Loop-and-allied QG bibliography

[marcus]

New paper by Ambjorn, Goerlich, Jurkiewicz, and Loll

http://arxiv.org/abs/0807.4481
The Nonperturbative Quantum de Sitter Universe
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
37 pages, many figures
(Submitted on 28 Jul 2008)

"The dynamical generation of a four-dimensional classical universe from nothing but fundamental quantum excitations at the Planck scale is a long-standing challenge to theoretical physicists. A candidate theory of quantum gravity which achieves this goal without invoking exotic ingredients or excessive fine-tuning is based on the nonperturbative and background-independent technique of Causal Dynamical Triangulations. We demonstrate in detail how in this approach a macroscopic de Sitter universe, accompanied by small quantum fluctuations, emerges from the full gravitational path integral, and how the effective action determining its dynamics can be reconstructed uniquely from Monte Carlo data. We also provide evidence that it may be possible to penetrate to the sub-Planckian regime, where the Planck length is large compared to the lattice spacing of the underlying regularization of geometry." http://arxiv.org/abs/0807.4192
Black Hole Entropy and the Problem of Universality
Steven Carlip
14 pages, Springer macros; to appear in 'Quantum Mechanics of Fundamental Systems: the Quest for Beauty and Simplicity' (Claudio Bunster Festschrift)
(Submitted on 25 Jul 2008)

"To derive black hole thermodynamics in any quantum theory of gravity, one must introduce constraints that ensure that a black hole is actually present. For a large class of black holes, the imposition of such 'horizon constraints' allows the use of conformal field theory methods to compute the density of states, reproducing the correct Bekenstein-Hawking entropy in a nearly model-independent manner. This approach may explain the 'universality' of black hole entropy, the fact that many inequivalent descriptions of quantum states all seem to give the same thermodynamic predictions. It also suggests an elegant picture of the relevant degrees of freedom, as Goldstone-boson-like excitations arising from symmetry breaking by a conformal anomaly induced by the horizon constraints.

Claudio Bunster, who is being celebrated by this collection of papers, used to be Claudio Teitelboim---a safer name to have in Chile during the Pinochet years. The name Bunster which he has now adopted was associated with opposition to military dictatorship. To find Bunster's research writings, search under his previous name.

http://arxiv.org/abs/0807.4152
The Constraint Algebra of Topologically Massive AdS Gravity
Steven Carlip
17 pages
(Submitted on 25 Jul 2008)

"Three-dimensional topologically massive AdS gravity has a complicated constraint algebra, making it difficult to count nonperturbative degrees of freedom. I show that a new choice of variables greatly simplifies this algebra, and confirm that the theory contains a single propagating mode for all values of the mass parameter and the cosmological constant. As an added benefit, I rederive the central charges and conformal weights of the boundary conformal field theory from an explicit analysis of the asymptotic algebra of constraints."

http://arxiv.org/abs/0807.4520
Black Hole Thermodynamics and Statistical Mechanics
Steven Carlip
35 pages, Springer macros; for the Proceedings of the 4th Aegean Summer School on Black Holes
(Submitted on 28 Jul 2008)

"We have known for more than thirty years that black holes behave as thermodynamic systems, radiating as black bodies with characteristic temperatures and entropies. This behavior is not only interesting in its own right; it could also, through a statistical mechanical description, cast light on some of the deep problems of quantizing gravity. In these lectures, I review what we currently know about black hole thermodynamics and statistical mechanics, suggest a rather speculative 'universal' characterization of the underlying states, and describe some key open questions."

http://arxiv.org/abs/0807.4468
The cosmological BCS mechanism and the Big Bang Singularity
Stephon Alexander, Tirthabir Biswas
11 pages, 1 figure
(Submitted on 28 Jul 2008)

"We provide a novel mechanism that resolves the Big Bang Singularity present in FRW space-times without the need for ghost fields. Building on the fact that a four-fermion interaction arises in General Relativity when fermions are covariantly coupled, we show that at early times the decrease in scale factor enhances the correlation between pairs of fermions. This enhancement leads to a BCS-like condensation of the fermions and opens a gap dynamically driving the Hubble parameter to zero and results in a non-singular bounce. We derive the four fermion interaction and the effective potential for the gap, demonstrating a consistency condition necessary for the non-singular bounce."

