe. In General Relativity, the Einstein static model with positive cosmological constant Lambda is unstable to homogeneous perturbations. We show that LQC modifications can lead to a centre of stability for a large enough positive value of Lambda."
http://arxiv.org/abs/0706.4452
What is the Mathematical Structure of Quantum Spacetime?
Louis Crane
25 pages
(Submitted on 29 Jun 2007)
"We survey indications from different branches of Physics that the fine scale structure of spacetime is not adequately described by a manifold. Based on the hints we accumulate, we propose a new structure, which we call a quantum topos. In the process of constructing a quantum topos for quantum gravity, we propose a new, operational approach to the problem of the obervables in quantum gravity, which leads to a new mathematical point of view on the state sum models."
http://arxiv.org/abs/0706.4481
Isogravity: Toward an Electroweak and Gravitational Unification
Stephon H.S. Alexander
(Submitted on 29 Jun 2007)
"We present a model that unites the electroweak interaction with general relativity without specifying a space-time metric. This is made possible by embedding the kinetic terms for gravity and electroweak theory using one SL connection variable. The gauge theory is specified without relying on a space-time metric. We show that once a symmetry breaking mechanism is implemented that selects a global time-like direction, the electroweak theory and general relativity emerges with their associated massless degrees of freedom; the spin 1 vector boson and the spin 2 graviton."
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http://arxiv.org/abs/0707.0588
Lattice Refining LQC and the Matter Hamiltonian
William Nelson, Mairi Sakellariadou (King's College, London)
14 pages, 3 figures
(Submitted on 4 Jul 2007)
"In the context of loop quantum cosmology, we parametrise the lattice refinement by a parameter, A, and the matter Hamiltonian by a parameter, delta. We then solve the Hamiltonian constraint for both a self-adjoint, and a non-self-adjoint Hamiltonian operator. Demanding that the solutions for the wave-functions obey certain physical restrictions, we impose constraints on the two-dimensional, (A,delta), parameter space, thereby restricting the types of matter content that can be supported by a particular lattice refinement model."brief mention of a curious paper, seemingly off beaten track, which however has been accepted for publication by Classical and Quantum Gravity
http://arxiv.org/abs/0707.0341
Considering boundary conditions for black hole entropy in loop quantum gravity
Takashi Tamaki
4 pages, final version to be published in CQG
(Submitted on 3 Jul 2007)
"We argue for black hole entropy in loop quantum gravity (LQG) by taking into account the interpretation that there is no other side of the horizon..."
Lattice Refining LQC and the Matter Hamiltonian
William Nelson, Mairi Sakellariadou (King's College, London)
14 pages, 3 figures
(Submitted on 4 Jul 2007)
"In the context of loop quantum cosmology, we parametrise the lattice refinement by a parameter, A, and the matter Hamiltonian by a parameter, delta. We then solve the Hamiltonian constraint for both a self-adjoint, and a non-self-adjoint Hamiltonian operator. Demanding that the solutions for the wave-functions obey certain physical restrictions, we impose constraints on the two-dimensional, (A,delta), parameter space, thereby restricting the types of matter content that can be supported by a particular lattice refinement model."brief mention of a curious paper, seemingly off beaten track, which however has been accepted for publication by Classical and Quantum Gravity
http://arxiv.org/abs/0707.0341
Considering boundary conditions for black hole entropy in loop quantum gravity
Takashi Tamaki
4 pages, final version to be published in CQG
(Submitted on 3 Jul 2007)
"We argue for black hole entropy in loop quantum gravity (LQG) by taking into account the interpretation that there is no other side of the horizon..."
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John Baez has a useful Derek Wise page
http://math.ucr.edu/home/baez/derek/
Cartanization is something that is likely to happen soon in quantum geometry/gravity.
Derek Wise thesis is a step in that direction and Baez page has links to talks and stuff related to that---preliminary research by him and Wise and others.
http://math.ucr.edu/home/baez/derek/
Cartanization is something that is likely to happen soon in quantum geometry/gravity.
Derek Wise thesis is a step in that direction and Baez page has links to talks and stuff related to that---preliminary research by him and Wise and others.
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Thomas Thiemann's big book comes out in September
https://www.amazon.com/dp/0521842638/?tag=pfamazon01-20
Cambridge University Press.
688 pages (!)
there is a very early draft (2001) on arxiv
but that may not bear much relation to what is to appear six years later
Here is the C.U.P. webpage about the book, which gives the table of contents and other information:
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521842631
these are some exerpts from the publisher's webpage:
Modern Canonical Quantum General Relativity
Cambridge Monographs on Mathematical Physics
Thomas Thiemann
Max-Planck-Institut für Gravitationsphysik, Germany
...
...In order to construct quantum gravity one must reformulate quantum theory in a background independent way. ... complete treatise of the canonical quantisation of general relativity... can be read by graduate students with basic knowledge of quantum field theory or general relativity.
...
Contents
Preface; Notation and conventions; Introduction;
Part I. Classical Foundations, Interpretation and the Canonical Quantisation Programme:
1. Classical Hamiltonian formulation of general relativity;
2. The problem of time, locality and the interpretation of quantum mechanics;
3. The programme of canonical quantisation;
4. The new canonical variables of Ashtekar for general relativity;
Part II. Foundations of Modern Canonical Quantum General Relativity:
5. Introduction;
6. Step I: the holonomy-flux algebra [P];
7. Step II: quantum algebra;
8. Step III: representation theory of [A];
9. Step IV: 1. Implementation and solution of the kinematical constraints;
10. Step V: 2. implementation and solution of the Hamiltonian constraint;
11. Step VI: semiclassical analysis;
Part III. Physical Applications:
12. Extension to standard matter;
13. Kinematical geometrical operators;
14. Spin foam models;
15. Quantum black hole physics;
16. Applications to particle physics and quantum cosmology;
17. Loop quantum gravity phenomenology;
Part IV. Mathematical Tools and their Connection to Physics:
18. Tools from general topology;
19. Differential, Riemannian, symplectic and complex geometry;
20. Semianalytical category;
21. Elements of fibre bundle theory;
22. Holonomies on non-trivial fibre bundles;
23. Geometric quantisation;
24. The Dirac algorithm for field theories with constraints;
25. Tools from measure theory;
26. Elementary introduction to Gelfand theory for Abelean C* algebras;
27. Bohr compactification of the real line;
28. Operator algebras and spectral theorem;
29. Refined algebraic quantisation (RAQ) and direct integral decomposition (DID);
30. Basics of harmonic analysis on compact Lie groups;
31. Spin network functions for SU(2);
32. Functional analytical description of classical connection dynamics;
Bibliography;
Index.
https://www.amazon.com/dp/0521842638/?tag=pfamazon01-20
Cambridge University Press.
688 pages (!)
there is a very early draft (2001) on arxiv
but that may not bear much relation to what is to appear six years later
Here is the C.U.P. webpage about the book, which gives the table of contents and other information:
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521842631
these are some exerpts from the publisher's webpage:
Modern Canonical Quantum General Relativity
Cambridge Monographs on Mathematical Physics
Thomas Thiemann
Max-Planck-Institut für Gravitationsphysik, Germany
...
...In order to construct quantum gravity one must reformulate quantum theory in a background independent way. ... complete treatise of the canonical quantisation of general relativity... can be read by graduate students with basic knowledge of quantum field theory or general relativity.
...
Contents
Preface; Notation and conventions; Introduction;
Part I. Classical Foundations, Interpretation and the Canonical Quantisation Programme:
1. Classical Hamiltonian formulation of general relativity;
2. The problem of time, locality and the interpretation of quantum mechanics;
3. The programme of canonical quantisation;
4. The new canonical variables of Ashtekar for general relativity;
Part II. Foundations of Modern Canonical Quantum General Relativity:
5. Introduction;
6. Step I: the holonomy-flux algebra [P];
7. Step II: quantum algebra;
8. Step III: representation theory of [A];
9. Step IV: 1. Implementation and solution of the kinematical constraints;
10. Step V: 2. implementation and solution of the Hamiltonian constraint;
11. Step VI: semiclassical analysis;
Part III. Physical Applications:
12. Extension to standard matter;
13. Kinematical geometrical operators;
14. Spin foam models;
15. Quantum black hole physics;
16. Applications to particle physics and quantum cosmology;
17. Loop quantum gravity phenomenology;
Part IV. Mathematical Tools and their Connection to Physics:
18. Tools from general topology;
19. Differential, Riemannian, symplectic and complex geometry;
20. Semianalytical category;
21. Elements of fibre bundle theory;
22. Holonomies on non-trivial fibre bundles;
23. Geometric quantisation;
24. The Dirac algorithm for field theories with constraints;
25. Tools from measure theory;
26. Elementary introduction to Gelfand theory for Abelean C* algebras;
27. Bohr compactification of the real line;
28. Operator algebras and spectral theorem;
29. Refined algebraic quantisation (RAQ) and direct integral decomposition (DID);
30. Basics of harmonic analysis on compact Lie groups;
31. Spin network functions for SU(2);
32. Functional analytical description of classical connection dynamics;
Bibliography;
Index.
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Judging by the toc, that looks extremely nice. Hope it will be really pitched at level of the beginner who has some knowledge in QFT and GR. Thielmann likes it more rigorous, am I right?
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to get an audio+slide presentation that gives a condensed Thiemann treatment and could give, in a way, a "taste" of the book, get the slides here:
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
at the end, he says since he went very fast over it, and was a little sloppy in places, that everybody should buy his book (so they would understand then thoroughly the details)
and the last slide is a picture of his book.
BTW Thiemann mentions this animation sequence in his slides. It is a picture of spinnetwork quantum states of geometry evolving----but for graphic purposes one can replace a spinnetwork picture by a dual triangulation picture---so each colored EDGE is replaced by a colored TRIANGLE, and four-valent NODES are replaced by (foursided) TETRAHEDRA. So then the combinatorial or mathematical content is exactly the same there is simply more color in the picture. One sees chunky colored blocks instead of skinny colored sticks. Perhaps as a demo some animation studio made a short sequence of this model of evolving geometry. I will put the link here and try it to see if it works:
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/index.html
well it takes two minutes (part of which is title and credits). to find the links to the animation you need to scroll down pretty far on the index page.