 

[MTd2]

http://arxiv.org/abs/0807.4574v1
Proof of the entropy bound on dynamical horizons
Authors: Sijie Gao, Xiaoning Wu
(Submitted on 29 Jul 2008)

"The entropy bound conjecture concerning black hole dynamical horizons is proved. The conjecture states, if a dynamical horizon, $D_H$, is bounded by two surfaces with areas of $A_B$ and $\abp$ ($\abp>A_B$), then the entropy, $S_D$, that crosses $D_H$ must satisfy $S_D\leq {1/4}(\abp-A_B)$. We show that this conjecture is implied by the generalized Bousso bound. Consequently, the generalized second law holds for dynamical horizons. Finally, we show that the lightlike bousso bound and its spacelike counterpart can be unified as one bound."

This is a simple but very insightful paper. By unifying both components, you can just now work with just the lightsheet of causal diamonds.

Now, this is something from my guts. If we consider that bound true, we can start thinking about a universe as a duality between the union of the domain of RP3 projections of matter and fields, because now we can the spacelike component of the causal diamond, and one holographic screen, which is the union of the images of all RP3.

 

[marcus]

http://arxiv.org/abs/0807.4748
Diffeomorphism invariance in spherically symmetric loop quantum gravity
Rodolfo Gambini, Jorge Pullin
5 pages, invited paper for a special issue of Advanced Science Letters
(Submitted on 29 Jul 2008)

"We study the issue of the recovery of diffeomorphism invariance in the recently introduced loop quantum gravity treatment of the exterior Schwarzschild space-time. Although the loop quantization agrees with the quantization in terms of metric variables in identifying the physical Hilbert space, we show that diffeomorphism invariance in space-time is recovered with certain limitations due to the use of holonomic variables in the loop treatment of the model. This resembles behaviors that are expected in the full theory."

http://arxiv.org/abs/0807.4874
Canonical Relativity and the Dimensionality of the World
Martin Bojowald
17 pages, 2 figures; Chapter in: Relativity and the Dimensionality of the World, Ed. V. Petkov (Springer, 2007), pp. 137-152
(Submitted on 30 Jul 2008)

"Different aspects of relativity, mainly in a canonical formulation, relevant for the question 'Is spacetime nothing more than a mathematical space (which describes the evolution in time of the ordinary three-dimensional world) or is it a mathematical model of a real four-dimensional world with time entirely given as the fourth dimension?' are presented. The availability as well as clarity of the arguments depend on which framework is being used, for which currently special relativity, general relativity and some schemes of quantum gravity are available. Canonical gravity provides means to analyze the field equations as well as observable quantities, the latter even in coordinate independent form. This allows a unique perspective on the question of dimensionality since the space-time manifold does not play a prominent role. After re-introducing a Minkowski background into the formalism, one can see how distinguished coordinates of special relativity arise, where also the nature of time is different from that in the general perspective. Just as it is of advantage to extend special to general relativity, general relativity itself has to be extended to some theory of quantum gravity. This suggests that a final answer has to await a thorough formulation and understanding of a fundamental theory of space-time. Nevertheless, we argue that current insights into quantum gravity do not change the picture of the role of time obtained from general relativity."

http://arxiv.org/abs/0807.4910
Gravitational dynamics in Bose Einstein condensates
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
15 pages
(Submitted on 30 Jul 2008)

"Analogue models for gravity intend to provide a framework where matter and gravity, as well as their intertwined dynamics, emerge from degrees of freedom that have a priori nothing to do with what we call gravity or matter. Bose Einstein condensates (BEC) are a natural example of analogue model since one can identify matter propagating on a (pseudo-Riemannian) metric with collective excitations above the condensate of atoms. However, until now, a description of the 'analogue gravitational dynamics' for such model was missing. We show here that in a BEC system with massive quasi-particles, the gravitational dynamics can be encoded in a modified (semi-classical) Poisson equation. In particular, gravity is of extreme short range (characterized by the healing length) and the cosmological constant appears from the non-condensed fraction of atoms in the quasi-particle vacuum. While some of these features make the analogue gravitational dynamics of our BEC system quite different from standard Newtonian gravity, we nonetheless show that it can be used to draw some interesting lessons about 'emergent gravity' scenarios."

 

[marcus]

http://arxiv.org/abs/0808.0025
A Note on B-observables in Ponzano-Regge 3d Quantum Gravity
Etera R. Livine, James P. Ryan
17 pages
(Submitted on 31 Jul 2008)

"We study the insertion and value of metric observables in the (discrete) path integral formulation of the Ponzano-Regge spinfoam model for 3d quantum gravity. In particular, we discuss the length spectrum and the relation between insertion of such B-observables and gauge fixing in the path integral."

 

[marcus]

http://arxiv.org/abs/0808.0190
Singularities in loop quantum cosmology
Thomas Cailleteau, Antonio Cardoso, Kevin Vandersloot, David Wands
5 pages, 3 figures
(Submitted on 1 Aug 2008)

"We show that simple scalar field models can give rise to curvature singularities in Loop Quantum Cosmology (LQC). We find singular solutions for spatially flat Friedmann-Robertson-Walker cosmologies with a canonical scalar field and a negative exponential potential, or with a phantom scalar field and a positive potential. While LQC avoids big bang or big rip type singularities, we find sudden singularities where the Hubble rate is bounded, but the Ricci curvature scalar diverges. We conclude that the effective equations of LQC are not in themselves sufficient to avoid the occurrence of singularities."

http://arxiv.org/abs/0808.0246
Categorified Symplectic Geometry and the Classical String
John C. Baez, Alexander E. Hoffnung, Christopher L. Rogers
28 pages
(Submitted on 2 Aug 2008)

"A Lie 2-algebra is a 'categorified' version of a Lie algebra: that is, a category equipped with structures analogous those of a Lie algebra, for which the usual laws hold up to isomorphism. In the classical mechanics of point particles, the phase space is often a symplectic manifold, and the Poisson bracket of functions on this space gives a Lie algebra of observables. Multisymplectic geometry describes an n-dimensional field theory using a phase space that is an 'n-plectic manifold': a finite-dimensional manifold equipped with a closed nondegenerate (n+1)-form. Here we consider the case n = 2. For any 2-plectic manifold, we construct a Lie 2-algebra of observables. We then explain how this Lie 2-algebra can be used to describe the dynamics of a classical bosonic string. Just as the presence of an electromagnetic field affects the symplectic structure for a charged point particle, the presence of a B field affects the 2-plectic structure for the string."

 

[marcus]

On 25 July Lee Smolin posted a revision of his December 2007 paper, containing new information worth noting. Thanks to MTd2 for alerting us to this.

http://arxiv.org/abs/0712.0977
The Plebanski action extended to a unification of gravity and Yang-Mills theory
Lee Smolin
12 pages, one figure
(Submitted on 6 Dec 2007 (v1), last revised 25 Jul 2008 (this version, v2))

"We study a unification of gravity with Yang-Mills fields based on a simple extension of the Plebanski action to a Lie group G which contains the local lorentz group. The Coleman-Mandula theorem is avoided because the theory has no global spacetime symmetry. This may be applied to Lisi's proposal of an E8 unified theory, giving a fully E8 invariant action. The extended form of the Plebanski action suggests a new class of spin foam models."

 

[marcus]

http://arxiv.org/abs/0808.0701
Effective theory for the cosmological generation of structure
Martin Bojowald, Aureliano Skirzewski
8 pages, 1 figure
(Submitted on 5 Aug 2008)

"The current understanding of structure formation in the early universe is mainly built on a magnification of quantum fluctuations in an initial vacuum state during an early phase of accelerated universe expansion. One usually describes this process by solving equations for a quantum state of matter on a given expanding background space-time, followed by decoherence arguments for the emergence of classical inhomogeneities from the quantum fluctuations. Here, we formulate the coupling of quantum matter fields to a dynamical gravitational background in an effective framework which allows the inclusion of back-reaction effects. It is shown how quantum fluctuations couple to classical inhomogeneities and can thus manage to generate cosmic structure in an evolving background. Several specific effects follow from a qualitative analysis of the back-reaction, including a likely reduction of the overall amplitude of power in the cosmic microwave background, the occurrence of small non-Gaussianities, and a possible suppression of power for odd modes on large scales without parity violation."

 

[marcus]

http://arxiv.org/abs/0808.0949
Critical behaviour in quantum gravitational collapse
Viqar Husain
8 pages, 3 figures
(Submitted on 7 Aug 2008)

"We study the gravitational collapse of an inhomogeneous scalar field with quantum gravity corrections associated with singularity avoidance. Numerical simulations indicate that there is critical behaviour at the onset of black hole formation as in the classical theory, but with the difference that black holes form with a mass gap."

http://arxiv.org/abs/0808.0990
Effective Scenario of Loop Quantum Cosmology
You Ding, Yongge Ma, Jinsong Yang
4 pages, 2 figures
(Submitted on 7 Aug 2008)

"Semiclassical states in isotropic loop quantum cosmology are employed to show that the improved dynamics has the correct classical limit. The effective Hamiltonian for the quantum cosmological model with a massless scalar field is thus obtained, which incorporates also the next to leading order quantum corrections. The possibility that the higher order correction terms may lead to significant departure from the leading order effective scenario is revealed. If the semiclassicality of the model is maintained in the large scale limit, even k=0 Friedmann expanding universe will undergo a collapse in the future due to the quantum gravity effect. Thus the quantum bounce and collapse may contribute a cyclic universe in the new scenario."

http://arxiv.org/abs/0808.1081
The kinematic origin of the cosmological redshift
Emory F. Bunn, David W. Hogg
14 pages. Submitted to Am. J. Phys
(Submitted on 7 Aug 2008)

"A common belief among cosmologists is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that of coordinate freedom, specifically the idea that it is always valid to choose a coordinate system that is locally Minkowski. We show that, in any spacetime, an observed frequency shift can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a family of observers along the photon's path. In the context of the expanding Universe, the kinematic interpretation corresponds to a family of comoving observers and hence seems to be the more natural one."