If you have a Mac then you can skip the index page and click directly on
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/spinfoam2.mov
to me it looks like an abstract cinematic artwork and not like I could learn from it, but maybe that's just me.
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
at the end, he says since he went very fast over it, and was a little sloppy in places, that everybody should buy his book (so they would understand then thoroughly the details)
and the last slide is a picture of his book.
BTW Thiemann mentions this animation sequence in his slides. It is a picture of spinnetwork quantum states of geometry evolving----but for graphic purposes one can replace a spinnetwork picture by a dual triangulation picture---so each colored EDGE is replaced by a colored TRIANGLE, and four-valent NODES are replaced by (foursided) TETRAHEDRA. So then the combinatorial or mathematical content is exactly the same there is simply more color in the picture. One sees chunky colored blocks instead of skinny colored sticks. Perhaps as a demo some animation studio made a short sequence of this model of evolving geometry. I will put the link here and try it to see if it works:
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/index.html
well it takes two minutes (part of which is title and credits). to find the links to the animation you need to scroll down pretty far on the index page.
If you have a Mac then you can skip the index page and click directly on
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/spinfoam2.mov
to me it looks like an abstract cinematic artwork and not like I could learn from it, but maybe that's just me.
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One of the most impressive talks at Loops '07 was probably the one given by Martin Reuter about "asymptotically safe" quantum gravity, specifically the QEG approach.
get the slides here:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
==============
I am not familiar with the next author, he is perhaps a new arrival in the LQC field
http://arxiv.org/abs/0707.1816
Open FRW model in Loop Quantum Cosmology
Lukasz Szulc
12 pages
(Submitted on 12 Jul 2007)
"Open FRW model in Loop Quantum Cosmology is under consideration. The left and right invariant vector fields and holonomies along them are studied. It is shown that in the hyperbolic geometry of k=-1 it is possible to construct a suitable loop which provides us with quantum scalar constraint originally introduced by Vandersloot. Such an operator has correct geometrical interpretation. The quantum scalar constraint operator with negative cosmological constant is proven to be essentially self-adjoint."
get the slides here:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
==============
I am not familiar with the next author, he is perhaps a new arrival in the LQC field
http://arxiv.org/abs/0707.1816
Open FRW model in Loop Quantum Cosmology
Lukasz Szulc
12 pages
(Submitted on 12 Jul 2007)
"Open FRW model in Loop Quantum Cosmology is under consideration. The left and right invariant vector fields and holonomies along them are studied. It is shown that in the hyperbolic geometry of k=-1 it is possible to construct a suitable loop which provides us with quantum scalar constraint originally introduced by Vandersloot. Such an operator has correct geometrical interpretation. The quantum scalar constraint operator with negative cosmological constant is proven to be essentially self-adjoint."
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Penrose "before big bang" talk
Penrose talk given on 7 November 2005 at the Isaac Newton Institute at Cambridge describing
some "crazy ideas" about what came before the big bang
http://www.Newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/
audio-and-slides show, great handdrawn pictures by Penrose
===========
http://arxiv.org/abs/0707.2548
The behavior of non-linear anisotropies in bouncing Bianchi I models of loop quantum cosmology
Dah-Wei Chiou, Kevin Vandersloot
15 pages, 10 figures
(Submitted on 17 Jul 2007)
"In homogeneous and isotropic loop quantum cosmology, gravity can behave repulsively at Planckian energy densities leading to the replacement of the big bang singularity with a big bounce. Yet in any bouncing scenario it is important to include non-linear effects from anisotropies which typically grow during the collapsing phase. We investigate the dynamics of a Bianchi I anisotropic model within the framework of loop quantum cosmology. Using effective semi-classical equations of motion to study the dynamics, we show that the big bounce is still predicted with only differences in detail arising from the inclusion of anisotropies. We show that the anisotropic shear term grows during the collapsing phase, but remains finite through the bounce. Immediately following the bounce, the anisotropies decay and with the inclusion of matter with equation of state w < +1, the universe isotropizes in the expanding phase."
=================================
http://arxiv.org/abs/0707.2153
Dark Energy from Structure - A Status Report
Thomas Buchert
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 58 pages, 2 figures
(Submitted on 14 Jul 2007)
"The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein's theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (`morphon field') modeling Dark Energy.
The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation.
The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches all fall short in correctly describing the Late Universe."
we now have FOUR preprints of articles which are to be included in the SPECIAL GRG ISSUE ON DARK ENERGY
the one that appeared earlier is by Bojowald, describing how the effect of accelerating expansion could be the result of a quantum correction in LQG dynamics---something which, if true, would lead to a distinctive expansion history different from what one would see in a simple cosmological constant model.
So besides Buchert, we have Bojowald, Koyama, and Padmanabhan:
1. arXiv:0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
2. arXiv:0706.1557
The cosmological constant and dark energy in braneworlds
Kazuya Koyama
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 22 pages, 13 figures
3. arXiv:0705.2533
Title: Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures
We also know that the team editing the special issue is George Ellis of Capetown, Roy Maartens of Portsmouth UK, and Hermann Nicolai of AEI Potsdam.
============
I am currently in doubt as to what to make of this paper by Philip Mannheim
http://arxiv.org/abs/0707.2283
Conformal Gravity Challenges String Theory
Philip D. Mannheim
8 pages. Proceedings write-up of talk presented at PASCOS-07, Imperial College London, July 2007
(Submitted on 16 Jul 2007)
"The cosmological constant problem and the compatibility of gravity with quantum mechanics are the two most pressing problems in all of gravitational theory. While string theory nicely addresses the latter, it has so far failed to provide any compelling solution to the former. On the other hand, while conformal gravity nicely addresses the cosmological constant problem (by naturally quenching the amount by which the cosmological constant gravitates rather than by quenching the cosmological constant itself), the fourth order derivative conformal theory has long been thought to possesses a ghost when quantized. However, it has recently been shown by Bender and Mannheim that not only do theories based on fourth order derivative equations of motion not have ghosts, they actually never had any to begin with, with the apparent presence of ghosts being due entirely to treating operators which were not Hermitian on the real axis as though they were. When this is taken care of via an underlying PT symmetry that such theories are found to possess, there are then no ghosts at all and the S-matrix is fully unitary. Conformal gravity is thus advanced as a fully consistent four-dimensional alternative to ten-dimensional string theory."
An essential reference in this paper is [1] another Mannheim article in Progress in Particle and Nuclear Physics 2005
http://arxiv.org/abs/astro-ph/0505266
Alternatives to Dark Matter and Dark Energy
Philip D. Mannheim (University of Connecticut)
87 pages, 3 figures. To appear in Progress in Particle and Nuclear Physics, 2005
(Submitted on 12 May 2005 (v1), last revised 1 Aug 2005 (this version, v2))
"We review the underpinnings of the standard Newton-Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy-momentum tensor which serves as the source of gravity. We identify the a priori assumption of the validity of standard gravity on all distance scales as the root cause of the dark matter and dark energy problems, and discuss how the freedom currently present in gravitational theory can enable us to construct candidate alternatives to the standard theory in which the dark matter and dark energy problems could then be resolved. We identify three generic aspects of these alternate approaches: that it is a universal acceleration scale which determines when a luminous Newtonian expectation is to fail to fit data, that there is a global cosmological effect on local galactic motions which can replace galactic dark matter, and that to solve the cosmological constant problem it is not necessary to quench the cosmological constant itself, but only the amount by which it gravitates."
Another key reference [2] is
http://arxiv.org/abs/0706.0207
No-ghost theorem for the fourth-order derivative Pais-Uhlenbeck oscillator model
Carl M. Bender, Philip D. Mannheim
4 pages
(Submitted on 1 Jun 2007)
"Contrary to common belief, it is shown that theories whose field equations are higher than second order in derivatives need not be stricken with ghosts. In particular, the prototypical fourth-order derivative Pais-Uhlenbeck oscillator model is shown to be free of states of negative energy or negative norm. When correctly formulated (as a PT symmetric theory), the theory determines its own Hilbert space and associated positive-definite inner product. In this Hilbert space the model is found to be a fully acceptable quantum-mechanical theory that exhibits unitary time evolution."
Penrose talk given on 7 November 2005 at the Isaac Newton Institute at Cambridge describing
some "crazy ideas" about what came before the big bang
http://www.Newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/
audio-and-slides show, great handdrawn pictures by Penrose
===========
http://arxiv.org/abs/0707.2548
The behavior of non-linear anisotropies in bouncing Bianchi I models of loop quantum cosmology
Dah-Wei Chiou, Kevin Vandersloot
15 pages, 10 figures
(Submitted on 17 Jul 2007)
"In homogeneous and isotropic loop quantum cosmology, gravity can behave repulsively at Planckian energy densities leading to the replacement of the big bang singularity with a big bounce. Yet in any bouncing scenario it is important to include non-linear effects from anisotropies which typically grow during the collapsing phase. We investigate the dynamics of a Bianchi I anisotropic model within the framework of loop quantum cosmology. Using effective semi-classical equations of motion to study the dynamics, we show that the big bounce is still predicted with only differences in detail arising from the inclusion of anisotropies. We show that the anisotropic shear term grows during the collapsing phase, but remains finite through the bounce. Immediately following the bounce, the anisotropies decay and with the inclusion of matter with equation of state w < +1, the universe isotropizes in the expanding phase."
=================================
http://arxiv.org/abs/0707.2153
Dark Energy from Structure - A Status Report
Thomas Buchert
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 58 pages, 2 figures
(Submitted on 14 Jul 2007)
"The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein's theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (`morphon field') modeling Dark Energy.
The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation.