 

[marcus]

http://arxiv.org/abs/0808.1107
Semiclassical regime of Regge calculus and spin foams
Eugenio Bianchi, Alejandro Satz
30 pages
(Submitted on 7 Aug 2008)

"Recent attempts to recover the graviton propagator from spin foam models involve the use of a boundary quantum state peaked on a classical geometry. The question arises whether beyond the case of a single simplex this suffices for peaking the interior geometry in a semiclassical configuration. In this paper we explore this issue in the context of quantum Regge calculus with a general triangulation. Via a stationary phase approximation, we show that the boundary state succeeds in peaking the interior in the appropriate configuration, and that boundary correlations can be computed order by order in an asymptotic expansion. Further, we show that if we replace at each simplex the exponential of the Regge action by its cosine -- as expected from the semiclassical limit of spin foam models -- then the contribution from the sign-reversed terms is suppressed in the semiclassical regime and the results match those of conventional Regge calculus."

http://arxiv.org/abs/0808.1223
Constraints and gauge transformations: Dirac's theorem is not always valid
Julian Barbour, Brendan Z. Foster
14 pages
(Submitted on 8 Aug 2008)

"A standard tenet of canonical quantum gravity is that evolution generated by a Hamiltonian constraint is just a gauge transformation on the phase space and therefore does not change the physical state. The basis for this belief is a theorem of Dirac that identifies primary first-class constraints as generators of physically irrelevant motions. We point out that certain assumptions on which Dirac based his argument do not hold for reparametrization invariant systems, and show that the primary Hamiltonian constraint of these systems does generate physical motion. We show explicitly how the argument fails for systems described by Jacobi's principle, which has a structure closely resembling that of general relativity. We defer discussion of general relativity and the implications for quantum gravity to a later paper."

 

[marcus]

Hongbao Zhang is at Beijing Normal, same place as Yongge Ma. We should watch the quantum gravity/cosmology group there. I have already noted several of their papers.
In this note Hongbao improves on Steven Weinberg's treatment of a very interesting problem---how does the expansion of the universe slow down massive particles? E.g. neutrinos originally produced "hot" in the early universe will have slowed down due to expansion.

Note on the thermal history of decoupled massive particles
Hongbao Zhang
JHEP style, 4 pages, to appear in CQG
(Submitted on 11 Aug 2008)

"This note provides an alternative approach to the momentum decay and thermal evolution of decoupled massive particles. Although the ingredients in our results have been addressed in Weinberg's Cosmology, the strategies employed here are simpler, and the results obtained here are more general."

==sample excerpt==

"As is well known, for the freely traveling massless particle like photon in an expanding
FLRW universe, the frequency or energy will vary inversely proportional to the scale
factor, which implies that the number density of massless particles still keeps its thermal
spectrum form with a redshifted effective temperature although these particles went
out of the thermal equilibrium into the free expansion as time passed. This is the
physical foundation for the cosmic microwave radiation background currently observed
by us. Now a natural question arises, namely, does the above fact also apply to the
massive particle? Not only does this question possesses a theoretical interest by itself,
but also acquires a practical implication in cosmology since neutrinos and antineutrinos
are believed to be massive. However, to my best knowledge, this issue has not been
addressed in literatures except in Weinberg’s cosmology book published recently[1].

The purpose of this note is to provide an alternative approach to this issue. The
strategies employed here are simpler, but the results obtained here are more general.
Notations and conventions follow Ref.[2]."

In case Yongge Ma's name is not familiar, a recent paper of his was noted in post #743, two posts back.