The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches all fall short in correctly describing the Late Universe."
we now have FOUR preprints of articles which are to be included in the SPECIAL GRG ISSUE ON DARK ENERGY
the one that appeared earlier is by Bojowald, describing how the effect of accelerating expansion could be the result of a quantum correction in LQG dynamics---something which, if true, would lead to a distinctive expansion history different from what one would see in a simple cosmological constant model.
So besides Buchert, we have Bojowald, Koyama, and Padmanabhan:
1. arXiv:0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
2. arXiv:0706.1557
The cosmological constant and dark energy in braneworlds
Kazuya Koyama
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 22 pages, 13 figures
3. arXiv:0705.2533
Title: Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures
We also know that the team editing the special issue is George Ellis of Capetown, Roy Maartens of Portsmouth UK, and Hermann Nicolai of AEI Potsdam.
============
I am currently in doubt as to what to make of this paper by Philip Mannheim
http://arxiv.org/abs/0707.2283
Conformal Gravity Challenges String Theory
Philip D. Mannheim
8 pages. Proceedings write-up of talk presented at PASCOS-07, Imperial College London, July 2007
(Submitted on 16 Jul 2007)
"The cosmological constant problem and the compatibility of gravity with quantum mechanics are the two most pressing problems in all of gravitational theory. While string theory nicely addresses the latter, it has so far failed to provide any compelling solution to the former. On the other hand, while conformal gravity nicely addresses the cosmological constant problem (by naturally quenching the amount by which the cosmological constant gravitates rather than by quenching the cosmological constant itself), the fourth order derivative conformal theory has long been thought to possesses a ghost when quantized. However, it has recently been shown by Bender and Mannheim that not only do theories based on fourth order derivative equations of motion not have ghosts, they actually never had any to begin with, with the apparent presence of ghosts being due entirely to treating operators which were not Hermitian on the real axis as though they were. When this is taken care of via an underlying PT symmetry that such theories are found to possess, there are then no ghosts at all and the S-matrix is fully unitary. Conformal gravity is thus advanced as a fully consistent four-dimensional alternative to ten-dimensional string theory."
An essential reference in this paper is [1] another Mannheim article in Progress in Particle and Nuclear Physics 2005
http://arxiv.org/abs/astro-ph/0505266
Alternatives to Dark Matter and Dark Energy
Philip D. Mannheim (University of Connecticut)
87 pages, 3 figures. To appear in Progress in Particle and Nuclear Physics, 2005
(Submitted on 12 May 2005 (v1), last revised 1 Aug 2005 (this version, v2))
"We review the underpinnings of the standard Newton-Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy-momentum tensor which serves as the source of gravity. We identify the a priori assumption of the validity of standard gravity on all distance scales as the root cause of the dark matter and dark energy problems, and discuss how the freedom currently present in gravitational theory can enable us to construct candidate alternatives to the standard theory in which the dark matter and dark energy problems could then be resolved. We identify three generic aspects of these alternate approaches: that it is a universal acceleration scale which determines when a luminous Newtonian expectation is to fail to fit data, that there is a global cosmological effect on local galactic motions which can replace galactic dark matter, and that to solve the cosmological constant problem it is not necessary to quench the cosmological constant itself, but only the amount by which it gravitates."
Another key reference [2] is
http://arxiv.org/abs/0706.0207
No-ghost theorem for the fourth-order derivative Pais-Uhlenbeck oscillator model
Carl M. Bender, Philip D. Mannheim
4 pages
(Submitted on 1 Jun 2007)
"Contrary to common belief, it is shown that theories whose field equations are higher than second order in derivatives need not be stricken with ghosts. In particular, the prototypical fourth-order derivative Pais-Uhlenbeck oscillator model is shown to be free of states of negative energy or negative norm. When correctly formulated (as a PT symmetric theory), the theory determines its own Hilbert space and associated positive-definite inner product. In this Hilbert space the model is found to be a fully acceptable quantum-mechanical theory that exhibits unitary time evolution."
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Benedetti's thesis (Loll CDT group at Utrecht)
http://arxiv.org/abs/0707.3070
Quantum Gravity from Simplices: Analytical Investigations of Causal Dynamical Triangulations
Dario Benedetti
116 pages, 42 figures; PhD thesis, Utrecht University, Advisor: Prof. Renate Loll
(Submitted on 20 Jul 2007)
"A potentially powerful approach to quantum gravity has been developed over the last few years under the name of Causal Dynamical Triangulations. Numerical simulations have given very interesting results in the cases of two, three and four spacetime dimension. The aim of this thesis is to give an introduction to the subject (Chapter 1), and try to push the analytical understanding of these models further. This is done by first studying (Chapter 2) the case of a (1+1)-dimensional spacetime coupled to matter, in the form of an Ising model, by means of high- and low-temperature expansions. And after (Chapter 3) by studying a specific model in (2+1) dimensions, whose solution and continuum limit are presented."
http://arxiv.org/abs/0707.3064
Water vapour in the atmosphere of a transiting extrasolar planet
Giovanna Tinetti, Alfred Vidal-Madjar, Mao-Chang Liang, Jean-Philippe Beaulieu, Yuk Yung, Sean Carey, Robert J. Barber, Jonathan Tennyson, Ignasi Ribas, Nicole Allard, Gilda E. Ballester, David K. Sing, Franck Selsis
Nature 2007, 448, p 163
(Submitted on 20 Jul 2007)
"Water is predicted to be among, if not the most abundant molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets (hot-Jupiters) Several attempts have been made to detect water on an exoplanet, but have failed to find compelling evidence for it or led to claims that should be taken with caution. Here we report an analysis of recent observations of the hot-Jupiter HD189733b taken during the transit, where the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6, 5.8 and 8 microns. The larger effective radius observed at visible wavelengths may be due to either star variability or the presence of clouds/hazes. We explain the most recent thermal infrared observations of the planet during secondary transit behind the star, reporting a non-detection of water on HD189733b, as being a consequence of the nearly isothermal vertical profile of the planet.s atmosphere. Our results show that water is detectable on extrasolar planets using the primary transit technique and that the infrared should be a better wavelength region than the visible, for such searches."
http://arxiv.org/abs/0707.3070
Quantum Gravity from Simplices: Analytical Investigations of Causal Dynamical Triangulations
Dario Benedetti
116 pages, 42 figures; PhD thesis, Utrecht University, Advisor: Prof. Renate Loll
(Submitted on 20 Jul 2007)
"A potentially powerful approach to quantum gravity has been developed over the last few years under the name of Causal Dynamical Triangulations. Numerical simulations have given very interesting results in the cases of two, three and four spacetime dimension. The aim of this thesis is to give an introduction to the subject (Chapter 1), and try to push the analytical understanding of these models further. This is done by first studying (Chapter 2) the case of a (1+1)-dimensional spacetime coupled to matter, in the form of an Ising model, by means of high- and low-temperature expansions. And after (Chapter 3) by studying a specific model in (2+1) dimensions, whose solution and continuum limit are presented."
http://arxiv.org/abs/0707.3064
Water vapour in the atmosphere of a transiting extrasolar planet
Giovanna Tinetti, Alfred Vidal-Madjar, Mao-Chang Liang, Jean-Philippe Beaulieu, Yuk Yung, Sean Carey, Robert J. Barber, Jonathan Tennyson, Ignasi Ribas, Nicole Allard, Gilda E. Ballester, David K. Sing, Franck Selsis
Nature 2007, 448, p 163
(Submitted on 20 Jul 2007)
"Water is predicted to be among, if not the most abundant molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets (hot-Jupiters) Several attempts have been made to detect water on an exoplanet, but have failed to find compelling evidence for it or led to claims that should be taken with caution. Here we report an analysis of recent observations of the hot-Jupiter HD189733b taken during the transit, where the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6, 5.8 and 8 microns. The larger effective radius observed at visible wavelengths may be due to either star variability or the presence of clouds/hazes. We explain the most recent thermal infrared observations of the planet during secondary transit behind the star, reporting a non-detection of water on HD189733b, as being a consequence of the nearly isothermal vertical profile of the planet.s atmosphere. Our results show that water is detectable on extrasolar planets using the primary transit technique and that the infrared should be a better wavelength region than the visible, for such searches."
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http://arxiv.org/abs/0707.4026
Renormalization and black hole entropy in Loop Quantum Gravity
Ted Jacobson
7 pages
(Submitted on 26 Jul 2007)
"Microscopic state counting for a black hole in Loop Quantum Gravity yields a result proportional to horizon area, and inversely proportional to Newton's constant and the Immirzi parameter. It is argued here that before this result can be compared to the Bekenstein-Hawking entropy of a macroscopic black hole, the scale dependence of both Newton's constant and the area must be accounted for. The two entropies could then agree for any value of the Immirzi parameter, if a certain renormalization property holds."
Jacobson's reference [15] is a Martin Reuter paper
[15] M. Reuter and J. M. Schwindt, “Scale-dependent metric and causal
structures in quantum Einstein gravity,” JHEP 0701, 049 (2007)
[arXiv:hep-th/0611294].
Renormalization and black hole entropy in Loop Quantum Gravity
Ted Jacobson
7 pages
(Submitted on 26 Jul 2007)
"Microscopic state counting for a black hole in Loop Quantum Gravity yields a result proportional to horizon area, and inversely proportional to Newton's constant and the Immirzi parameter. It is argued here that before this result can be compared to the Bekenstein-Hawking entropy of a macroscopic black hole, the scale dependence of both Newton's constant and the area must be accounted for. The two entropies could then agree for any value of the Immirzi parameter, if a certain renormalization property holds."
Jacobson's reference [15] is a Martin Reuter paper
[15] M. Reuter and J. M. Schwindt, “Scale-dependent metric and causal
structures in quantum Einstein gravity,” JHEP 0701, 049 (2007)
[arXiv:hep-th/0611294].