I'm told that next year's Loop or QG conference will be in Beijing. I should be watching the QG research output of people like Yongge Ma and Hongbao Zhang. Here are three researchers at Hunan Normal, whose paper has already been accepted for publication in Physical Review D. and their names are new to me. Hongwei Yu is the senior researcher.

http://arxiv.org/abs/0808.1382
Dynamics of interacting phantom scalar field dark energy in Loop Quantum Cosmology
Xiangyun Fu, Hongwei Yu, Puxun Wu
18 pages, 7 figures, to be published in PRD
(Submitted on 10 Aug 2008)

"We study the dynamics of a phantom scalar field dark energy interacting with dark matter in loop quantum cosmology (LQC). Two kinds of coupling of the form $$\alpha{\rho_m}{\dot\phi}$$ (case I) and $$3\beta H (\rho_\phi +\rho_m)$$ (case II) between the phantom energy and dark matter are examined with the potential for the phantom field taken to be exponential. For both kinds of interactions, we find that the future singularity appearing in the standard FRW cosmology can be avoided by loop quantum gravity effects. In case II, if the phantom field is initially rolling down the potential, the loop quantum effect has no influence on the cosmic late time evolution and the universe will accelerate forever with a constant energy ratio between the dark energy and dark matter."

 

[marcus]

http://arxiv.org/abs/0808.1971
Intertwiner dynamics in the flipped vertex
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
12 pages, 7 figures
(Submitted on 14 Aug 2008)

"We continue the semiclassical analysis, started in a previous paper, of the intertwiner sector of the flipped vertex spinfoam model. We use independently both a semi-analytical and a purely numerical approach, finding the correct behavior of wave packet propagation and physical expectation values. In the end, we show preliminary results about correlation functions."

brief mention
http://arxiv.org/abs/0808.1913
Transiting Exoplanets with JWST
S. Seager (MIT), D. Deming (NASA/GSFC), J. A. Valenti (STScI)
22 pages, 11 figures. In press in "Astrophysics in the Next Decade: JWST and Concurrent Facilities, Astrophysics & Space Science Library, Thronson, H. A., Tielens, A., Stiavelli, M., eds., Springer: Dordrecht (2008)."
(Submitted on 13 Aug 2008)

 

[francesca]

http://arxiv.org/abs/0808.2069"
Action and Hamiltonians in higher dimensional general relativity: First order framework
Abhay Ashtekar, David Sloan
12 pages, no figures
(Submitted on 18 Aug 08)
We consider d>4 dimensional space-times which are asymptotically flat at spatial infinity and show that, in the first order framework, the action principle is well-defined without the need of infinite counter terms. It naturally leads to a covariant phase space in which the Hamiltonians generating asymptotic symmetries provide the total energy-momentum and angular momentum of the isolated system. This work runs parallel to our previous analysis in four dimensions. The higher dimensional analysis is in fact simpler because of absence of logarithmic and super translation ambiguities.

 

[marcus]

http://arxiv.org/abs/0808.2259
Time delay of light signals in an energy-dependent spacetime metric
A. F. Grillo, E. Luzio, F. Mendez
5 pages. Physical Review D77, 104033 (2008)
(Submitted on 16 Aug 2008)

"In this note we review the problem of time delay of photons propagating in a spacetime with a metric that explicitly depends on the energy of the particles (Gravity-Rainbow approach). We show that corrections due to this approach -- which is closely related to DSR proposal -- produce for small redshifts (z<<1) smaller time delays than in the generic Lorentz Invariance Violating case."

 

[marcus]

http://arxiv.org/abs/0808.2842
Unitarity and Holography in Gravitational Physics
Donald Marolf
14 pages
(Submitted on 21 Aug 2008)

http://arxiv.org/abs/0808.2845
Holographic Thought Experiments
Donald Marolf
24 pages
(Submitted on 21 Aug 2008)

 

[marcus]

http://arxiv.org/abs/0808.3124
Planck Scale Cosmology in Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
7 pages
(Submitted on 22 Aug 2008)

"We show that, by using resummation techniques based on the extension of the methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's theory, we get quantum field theoretic predictions for the UV fixed-point values of the dimensionless gravitational and cosmological constants. Connections to the phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter are discussed."

Asymptotic safety papers cited by Reuter et al., Percacci et al.
Expectation mentioned of possible observational test. Running (Newton and cosmological) G and Lambda constants. UV limit of Lambda derived.
Conclusions mention a followup paper. Bennie Ward has over 300 papers at Spires going back to around 1973. Seems to be new arrival in asymptotic safety-related QG.