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http://arxiv.org/abs/0707.4513
Linearized dynamics from the 4-simplex Regge action
Bianca Dittrich, Laurent Freidel, Simone Speziale
16 (+9 Appendix) pages, 1 figure
(Submitted on 31 Jul 2007)
"We study the relation between the hessian matrix of the riemannian Reggae action on a 4-simplex and linearized quantum gravity. We give an explicit formula for the hessian as a function of the geometry, and show that it has a single zero mode. We then use a 3d lattice model to show that (i) the zero mode is a remnant of the continuum diffeomorphism invariance, and (ii) we recover the complete free graviton propagator in the continuum limit. The results help clarify the structure of the boundary state needed in the recent calculations of the graviton propagator in loop quantum gravity, and in particular its role in fixing the gauge."
http://arxiv.org/abs/0708.0037
The Immirzi Parameter as a topological quantization ambiguity
Simone Mercuri
5 pages
(Submitted on 31 Jul 2007)
"The Immirzi ambiguity is traced back to the non-trivial behavior of the state functional under large gauge transformations of the spatial rotations group, emphasizing the role that the Nieh-Yan class plays in gravity when spinor matter is considered."
http://arxiv.org/abs/0707.4568
Emergent Quantum Mechanics and Emergent Symmetries
Gerard 't Hooft
10 pages, 1 figure. Presented at PASCOS 13, Imperial College, London, July 6, 2007
(Submitted on 31 Jul 2007)
"Quantum mechanics is 'emergent' if a statistical treatment of large scale phenomena in a locally deterministic theory requires the use of quantum operators. These quantum operators may allow for symmetry transformations that are not present in the underlying deterministic system. Such theories allow for a natural explanation of the existence of gauge equivalence classes (gauge orbits), including the equivalence classes generated by general coordinate transformations. Thus, local gauge symmetries and general coordinate invariance could be emergent symmetries, and this might lead to new alleys towards understanding the flatness problem of the Universe."
http://arxiv.org/abs/0707.4572
The Grand View of Physics
Gerard 't Hooft
5 pages, 1 figure. Presented at Salam +50, Imperial College, London, July 7, 2007
(Submitted on 31 Jul 2007)
"Abdus Salam was known for his `grand views', grand views of science as well as grand views of society. In this talk the grand view of theoretical physics is put in perspective."
Linearized dynamics from the 4-simplex Regge action
Bianca Dittrich, Laurent Freidel, Simone Speziale
16 (+9 Appendix) pages, 1 figure
(Submitted on 31 Jul 2007)
"We study the relation between the hessian matrix of the riemannian Reggae action on a 4-simplex and linearized quantum gravity. We give an explicit formula for the hessian as a function of the geometry, and show that it has a single zero mode. We then use a 3d lattice model to show that (i) the zero mode is a remnant of the continuum diffeomorphism invariance, and (ii) we recover the complete free graviton propagator in the continuum limit. The results help clarify the structure of the boundary state needed in the recent calculations of the graviton propagator in loop quantum gravity, and in particular its role in fixing the gauge."
http://arxiv.org/abs/0708.0037
The Immirzi Parameter as a topological quantization ambiguity
Simone Mercuri
5 pages
(Submitted on 31 Jul 2007)
"The Immirzi ambiguity is traced back to the non-trivial behavior of the state functional under large gauge transformations of the spatial rotations group, emphasizing the role that the Nieh-Yan class plays in gravity when spinor matter is considered."
http://arxiv.org/abs/0707.4568
Emergent Quantum Mechanics and Emergent Symmetries
Gerard 't Hooft
10 pages, 1 figure. Presented at PASCOS 13, Imperial College, London, July 6, 2007
(Submitted on 31 Jul 2007)
"Quantum mechanics is 'emergent' if a statistical treatment of large scale phenomena in a locally deterministic theory requires the use of quantum operators. These quantum operators may allow for symmetry transformations that are not present in the underlying deterministic system. Such theories allow for a natural explanation of the existence of gauge equivalence classes (gauge orbits), including the equivalence classes generated by general coordinate transformations. Thus, local gauge symmetries and general coordinate invariance could be emergent symmetries, and this might lead to new alleys towards understanding the flatness problem of the Universe."
http://arxiv.org/abs/0707.4572
The Grand View of Physics
Gerard 't Hooft
5 pages, 1 figure. Presented at Salam +50, Imperial College, London, July 7, 2007
(Submitted on 31 Jul 2007)
"Abdus Salam was known for his `grand views', grand views of science as well as grand views of society. In this talk the grand view of theoretical physics is put in perspective."
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http://arxiv.org/abs/0708.0250
Holography in spherically symmetric loop quantum gravity
Rodolfo Gambini, Jorge Pullin
5 pages
(Submitted on 2 Aug 2007)
"We show that holography arises naturally in the context of spherically symmetric loop quantum gravity. The result is not dependent on detailed assumptions about the dynamics of the theory being considered. It ties strongly the amount of information contained in a region of space to the tight mathematical underpinnings of loop quantum geometry, at least in this particular context."
http://arxiv.org/abs/0708.0062
On Information Theory, Spectral Geometry and Quantum Gravity
Achim Kempf, Robert Martin
4 pages
(Submitted on 1 Aug 2007)
"We show that there exists a deep link between the two disciplines of information theory and spectral geometry. This allows us to obtain new results on a well known quantum gravity motivated natural ultraviolet cutoff which describes an upper bound on the spatial density of information. Concretely, we show that, together with an infrared cutoff, this natural ultraviolet cutoff beautifully reduces the path integral of quantum field theory on curved space to a finite number of ordinary integrations. We then show, in particular, that the subsequent removal of the infrared cutoff is safe."
Although I could not evaluate this paper, it sounded too interesting not to mention.
Holography in spherically symmetric loop quantum gravity
Rodolfo Gambini, Jorge Pullin
5 pages
(Submitted on 2 Aug 2007)
"We show that holography arises naturally in the context of spherically symmetric loop quantum gravity. The result is not dependent on detailed assumptions about the dynamics of the theory being considered. It ties strongly the amount of information contained in a region of space to the tight mathematical underpinnings of loop quantum geometry, at least in this particular context."
http://arxiv.org/abs/0708.0062
On Information Theory, Spectral Geometry and Quantum Gravity
Achim Kempf, Robert Martin
4 pages
(Submitted on 1 Aug 2007)
"We show that there exists a deep link between the two disciplines of information theory and spectral geometry. This allows us to obtain new results on a well known quantum gravity motivated natural ultraviolet cutoff which describes an upper bound on the spatial density of information. Concretely, we show that, together with an infrared cutoff, this natural ultraviolet cutoff beautifully reduces the path integral of quantum field theory on curved space to a finite number of ordinary integrations. We then show, in particular, that the subsequent removal of the infrared cutoff is safe."
Although I could not evaluate this paper, it sounded too interesting not to mention.
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http://arxiv.org/abs/0708.0573
The Height of a Giraffe
Don N. Page
12 pages
(Submitted on 3 Aug 2007 (v1), last revised 3 Aug 2007 (this version, v2))
"A minor modification of the arguments of Press and Lightman leads to an estimate of the height of the tallest running, breathing organism on a habitable planet as the Bohr radius multiplied by the three-tenths power of the ratio of the electrical to gravitational forces between two protons (rather than the one-quarter power that Press got for the largest animal that would not break in falling over, after making an assumption of unreasonable brittleness). My new estimate gives a height of about 3.6 meters rather than Press's original estimate of about 2.6 cm. It also implies that the number of atoms in the tallest runner is very roughly of the order of the nine-tenths power of the ratio of the electrical to gravitational forces between two protons, which is about 3 x 10^32."
http://arxiv.org/abs/0708.0429
Observing the temperature of the Big Bang through large scale structure
Pedro Ferreira, Joao Magueijo
(Submitted on 2 Aug 2007)
"It is widely accepted that the Universe underwent a period of thermal equilibrium at very early times. One expects a residue of this primordial state to be imprinted on the large scale structure of space time. In this paper we study the morphology of this thermal residue in a universe whose early dynamics is governed by a scalar field. We calculate the amplitude of fluctuations on large scales and compare it to the imprint of vacuum fluctuations. We then use the observed power spectrum of fluctuations on the cosmic microwave background to place a constraint on the temperature of the Universe before and during inflation. We also present an alternative scenario where the fluctuations are predominantly thermal and near scale-invariant."
http://arxiv.org/abs/0708.0750
On the q-quantum gravity loop algebra
Seth Major
(Submitted on 6 Aug 2007)
"A class of deformations of the q-quantum gravity loop algebra is shown to be incompatible with the combinatorics of Temperley-Lieb recoupling theory with deformation parameter at a root of unity. This incompatibility appears to extend to more general deformation parameters."
The Height of a Giraffe
Don N. Page
12 pages
(Submitted on 3 Aug 2007 (v1), last revised 3 Aug 2007 (this version, v2))
"A minor modification of the arguments of Press and Lightman leads to an estimate of the height of the tallest running, breathing organism on a habitable planet as the Bohr radius multiplied by the three-tenths power of the ratio of the electrical to gravitational forces between two protons (rather than the one-quarter power that Press got for the largest animal that would not break in falling over, after making an assumption of unreasonable brittleness). My new estimate gives a height of about 3.6 meters rather than Press's original estimate of about 2.6 cm. It also implies that the number of atoms in the tallest runner is very roughly of the order of the nine-tenths power of the ratio of the electrical to gravitational forces between two protons, which is about 3 x 10^32."
http://arxiv.org/abs/0708.0429
Observing the temperature of the Big Bang through large scale structure
Pedro Ferreira, Joao Magueijo
(Submitted on 2 Aug 2007)
"It is widely accepted that the Universe underwent a period of thermal equilibrium at very early times. One expects a residue of this primordial state to be imprinted on the large scale structure of space time. In this paper we study the morphology of this thermal residue in a universe whose early dynamics is governed by a scalar field. We calculate the amplitude of fluctuations on large scales and compare it to the imprint of vacuum fluctuations. We then use the observed power spectrum of fluctuations on the cosmic microwave background to place a constraint on the temperature of the Universe before and during inflation. We also present an alternative scenario where the fluctuations are predominantly thermal and near scale-invariant."
http://arxiv.org/abs/0708.0750
On the q-quantum gravity loop algebra
Seth Major
(Submitted on 6 Aug 2007)
"A class of deformations of the q-quantum gravity loop algebra is shown to be incompatible with the combinatorics of Temperley-Lieb recoupling theory with deformation parameter at a root of unity. This incompatibility appears to extend to more general deformation parameters."