 

[marcus]

http://arxiv.org/abs/0808.3505
A note on DSR
Carlo Rovelli
This is a note circulated privately for a while. 5 pages, 2 figures
(Submitted on 26 Aug 2008)

"I study the physical meaning of Doubly Special Relativity (DSR) and argue that DSR can be physically relevant in a certain large distance limit. I present a direct physical interpretation of the 5-dimensional Girelli-Livine DSR formalism, by deriving it explicitly from a concrete physical effect: the gravitational time contraction and its effect on the dynamics of massive point particles. I give a physical interpretation to the corresponding 5d spacetime. I also present some speculations on the interpretation of the corresponding SO(4,1) transformations."

http://arxiv.org/abs/0808.3482
Dynamics of interacting dark energy model in Einstein and Loop Quantum Cosmology
Songbai Chen, Bin Wang, Jiliang Jing
15 pages, 4 figures
(Submitted on 26 Aug 2008)

"We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by Einstein cosmology and Loop Quantum Cosmology. We introduce a new general form of dark sector coupling, which presents us a more complicated dynamical phase space. Differences in the phase space in obtaining the accelerated scaling attractor in Einstein cosmology and Loop Quantum Cosmology are disclosed."

 

[marcus]

http://arxiv.org/abs/0808.3765
Could deformed special relativity naturally arise from the semiclassical limit of quantum gravity?
Lee Smolin
12 pages
(Submitted on 27 Aug 2008)

"A argument is described for how deformed or doubly special relativity may arise in the semiclassical limit of a quantum theory of gravity. We consider a generic quantum theory of gravity coupled to matter, from which we use only the assumption that a Hamiltonian constraint is imposed. We study circumstances in which Lambda, G and hbar all may be separately neglected, but there may arise terms in the ratio of particle energies to the Planck mass which are small but measurable. Such cases include probes of an energy dependent speed of light such as are possible in experiments such as MAGIC and GLAST. We show that in such cases the leading order effect of quantum gravity will, if certain scaling relations are satisfied, be to deform the metric in the effective Hamiltonian of the matter quantum field theory by terms linear in energies. As the Hamiltonian constraint has been imposed there can be no preferred time coordinate or frame of reference, hence this is a modification rather than a breaking of special relativity."

http://arxiv.org/abs/0808.3725
New Positron Spectral Features from Supersymmetric Dark Matter - a Way to Explain the PAMELA Data?
Lars Bergstrom, Torsten Bringmann, Joakim Edsjo
6 pages, 4 figures
(Submitted on 27 Aug 2008)

"The space-borne antimatter experiment PAMELA has recently reported a surprising rise in the positron to electron ratio at high energies. It has also recently been found that electromagnetic radiative corrections in some cases may boost the gamma-ray yield from supersymmetric dark matter annihilations in the galactic halo by up to three or four orders of magnitude, providing distinct spectral signatures for indirect dark matter searches to look for. Here, we investigate whether the same type of corrections can also lead to sizeable enhancements in the positron yield. We find that this is indeed the case, albeit for a smaller region of parameter space than for gamma rays; selecting models with a small mass difference between the neutralino and sleptons, like in the stau coannihilation region in mSUGRA, the effect becomes more pronounced. The resulting, rather hard positron spectrum with a relatively sharp cutoff may potentially fit the rising positron ratio measured by the PAMELA satellite. To do so, however, as seems also to be the case for most other dark matter models, very large 'boost factors' have to be invoked that are not expected in current models of halo structure. If the predicted cutoff would also be confirmed by later PAMELA data or upcoming experiments, one could either assume non-thermal production in the early universe or non-standard halo formation to explain such a spectral feature as an effect of dark matter annihilation. At the end of the paper, we briefly comment on the impact of radiative corrections on other annihilation channels, in particular antiprotons and neutrinos."

 

[marcus]

http://arxiv.org/abs/0808.4056
Black hole entropy for the general area spectrum
Tomo Tanaka, Takashi Tamaki
5 pages
(Submitted on 29 Aug 2008)

"We consider the possibility that the horizon area is expressed by the general area spectrum in loop quantum gravity and calculate the black hole entropy by counting the degrees of freedom in spin-network states related to its area. Although the general area spectrum has a complex expression, we succeeded in obtaining the result that the black hole entropy is proportional to its area as in previous works where the simplified area formula has been used. This gives new values for the Barbero-Immirzi parameter ($$\gamma$$ =0.5802... or 0.7847...) which are larger than that of previous works."

both of the authors are new to me.

 

[marcus]

http://arxiv.org/abs/0809.0304
Towards a spin foam model description of black hole entropy
J.Manuel Garcia-Islas
5 pages, 1 figure
(Submitted on 1 Sep 2008)

"We propose a way to describe the origin of black hole entropy in the spin foam models of quantum gravity. This stimulates a new way to study the relation of spin foam models and loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0809.0469
Dark spinors with torsion in cosmology
Christian G. Boehmer, James Burnett
7 pages, 3 figures
(Submitted on 2 Sep 2008)

"We solve one of the open problems in Einstein-Cartan theory, namely we find a natural matter source whose spin angular momentum tensor is compatible with the cosmological principle. We analyze the resulting evolution equations and find that an epoch of accelerated expansion is an attractor. The torsion field quickly decays in that period. Our results are interpreted in the context of the standard model of cosmology."

Christian Boehmer has already co-authored in loop cosmology. Although this particular paper of his is formally classical, I think it likely to contribute to research in quantum cosmology, possible causes of inflation, and the dark energy effect.

http://arxiv.org/abs/0804.0616
Dark spinor inflation -- theory primer and dynamics
Christian G. Boehmer
13 pages Phys. Rev. D 77 (2008) 123535
(Submitted on 3 Apr 2008)

"Inflation driven by a single dark spinor field is discussed. We define the notion of a dark spinor field and derive the cosmological field equations for such a matter source. The conditions for inflation are determined and an exactly solvable model is presented. We find the power spectrum of the quantum fluctuation of this field and compare the results with scalar field inflation."

 

[marcus]

http://arxiv.org/abs/0809.2022
Cosmology without inflation
Patrick Peter, Nelson Pinto-Neto
8 pages, Phys. Rev. D 78, 063506 (2008)
(Submitted on 11 Sep 2008)

"We propose a new cosmological paradigm in which our observed expanding phase is originated from an initially large contracting Universe that subsequently experienced a bounce. This category of models, being geodesically complete, is non-singular and horizon-free, and can be made to prevent any relevant scale to ever have been smaller than the Planck length. In this scenario, one can find new ways to solve the standard cosmological puzzles. One can also obtain scale invariant spectra for both scalar and tensor perturbations: this will be the case, for instance, if the contracting Universe is dust-dominated at the time at which large wavelength perturbations get larger than the curvature scale. We present a particular example based on a dust fluid classically contracting model, where a bounce occurs due to quantum effects, in which these features are explicit."

 

[marcus]

Freidel-Conrady on semiclassical limit

http://arxiv.org/abs/0809.2280
On the semiclassical limit of 4d spin foam models
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
32 pages, 5 figures
(Submitted on 15 Sep 2008)

"We study the semiclassical properties of the Riemannian spin foam models with Immirzi parameter that are constructed via coherent states. We show that in the semiclassical limit the quantum spin foam amplitudes of an arbitrary triangulation are exponentially suppressed, if the face spins do not correspond to a discrete geometry. When they do arise from a geometry, the amplitudes reduce to the exponential of i times the Regge action. Remarkably, the dependence on the Immirzi parameter disappears in this limit."

http://arxiv.org/abs/0809.2590
Physical time and other conceptual issues of QG on the example of LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
18 pages, 1 figure
(Submitted on 15 Sep 2008)

"Several conceptual aspects of quantum gravity are studied on the example of the homogeneous isotropic LQC model. In particular: (i) The relativistic time of the co-moving observers is showed to be a quantum operator and a quantum spacetime metric tensor operator is derived. (ii) Solutions of the quantum scalar constraint for two different choices of the lapse function are compared and contrasted. (iii) The mechanism of the singularity avoidance is analyzed via detailed studies of an energy density operator. (iv) The relation between the kinematical and the physical quantum geometry is discussed on the level of relation between observables."


http://arxiv.org/abs/0809.2469
Multi-fluid potential in the loop cosmology
Jakub Mielczarek
4 pages, 5 figures
(Submitted on 15 Sep 2008)

"The scalar field can behaves like a fluid with equation of state $$p_{\phi}=w\rho_{\phi}$$, where $$w \in [-1,1]$$. In this letter we derive a class of the scalar field potentials for which w= const. Scalar field with such a potential can mimic ordinary matter, radiation, cosmic strings etc. We perform our calculations in the framework of the loop cosmology with a holonomy corrections. We solve the model analytically for the whole parameter space. Subsequently, we perform similar consideration for the model with a phantom field (w<-1). We show that scalar field is monotonic function in both cases. This indicate that it can be treated as a well defined internal time for these models."

 

[francesca]

Quaternion-Loop Quantum Gravity

http://arxiv.org/abs/0809.2916"
Quaternion-Loop Quantum Gravity
M. D. Maia, S. S e Almeida Silva, F. S. Carvalho
Comments: 6 pages, latex, no figures

We present a simple realization of loop quantum gravity in terms of quaternion operators acting on the spinor space of the triad holonomy group. We find that the Riemannian curvature of the 3-dimensional hypersurfaces in space-time is induced by the SU(2) gauge potential, but it is constructed differently from the usual gauge curvature. Therefore, quaternion-loop quantum gravity is not a gauge theory of gravitation of the triad holonomy group, but is provides a natural justification for quaternion quantum gravity.