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Alejandro Satz has a series of four posts about the Loops 07 conference. The first had a number of photographs and the other three reported on presentations and discussion. Here's a sample from the last report:
==quote from Reality Conditions blog==
And now the last question. It asked, to all plenary speakers, to say they "dream for Loops '17"; that is, on their most optimistic possible view, what is the title and abstract of the talk they imagine themselves presenting within ten years?
Many of the answers were predictable and variations of a basic template: abstracts saying "we present a complete theory of quantum gravity with testable (or, in the most ambitious cases, confirmed) predictions." ... Reuter had one of the most concrete dreams: "It is shown that LQG is equivalent to Asymptotic Safety, and that that the quantization ambiguities in it are finite in number and equivalent to the dimensionality of the Non-Gaussian Fixed Point." And finally, there was an extremely amusing exchange between...
==endquote==
Loops 17 means the Quantum Geometry/Quantum Gravity conference of 2017, only ten years out from now. Reuter's program is a bold one---joining two fertile lines of QG/QG research. My sense of him doesn't compat with his saying something merely as pleasantry or diplomatic grace-note. there's probably some serious longrange vision in his "dream for 2017"/
You might be interested to read all three of Satz' reports. Garrett Lisi has an interesting one as well. I think for various reasons this conference will be remembered by those who had the good sense and fortune to be there.
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-3.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-2.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-1.html
Here's a PF post from Garrett while he was at the conference:
https://www.physicsforums.com/showpost.php?p=1365902&postcount=4
The paper he delivered at the conference, and a report containing lots more personal impressions is at his website.
Bee Hossenfelder did an outstanding email interview with Garrett
http://backreaction.blogspot.com/2007/08/garrett-lisis-inspiration.html
and for the interview she prepared a valuable footnote which gives a links-thumbnail-bio
containing "All URL need to know" about Garrett Lisi and his E8 ToE (exceptional Liegroup number eight theory-of-basically-everything)
==Bee's links footnote==
http://interstice.com/~aglisi/Physics/CV.html
Garrett Lisi is a wandering surfer-physicist, working on nomothetic unification while searching for the perfect wave. After graduating UCLA at the top of his class and getting his Ph.D. from UC San Diego, Garrett took off for Maui to windsurf and do physics on his own. Last year Garrett won a research grant from http://www.fqxi.org/ FQXi, which he spent on food, a laptop, and a new snowboard.
His work on unifying general relativity and the standard model as an E8 principal bundle was featured as a recent http://math.ucr.edu/home/baez/week253.html
This Week's Find by John Baez . Impatient with the slow progress of technology, Garrett has been manually uploading his brain to the web as an open-source theoretical research wiki: http://deferentialgeometry.org/ Deferential Geometry. He also blogs occasionally at http://www.fqxi.org/community/blogs.php FQXi blogs and has a semi-secret personal journal.
Garrett recently presented his work at conferences in http://www.matmor.unam.mx/eventos/loops07/ Mexico and http://interstice.com/~aglisi/albums/Iceland/Iceland.html Iceland, is currently hopping around California, and is looking forward to visiting the Perimeter Institute in October.
==endquote==
==quote from Reality Conditions blog==
And now the last question. It asked, to all plenary speakers, to say they "dream for Loops '17"; that is, on their most optimistic possible view, what is the title and abstract of the talk they imagine themselves presenting within ten years?
Many of the answers were predictable and variations of a basic template: abstracts saying "we present a complete theory of quantum gravity with testable (or, in the most ambitious cases, confirmed) predictions." ... Reuter had one of the most concrete dreams: "It is shown that LQG is equivalent to Asymptotic Safety, and that that the quantization ambiguities in it are finite in number and equivalent to the dimensionality of the Non-Gaussian Fixed Point." And finally, there was an extremely amusing exchange between...
==endquote==
Loops 17 means the Quantum Geometry/Quantum Gravity conference of 2017, only ten years out from now. Reuter's program is a bold one---joining two fertile lines of QG/QG research. My sense of him doesn't compat with his saying something merely as pleasantry or diplomatic grace-note. there's probably some serious longrange vision in his "dream for 2017"/
You might be interested to read all three of Satz' reports. Garrett Lisi has an interesting one as well. I think for various reasons this conference will be remembered by those who had the good sense and fortune to be there.
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-3.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-2.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-1.html
Here's a PF post from Garrett while he was at the conference:
https://www.physicsforums.com/showpost.php?p=1365902&postcount=4
The paper he delivered at the conference, and a report containing lots more personal impressions is at his website.
Bee Hossenfelder did an outstanding email interview with Garrett
http://backreaction.blogspot.com/2007/08/garrett-lisis-inspiration.html
and for the interview she prepared a valuable footnote which gives a links-thumbnail-bio
containing "All URL need to know" about Garrett Lisi and his E8 ToE (exceptional Liegroup number eight theory-of-basically-everything)
==Bee's links footnote==
http://interstice.com/~aglisi/Physics/CV.html
Garrett Lisi is a wandering surfer-physicist, working on nomothetic unification while searching for the perfect wave. After graduating UCLA at the top of his class and getting his Ph.D. from UC San Diego, Garrett took off for Maui to windsurf and do physics on his own. Last year Garrett won a research grant from http://www.fqxi.org/ FQXi, which he spent on food, a laptop, and a new snowboard.
His work on unifying general relativity and the standard model as an E8 principal bundle was featured as a recent http://math.ucr.edu/home/baez/week253.html
This Week's Find by John Baez . Impatient with the slow progress of technology, Garrett has been manually uploading his brain to the web as an open-source theoretical research wiki: http://deferentialgeometry.org/ Deferential Geometry. He also blogs occasionally at http://www.fqxi.org/community/blogs.php FQXi blogs and has a semi-secret personal journal.
Garrett recently presented his work at conferences in http://www.matmor.unam.mx/eventos/loops07/ Mexico and http://interstice.com/~aglisi/albums/Iceland/Iceland.html Iceland, is currently hopping around California, and is looking forward to visiting the Perimeter Institute in October.
==endquote==
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http://arxiv.org/abs/0708.0883
The complete LQG propagator: I. Difficulties with the Barrett-Crane vertex
Emanuele Alesci, Carlo Rovelli
31 pages
(Submitted on 7 Aug 2007)
"Some components of the graviton two-point function have been recently computed in the context of loop quantum gravity, using the spinfoam Barrett-Crane vertex. We complete the calculation of the remaining components. We find that, under our assumptions, the Barrett-Crane vertex does not yield the correct long distance limit. We argue that the problem is general and can be traced to the intertwiner-independence of the Barrett-Crane vertex, and therefore to the well-known mismatch between the Barrett-Crane formalism and the standard canonical spin networks. In a companion paper we illustrate the asymptotic behavior of a vertex amplitude that can correct this difficulty."
Part 2, which is TO APPEAR, is their reference [13]
[13] E. Alesci, C. Rovelli, “The complete LQG propagator: II. Asymptotics of the vertex” to appear.
This paper is actually shorter than you might expect. The main body is only pages 1 - 15 and the rest is a technical appendix and bibliography. On page 15, at the end of the Conclusions section, it says
"In the companion paper [13], we show that, perhaps surprisingly, a vertex with a suitable asymptotic behavior can overcame all these difficulties."
the paper also proposes some possibly interesting topics for young researchers. On page 2 it says "Recently, a vertex amplitude that modifies the BC amplitude, and which addresses precisely the problems that we find here, has been proposed [16, 17], see also [18]. It would be of great interest to repeat the calculation presented here for the new vertex proposed in those papers."
the way I read this is that there still good entry-level research topics in the field. it seems to say "repeat our calcuations but in this other case and you have a PhD thesis" whether you get a positive negative result doesn't matter---it needs to be checked. it's nice---the field is producing plenty of problems and it is still not overcrowded. References [17,18] are to vertex formulas proposed by Livine Speziale and by Alexandrov, not by Rovelli himself et al.
===============
The September 2007 issue of the monthly Notices of the American Mathematical Society has a perceptive book review of Smolin's The Trouble with Physics.
http://www.ams.org/notices/200708/tx070800990p.pdf
Since the exceptional Liegroup E8 plays an important role in Garrett Lisi's unification work that we've been hearing about, I pass on a link that Peter Woit found to an exponsitory piece about E8.
http://www-math.mit.edu/~dav/notices07.pdf
In case anyone wants to check out the whole issue, the TOC link to the current AMS Notices is http://www.ams.org/notices/200708/
The complete LQG propagator: I. Difficulties with the Barrett-Crane vertex
Emanuele Alesci, Carlo Rovelli
31 pages
(Submitted on 7 Aug 2007)
"Some components of the graviton two-point function have been recently computed in the context of loop quantum gravity, using the spinfoam Barrett-Crane vertex. We complete the calculation of the remaining components. We find that, under our assumptions, the Barrett-Crane vertex does not yield the correct long distance limit. We argue that the problem is general and can be traced to the intertwiner-independence of the Barrett-Crane vertex, and therefore to the well-known mismatch between the Barrett-Crane formalism and the standard canonical spin networks. In a companion paper we illustrate the asymptotic behavior of a vertex amplitude that can correct this difficulty."
Part 2, which is TO APPEAR, is their reference [13]
[13] E. Alesci, C. Rovelli, “The complete LQG propagator: II. Asymptotics of the vertex” to appear.
This paper is actually shorter than you might expect. The main body is only pages 1 - 15 and the rest is a technical appendix and bibliography. On page 15, at the end of the Conclusions section, it says
"In the companion paper [13], we show that, perhaps surprisingly, a vertex with a suitable asymptotic behavior can overcame all these difficulties."
the paper also proposes some possibly interesting topics for young researchers. On page 2 it says "Recently, a vertex amplitude that modifies the BC amplitude, and which addresses precisely the problems that we find here, has been proposed [16, 17], see also [18]. It would be of great interest to repeat the calculation presented here for the new vertex proposed in those papers."
the way I read this is that there still good entry-level research topics in the field. it seems to say "repeat our calcuations but in this other case and you have a PhD thesis" whether you get a positive negative result doesn't matter---it needs to be checked. it's nice---the field is producing plenty of problems and it is still not overcrowded. References [17,18] are to vertex formulas proposed by Livine Speziale and by Alexandrov, not by Rovelli himself et al.
===============
The September 2007 issue of the monthly Notices of the American Mathematical Society has a perceptive book review of Smolin's The Trouble with Physics.
http://www.ams.org/notices/200708/tx070800990p.pdf
Since the exceptional Liegroup E8 plays an important role in Garrett Lisi's unification work that we've been hearing about, I pass on a link that Peter Woit found to an exponsitory piece about E8.
http://www-math.mit.edu/~dav/notices07.pdf
In case anyone wants to check out the whole issue, the TOC link to the current AMS Notices is http://www.ams.org/notices/200708/
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http://arxiv.org/abs/0708.1236
Flipped spinfoam vertex and loop gravity
Jonathan Engle, Roberto Pereira, Carlo Rovelli
37 pages, 4 figures
(Submitted on 9 Aug 2007)
"We introduce a vertex amplitude for 4d loop quantum gravity. We derive it from a conventional quantization of a Regge discretization of euclidean general relativity. This yields a spinfoam sum that corrects some difficulties of the Barrett-Crane theory. The second class simplicity constraints are imposed weakly, and not strongly as in Barrett-Crane theory. Thanks to a flip in the quantum algebra, the boundary states turn out to match those of SO(3) loop quantum gravity -- the two can be identified as eigenstates of the same physical quantities -- providing a solution to the problem of connecting the covariant SO(4) spinfoam formalism with the canonical SO(3) spin-network one. The vertex amplitude is SO(3) and SO(4)-covariant. It rectifies the triviality of the intertwiner dependence of the Barrett-Crane vertex, which is responsible for its failure to yield the correct propagator tensorial structure. The construction provides also an independent derivation of the kinematics of loop quantum gravity and of the result that geometry is quantized."http://arxiv.org/abs/0708.1317
Functional Renormalization Group Equations, Asymptotic Safety, and Quantum Einstein Gravity
Martin Reuter, Frank Saueressig
Based on lectures given by M.R. at the 'First Quantum Geometry and Quantum Gravity School', Zakopane, Poland, March 2007, and the 'Summer School on Geometric and Topological Methods for Quantum Field Theory', Villa de Leyva, Colombia, July 2007, and by F.S. at NIKHEF, Amsterdam, The Netherlands, June 2006
(Submitted on 9 Aug 2007)
"These lecture notes provide a pedagogical introduction to a specific continuum implementation of the Wilsonian renormalization group, the effective average action. Its general properties and, in particular, its functional renormalization group equation are explained in a simple scalar setting. The approach is then applied to Quantum Einstein Gravity (QEG). The possibility of constructing a fundamental theory of quantum gravity in the framework of Asymptotic Safety is discussed and the supporting evidence is summarized." http://arxiv.org/abs/0708.1261
Super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
10 pages
(Submitted on 9 Aug 2007)
"We investigate the dynamics of super-inflation in two versions of Loop Quantum Cosmology, one in which the Friedmann equation is modified by the presence of inverse volume corrections, and one in which quadratic corrections are important. Computing the tilt of the power spectrum of the perturbed scalar field in terms of fast-roll parameters, we conclude that the first case leads to a power spectrum that is scale invariant for steep power law negative potentials and for the second case, scale invariance is obtained for positive potentials that asymptote to a constant value for large values of the scalar field. It is found that in both cases, the horizon problem is solved with only a few e-folds of super-inflationary evolution." http://arxiv.org/abs/0708.1264
Testing modified gravity with motion of satellites around galaxies
J. W. Moffat, V. T. Toth
5 pages, 1 figure
(Submitted on 9 Aug 2007)
"A modified gravity (MOG) theory that has been successfully fitted to galaxy rotational velocity data, cluster data and the Bullet Cluster 1E0657-56 is fitted to the motion of satellite galaxies around host galaxies at distances 50-400 kpc providing a new sensitive test to the MOG. We show that observational data from the Sloan Digital Sky Survey strongly favor the MOG, while Milgrom's MOND fails on these scales by predicting nearly constant rms velocities of satellites."
Flipped spinfoam vertex and loop gravity
Jonathan Engle, Roberto Pereira, Carlo Rovelli
37 pages, 4 figures
(Submitted on 9 Aug 2007)
"We introduce a vertex amplitude for 4d loop quantum gravity. We derive it from a conventional quantization of a Regge discretization of euclidean general relativity. This yields a spinfoam sum that corrects some difficulties of the Barrett-Crane theory. The second class simplicity constraints are imposed weakly, and not strongly as in Barrett-Crane theory. Thanks to a flip in the quantum algebra, the boundary states turn out to match those of SO(3) loop quantum gravity -- the two can be identified as eigenstates of the same physical quantities -- providing a solution to the problem of connecting the covariant SO(4) spinfoam formalism with the canonical SO(3) spin-network one. The vertex amplitude is SO(3) and SO(4)-covariant. It rectifies the triviality of the intertwiner dependence of the Barrett-Crane vertex, which is responsible for its failure to yield the correct propagator tensorial structure. The construction provides also an independent derivation of the kinematics of loop quantum gravity and of the result that geometry is quantized."http://arxiv.org/abs/0708.1317
Functional Renormalization Group Equations, Asymptotic Safety, and Quantum Einstein Gravity
Martin Reuter, Frank Saueressig
Based on lectures given by M.R. at the 'First Quantum Geometry and Quantum Gravity School', Zakopane, Poland, March 2007, and the 'Summer School on Geometric and Topological Methods for Quantum Field Theory', Villa de Leyva, Colombia, July 2007, and by F.S. at NIKHEF, Amsterdam, The Netherlands, June 2006
(Submitted on 9 Aug 2007)
"These lecture notes provide a pedagogical introduction to a specific continuum implementation of the Wilsonian renormalization group, the effective average action. Its general properties and, in particular, its functional renormalization group equation are explained in a simple scalar setting. The approach is then applied to Quantum Einstein Gravity (QEG). The possibility of constructing a fundamental theory of quantum gravity in the framework of Asymptotic Safety is discussed and the supporting evidence is summarized." http://arxiv.org/abs/0708.1261
Super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
10 pages
(Submitted on 9 Aug 2007)
"We investigate the dynamics of super-inflation in two versions of Loop Quantum Cosmology, one in which the Friedmann equation is modified by the presence of inverse volume corrections, and one in which quadratic corrections are important. Computing the tilt of the power spectrum of the perturbed scalar field in terms of fast-roll parameters, we conclude that the first case leads to a power spectrum that is scale invariant for steep power law negative potentials and for the second case, scale invariance is obtained for positive potentials that asymptote to a constant value for large values of the scalar field. It is found that in both cases, the horizon problem is solved with only a few e-folds of super-inflationary evolution." http://arxiv.org/abs/0708.1264
Testing modified gravity with motion of satellites around galaxies
J. W. Moffat, V. T. Toth
5 pages, 1 figure
(Submitted on 9 Aug 2007)
"A modified gravity (MOG) theory that has been successfully fitted to galaxy rotational velocity data, cluster data and the Bullet Cluster 1E0657-56 is fitted to the motion of satellite galaxies around host galaxies at distances 50-400 kpc providing a new sensitive test to the MOG. We show that observational data from the Sloan Digital Sky Survey strongly favor the MOG, while Milgrom's MOND fails on these scales by predicting nearly constant rms velocities of satellites."
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Steve1
- 4
- 0
Marcus,
How was the Loop Quantum congres last week between 9 and the 11 of august.
How was the Loop Quantum congres last week between 9 and the 11 of august.
- #627
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Thanks for asking, Steve. I wasnt at the inaugural IGC conference (9-11 August) but I should post links to the program of speakers.
http://www.gravity.psu.edu/igc/conf_files/prelim_agenda.html
At the moment this has not been updated since 8 August, and there is no indication that there are downloads available of the slides and audio.
Here is the start of a thread on the IGC conference:
https://www.physicsforums.com/showthread.php?t=179185
The main Loops conference for the year was "Loops 07" which took place in June. Here is the program
http://www.matmor.unam.mx/eventos/loops07/program.html
which has links to the plenary talks page and the contributed talks page
these have links to SLIDES AND AUDIO that you can download
the homepage for Loops 07 is here:
http://www.matmor.unam.mx/eventos/loops07/
=============
the IGC conference at Penn State 9-11 August was not exactly a Loops conference but it was interesting because of what it stands for. It inaugurated a NEW INSTITUTE that will bring together observational cosmologists with quantum gravity people and other theorists. If you refer to that thread or if you download the PDF files listing the names of the talks and speakers you will see that the focus is very broad and inclusive. So both the Institute for Gravitation and the Cosmos (IGC) and its inaugural celebration will serve as a MEETING GROUND for people in quite a range of different specialties
(quantum gravity, gravitational wave observation, cosmic ray observation, mainstream or classical cosmology, quantum cosmology...)
Ashtekar, the director of the new Institute, is a Loop researcher with current emphasis on Loop Quantum Cosmology (LQC) but this does not mean that the IGC will be specializing in LQC! His vision is very broad and inclusive.
There is a lot of action on all fronts in cosmology and the IGC will probably connect with all of it.
Besides the fact that I'm happy about the creation of the new Institute, I can't tell you very much about the Birthday Party. What little I know about it is in that other thread.
here's the homepage for the Institute for Gravitation and the Cosmos conference
http://www.gravity.psu.edu/igc/
http://www.gravity.psu.edu/igc/conf_files/prelim_agenda.html
At the moment this has not been updated since 8 August, and there is no indication that there are downloads available of the slides and audio.
Here is the start of a thread on the IGC conference:
https://www.physicsforums.com/showthread.php?t=179185
The main Loops conference for the year was "Loops 07" which took place in June. Here is the program
http://www.matmor.unam.mx/eventos/loops07/program.html
which has links to the plenary talks page and the contributed talks page
these have links to SLIDES AND AUDIO that you can download
the homepage for Loops 07 is here:
http://www.matmor.unam.mx/eventos/loops07/
=============
the IGC conference at Penn State 9-11 August was not exactly a Loops conference but it was interesting because of what it stands for. It inaugurated a NEW INSTITUTE that will bring together observational cosmologists with quantum gravity people and other theorists. If you refer to that thread or if you download the PDF files listing the names of the talks and speakers you will see that the focus is very broad and inclusive. So both the Institute for Gravitation and the Cosmos (IGC) and its inaugural celebration will serve as a MEETING GROUND for people in quite a range of different specialties
(quantum gravity, gravitational wave observation, cosmic ray observation, mainstream or classical cosmology, quantum cosmology...)
Ashtekar, the director of the new Institute, is a Loop researcher with current emphasis on Loop Quantum Cosmology (LQC) but this does not mean that the IGC will be specializing in LQC! His vision is very broad and inclusive.
There is a lot of action on all fronts in cosmology and the IGC will probably connect with all of it.
Besides the fact that I'm happy about the creation of the new Institute, I can't tell you very much about the Birthday Party. What little I know about it is in that other thread.
here's the homepage for the Institute for Gravitation and the Cosmos conference
http://www.gravity.psu.edu/igc/
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http://arxiv.org/abs/0708.1595
A New Spin Foam Model for 4d Gravity
Laurent Freidel, Kirill Krasnov
40 pages
(Submitted on 13 Aug 2007)
"Starting from the Plebanski formulation of gravity as a constrained BF theory we propose a new spin foam model for 4d Riemmanian quantum gravity that generalises the well-known model of Barrett-Crane and resolves the ultralocality problem that this model is known to possess. It is well known that the BF formulation of 4d gravity possesses two sectors: one corresponding to gravity and the other topological. The model presented here is shown to give a quantisation of the gravitational sector. The present model is dual to the recently proposed spin foam model of Engle et al. which, we show, corresponds to the topological sector of the theory. One important outcome of our approach is that it also allow us to introduce the Immirzi parameter into the framework of spin foam quantisation. We generalize some of our considerations to the Lorentzian setting and obtain a new spin foam model in that context as well."http://arxiv.org/abs/0708.1721
Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?
Bianca Dittrich, Thomas Thiemann
12 pages
(Submitted on 13 Aug 2007)
"One of the celebrated results of Loop Quantum Gravity (LQG) is the discreteness of the spectrum of geometrical operators such as length, area and volume operators. This is an indication that Planck scale geometry in LQG is discontinuous rather than smooth. However, there is no rigorous proof thereof at present, because the afore mentioned operators are not gauge invariant, they do not commute with the quantum constraints. The relational formalism in the incarnation of Rovelli's partial and complete observables provides a possible mechanism for turning a non gauge invariant operator into a gauge invariant one. In this paper we investigate whether the spectrum of such a physical, that is gauge invariant, observable can be predicted from the spectrum of the corresponding gauge variant observables. We will not do this in full LQG but rather consider much simpler examples where field theoretical complications are absent. We find, even in those simpler cases, that kinematical discreteness of the spectrum does not necessarily survive at the gauge invariant level. Whether or not this happens depends crucially on how the gauge invariant completion is performed. This indicates that 'fundamental discreteness at Planck scale in LQG' is an empty statement. To prove it, one must provide the detailed construction of gauge invariant versions of geometrical operators."
http://arxiv.org/abs/0708.1915
Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity
Etera R. Livine, Simone Speziale
6 pages, 2 figures
(Submitted on 14 Aug 2007 (v1), last revised 14 Aug 2007 (this version, v2))
"We give an independent derivations of the Engle-Pereira-Rovelli spinfoam model for quantum gravity which appeared in arXiv:0705.2388. Using the coherent state techniques we introduced in arXiv:0705.0674, we show that the EPR model realizes a consistent imposition of the simplicity constraints." http://arxiv.org/abs/0708.1561
Towards Quantum Noncommutative kappa-deformed Field Theory
M. Daszkiewicz (IFT, Wroclaw University), J. Lukierski (IFT, Wroclaw University), M. Woronowicz (IFT, Wroclaw University)
15 pages
(Submitted on 11 Aug 2007)
"We introduce new quantum kappa-star product describing the multiplication of quantized kappa-deformed free fields. The kappa-deformation of local free quantum fields originates from two sources: noncommutativity of space-time and the kappa-deformation of field oscillators algebra. We demonstrate that for suitable choice of kappa-deformed field oscillators the kappa-deformed version of microcausality condition is satisfied, and it leads to the deformation of the Pauli-Jordan commutation function defined by the kappa-deformed mass shell. We show by constructing the kappa-deformed Fock space that effectively the kappa-deformed oscillator algebra does not change the bosonic statistics of n-particle states. The proposed star product is extended to the product of n fields, which for n=4 defines the interaction vertex in perturbative description of noncommutative quantum lambda phi^4 field theory. It follows that the classical fourmomentum conservation law is satisfied at the interaction vertices."
A New Spin Foam Model for 4d Gravity
Laurent Freidel, Kirill Krasnov
40 pages
(Submitted on 13 Aug 2007)
"Starting from the Plebanski formulation of gravity as a constrained BF theory we propose a new spin foam model for 4d Riemmanian quantum gravity that generalises the well-known model of Barrett-Crane and resolves the ultralocality problem that this model is known to possess. It is well known that the BF formulation of 4d gravity possesses two sectors: one corresponding to gravity and the other topological. The model presented here is shown to give a quantisation of the gravitational sector. The present model is dual to the recently proposed spin foam model of Engle et al. which, we show, corresponds to the topological sector of the theory. One important outcome of our approach is that it also allow us to introduce the Immirzi parameter into the framework of spin foam quantisation. We generalize some of our considerations to the Lorentzian setting and obtain a new spin foam model in that context as well."http://arxiv.org/abs/0708.1721
Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?
Bianca Dittrich, Thomas Thiemann
12 pages
(Submitted on 13 Aug 2007)
"One of the celebrated results of Loop Quantum Gravity (LQG) is the discreteness of the spectrum of geometrical operators such as length, area and volume operators. This is an indication that Planck scale geometry in LQG is discontinuous rather than smooth. However, there is no rigorous proof thereof at present, because the afore mentioned operators are not gauge invariant, they do not commute with the quantum constraints. The relational formalism in the incarnation of Rovelli's partial and complete observables provides a possible mechanism for turning a non gauge invariant operator into a gauge invariant one. In this paper we investigate whether the spectrum of such a physical, that is gauge invariant, observable can be predicted from the spectrum of the corresponding gauge variant observables. We will not do this in full LQG but rather consider much simpler examples where field theoretical complications are absent. We find, even in those simpler cases, that kinematical discreteness of the spectrum does not necessarily survive at the gauge invariant level. Whether or not this happens depends crucially on how the gauge invariant completion is performed. This indicates that 'fundamental discreteness at Planck scale in LQG' is an empty statement. To prove it, one must provide the detailed construction of gauge invariant versions of geometrical operators."
http://arxiv.org/abs/0708.1915
Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity
Etera R. Livine, Simone Speziale
6 pages, 2 figures
(Submitted on 14 Aug 2007 (v1), last revised 14 Aug 2007 (this version, v2))
"We give an independent derivations of the Engle-Pereira-Rovelli spinfoam model for quantum gravity which appeared in arXiv:0705.2388. Using the coherent state techniques we introduced in arXiv:0705.0674, we show that the EPR model realizes a consistent imposition of the simplicity constraints." http://arxiv.org/abs/0708.1561
Towards Quantum Noncommutative kappa-deformed Field Theory
M. Daszkiewicz (IFT, Wroclaw University), J. Lukierski (IFT, Wroclaw University), M. Woronowicz (IFT, Wroclaw University)
15 pages
(Submitted on 11 Aug 2007)
"We introduce new quantum kappa-star product describing the multiplication of quantized kappa-deformed free fields. The kappa-deformation of local free quantum fields originates from two sources: noncommutativity of space-time and the kappa-deformation of field oscillators algebra. We demonstrate that for suitable choice of kappa-deformed field oscillators the kappa-deformed version of microcausality condition is satisfied, and it leads to the deformation of the Pauli-Jordan commutation function defined by the kappa-deformed mass shell. We show by constructing the kappa-deformed Fock space that effectively the kappa-deformed oscillator algebra does not change the bosonic statistics of n-particle states. The proposed star product is extended to the product of n fields, which for n=4 defines the interaction vertex in perturbative description of noncommutative quantum lambda phi^4 field theory. It follows that the classical fourmomentum conservation law is satisfied at the interaction vertices."
Last edited:
- #629
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http://arxiv.org/abs/0708.2481
Comment on "Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?" by B. Dittrich and T. Thiemann
Carlo Rovelli
6 pages, 1 figure
(Submitted on 20 Aug 2007)
"I argue that the prediction of physical discreteness at the Planck scale in loop gravity is a reasonable conclusion that derives from a sensible ensemble of hypotheses, in spite of some contrary arguments considered in an interesting recent paper by Dittrich and Thiemann. The counter-example presented by Dittrich and Thiemann illustrates a pathology which does not seem to be present in gravity. I also point out a common confusion between two distinct frameworks for the interpretation of general-covariant quantum theory, and observe that within one of these, the derivation of physical discreteness is immediate, and not in contradiction with gauge invariance."
brief mention, the following might also be of interest
http://arxiv.org/abs/0708.2738
The Physical Process First Law for Bifurcate Killing Horizons
Aaron J. Amsel, Donald Marolf, Amitabh Virmani
19 pages
(Submitted on 20 Aug 2007)
"The physical process version of the first law for black holes states that the passage of energy and angular momentum through the horizon results in a change in area [tex]\frac{\kappa}{8 \pi} \Delta A = \Delta E - \Omega \Delta J[/tex], so long as this passage is quasi-stationary. A similar physical process first law can be derived for any bifurcate Killing horizon in any spacetime dimension d >=3 using much the same argument. However, to make this law non-trivial, one must show that sufficiently quasi-stationary processes do in fact occur. In particular, one must show that processes exist for which the shear and expansion remain small, and in which no new generators are added to the horizon. Thorne, MacDonald, and Price considered related issues when an object falls across a d=4 black hole horizon. By generalizing their argument to arbitrary d >=3 and to any bifurcate Killing horizon, we derive a condition under which these effects are controlled and the first law applies. In particular, by providing a non-trivial first law for Rindler horizons, our work completes the parallel between the mechanics of such horizons and those of black holes for d >=3. We also comment on the situation for d=2. "
http://arxiv.org/abs/0708.2639
Black holes in the TeVeS theory of gravity and their thermodynamics
Eva Sagi, Jacob D. Bekenstein
11 pages
(Submitted on 20 Aug 2007)
TeVeS, a relativistic theory of gravity, was designed to provide a basis for the modified Newtonian dynamics. Since TeVeS differs from general relativity (e.g., it has two metrics, an Einstein metric and a physical metric), black hole solutions of it would be valuable for a number of endeavors ranging from astrophysical modeling to investigations into the interrelation between gravity and thermodynamics. Giannios has recently found a TeVeS analogue of the Schwarzschild black hole solution. We proceed further with the program by analytically solving the TeVeS equations for a static spherically symmetric and asymptotically flat system of electromagnetic and gravity fields. We show that one solution is provided by the Reissner-Nordström metric as physical metric, the TeVeS vector field pointing in the time direction, and a TeVeS scalar field positive everywhere (the last feature protects from superluminal propagation of disturbances in the fields). We work out black hole thermodynamics in TeVeS using the physical metric; black hole entropy, temperature and electric potential turn out to be identical to those in general relativity. We find it inconsistent to base thermodynamics on the Einstein metric. Consequently the two temperatures Dubovsky--Sibiryakov scenario for violating the second law of thermodynamics cannot be set up in TeVeS." http://arxiv.org/abs/0708.2428
A macroscopic test of the Aharonov-Bohm effect
Adam Caprez, Brett Barwick, Herman Batelaan
13 pages, 4 figures
(Submitted on 17 Aug 2007)
Comment on "Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?" by B. Dittrich and T. Thiemann
Carlo Rovelli
6 pages, 1 figure
(Submitted on 20 Aug 2007)
"I argue that the prediction of physical discreteness at the Planck scale in loop gravity is a reasonable conclusion that derives from a sensible ensemble of hypotheses, in spite of some contrary arguments considered in an interesting recent paper by Dittrich and Thiemann. The counter-example presented by Dittrich and Thiemann illustrates a pathology which does not seem to be present in gravity. I also point out a common confusion between two distinct frameworks for the interpretation of general-covariant quantum theory, and observe that within one of these, the derivation of physical discreteness is immediate, and not in contradiction with gauge invariance."
brief mention, the following might also be of interest
http://arxiv.org/abs/0708.2738
The Physical Process First Law for Bifurcate Killing Horizons
Aaron J. Amsel, Donald Marolf, Amitabh Virmani
19 pages
(Submitted on 20 Aug 2007)
"The physical process version of the first law for black holes states that the passage of energy and angular momentum through the horizon results in a change in area [tex]\frac{\kappa}{8 \pi} \Delta A = \Delta E - \Omega \Delta J[/tex], so long as this passage is quasi-stationary. A similar physical process first law can be derived for any bifurcate Killing horizon in any spacetime dimension d >=3 using much the same argument. However, to make this law non-trivial, one must show that sufficiently quasi-stationary processes do in fact occur. In particular, one must show that processes exist for which the shear and expansion remain small, and in which no new generators are added to the horizon. Thorne, MacDonald, and Price considered related issues when an object falls across a d=4 black hole horizon. By generalizing their argument to arbitrary d >=3 and to any bifurcate Killing horizon, we derive a condition under which these effects are controlled and the first law applies. In particular, by providing a non-trivial first law for Rindler horizons, our work completes the parallel between the mechanics of such horizons and those of black holes for d >=3. We also comment on the situation for d=2. "
http://arxiv.org/abs/0708.2639
Black holes in the TeVeS theory of gravity and their thermodynamics
Eva Sagi, Jacob D. Bekenstein
11 pages
(Submitted on 20 Aug 2007)
TeVeS, a relativistic theory of gravity, was designed to provide a basis for the modified Newtonian dynamics. Since TeVeS differs from general relativity (e.g., it has two metrics, an Einstein metric and a physical metric), black hole solutions of it would be valuable for a number of endeavors ranging from astrophysical modeling to investigations into the interrelation between gravity and thermodynamics. Giannios has recently found a TeVeS analogue of the Schwarzschild black hole solution. We proceed further with the program by analytically solving the TeVeS equations for a static spherically symmetric and asymptotically flat system of electromagnetic and gravity fields. We show that one solution is provided by the Reissner-Nordström metric as physical metric, the TeVeS vector field pointing in the time direction, and a TeVeS scalar field positive everywhere (the last feature protects from superluminal propagation of disturbances in the fields). We work out black hole thermodynamics in TeVeS using the physical metric; black hole entropy, temperature and electric potential turn out to be identical to those in general relativity. We find it inconsistent to base thermodynamics on the Einstein metric. Consequently the two temperatures Dubovsky--Sibiryakov scenario for violating the second law of thermodynamics cannot be set up in TeVeS." http://arxiv.org/abs/0708.2428
A macroscopic test of the Aharonov-Bohm effect
Adam Caprez, Brett Barwick, Herman Batelaan
13 pages, 4 figures
(Submitted on 17 Aug 2007)
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http://arxiv.org/abs/0708.3051
Topological Higher Gauge Theory - from BF to BFCG theory
F. Girelli, H. Pfeiffer, E. M. Popescu
15 pages
(Submitted on 22 Aug 2007)
"We study generalizations of 3- and 4-dimensional BF-theory in the context of higher gauge theory. First, we construct topological higher gauge theories as discrete state sum models and explain how they are related to the state sums of Yetter, Mackaay, and Porter. Under certain conditions, we can present their corresponding continuum counterparts in terms of classical Lagrangians. We then explain that two of these models are already familiar from the literature: the SigmaPhiEA-model of 3-dimensional gravity coupled to topological matter, and also a 4-dimensional model of BF-theory coupled to topological matter."
http://arxiv.org/abs/0708.2935
Loss of entanglement in quantum mechanics due to the use of realistic measuring rods
Rodolfo Gambini, Rafael A. Porto, Jorge Pullin
5 pages
(Submitted on 21 Aug 2007)
"We show that the use of real measuring rods in quantum mechanics places a fundamental gravitational limit to the level of entanglement that one can ultimately achieve in quantum systems. The result can be seen as a direct consequence of the fundamental gravitational limitations in the measurements of length and time in realistic physical systems. The effect may have implications for long distance teleportation and the measurement problem in quantum mechanics."
brief mention:
http://arxiv.org/abs/0708.2943
Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW?
Mustapha Ishak, James Richardson, Delilah Whittington, David Garred (The University of Texas at Dallas)
5 pages, 1 figure
(Submitted on 22 Aug 2007)
"We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit the supernova data as well as the LCDM model without requiring any dark energy component. The Szekeres models were originally derived as an exact solution to Einstein's equations with a general metric that has no symmetries and are regarded in the field as good candidates to represent the true lumpy universe that we observe. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. While more work remains, the result presented in this first paper appears to support the possibility of apparent acceleration."
http://arxiv.org/abs/0708.3017
Black Holes and Quantum Gravity at the LHC
Patrick Meade, Lisa Randall
Topological Higher Gauge Theory - from BF to BFCG theory
F. Girelli, H. Pfeiffer, E. M. Popescu
15 pages
(Submitted on 22 Aug 2007)
"We study generalizations of 3- and 4-dimensional BF-theory in the context of higher gauge theory. First, we construct topological higher gauge theories as discrete state sum models and explain how they are related to the state sums of Yetter, Mackaay, and Porter. Under certain conditions, we can present their corresponding continuum counterparts in terms of classical Lagrangians. We then explain that two of these models are already familiar from the literature: the SigmaPhiEA-model of 3-dimensional gravity coupled to topological matter, and also a 4-dimensional model of BF-theory coupled to topological matter."
http://arxiv.org/abs/0708.2935
Loss of entanglement in quantum mechanics due to the use of realistic measuring rods
Rodolfo Gambini, Rafael A. Porto, Jorge Pullin
5 pages
(Submitted on 21 Aug 2007)
"We show that the use of real measuring rods in quantum mechanics places a fundamental gravitational limit to the level of entanglement that one can ultimately achieve in quantum systems. The result can be seen as a direct consequence of the fundamental gravitational limitations in the measurements of length and time in realistic physical systems. The effect may have implications for long distance teleportation and the measurement problem in quantum mechanics."
brief mention:
http://arxiv.org/abs/0708.2943
Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW?
Mustapha Ishak, James Richardson, Delilah Whittington, David Garred (The University of Texas at Dallas)
5 pages, 1 figure
(Submitted on 22 Aug 2007)
"We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit the supernova data as well as the LCDM model without requiring any dark energy component. The Szekeres models were originally derived as an exact solution to Einstein's equations with a general metric that has no symmetries and are regarded in the field as good candidates to represent the true lumpy universe that we observe. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. While more work remains, the result presented in this first paper appears to support the possibility of apparent acceleration."
http://arxiv.org/abs/0708.3017
Black Holes and Quantum Gravity at the LHC
Patrick Meade, Lisa Randall