 

[francesca]

Evaluation of new spin foam vertex amplitudes

http://arxiv.org/abs/00809.3190"
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
Comments: 19 pages, 4 figures, amsrefs

A new numerical evaluation algorithm is proposed for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov. The algorithm is applied to compute the high spin behavior of the new vertex amplitudes. Their asymptotics exhibit non-oscillatory, power-law decay, similar to that of the Barrett-Crane model, but with different exponents.

 

[marcus]

http://arxiv.org/abs/00809.3190"
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
Comments: 19 pages, 4 figures, amsrefs

A new numerical evaluation algorithm is proposed for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov. The algorithm is applied to compute the high spin behavior of the new vertex amplitudes. Their asymptotics exhibit non-oscillatory, power-law decay, similar to that of the Barrett-Crane model, but with different exponents.

Fixed broken link:
http://arxiv.org/abs/0809.3190
excellent choices IMHO, thanks Francesca!

 

[marcus]

http://arxiv.org/abs/0809.3718
Asymptotics of LQG fusion coefficients
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
14 pages
(Submitted on 22 Sep 2008)

"The fusion coefficients from SO(3) to SO(4) play a key role in the definition of spin foam models for the dynamics in Loop Quantum Gravity. In this paper we give a simple analytic formula of the EPRL fusion coefficients. We study the large spin asymptotics and show that they map SO(3) semiclassical intertwiners into $$SU(2)_L\times SU(2)_R$$ semiclassical intertwiners. This non-trivial property opens the possibility for an analysis of the semiclassical behavior of the model."

 

[marcus]

http://arxiv.org/abs/0809.3850
Black holes and black hole thermodynamics without event horizons
Alex B. Nielsen
Invited review article for General Relativity and Gravitation. 43 pages
(Submitted on 23 Sep 2008)

"We investigate whether black holes can be defined without using event horizons. In particular we focus on the thermodynamic properties of event horizons and the alternative, locally defined horizons. We discuss the assumptions and limitations of the proofs of the zeroth, first and second laws of black hole mechanics for both event horizons and trapping horizons. This leads to the possibility that black holes may be more usefully defined in terms of trapping horizons. We also show how Hawking radiation can also be seen to arise from trapping horizons and discuss which horizon area should be associated with the gravitational entropy."

The black hole "event horizon" is poorly defined in some situations because its definition assumes we know the future with certainty, to infinite time. In quantum geometry/gravity there is geometrical uncertainty and limitation on what an observer knows. Several authors have tackled the problem of defining a different kind of horizon which can serve equally well but which at least in principle is defined based on locally observed and measured stuff. Ashtekar has several papers about this. This paper of Nielsen could provide a useful up-to-date review of where this business stands.

http://arxiv.org/abs/0809.4235
Conditional probabilities with Dirac observables and the problem of time in quantum gravity
Rodolfo Gambini, Rafael Porto, Sebastian Torterolo, Jorge Pullin
4 pages
(Submitted on 24 Sep 2008)

"We combine the 'evolving constants' approach to the construction of observables in canonical quantum gravity with the Page--Wootters formulation of quantum mechanics with a relational time for generally covariant systems. This overcomes the objections levied by Kuchar against the latter formalism. The construction is formulated entirely in terms of Dirac observables, avoiding in all cases the physical observation of quantities that do not belong in the physical Hilbert space. We work out explicitly the example of the parameterized particle, including the calculation of the propagator. The resulting theory also predicts a fundamental mechanism of decoherence."

http://arxiv.org/abs/0809.4170
Fine-grained state counting for black holes in loop quantum gravity
A. Ghosh, P. Mitra
5 pages,
(Submitted on 24 Sep 2008)

"The degeneracy of a black hole corresponding to a definite area eigenvalue is counted within the framework of loop quantum gravity. This is shown to involve several microscopic parameters depending on the area. Their fluctuations yield an explanation of the appearance of a fine structure seen in numerical calculations of the number of states."

 

[marcus]

http://arxiv.org/abs/0809.4464
Effective Theory of Braid Excitations of Quantum Geometry in terms of Feynman Diagrams
Yidun Wan
24 pages, 7 figures
(Submitted on 25 Sep 2008)

"We study interactions amongst topologically conserved excitations of quantum theories of gravity, in particular the braid excitations of four-valent spin networks. These have been shown previously to propagate and interact under evolution rules of spin foam models. We show that the dynamics of these braid excitations can be described by an effective theory based on Feynman diagrams. In this language, braids which are actively interacting are analogous to bosons, in that the topological conservation laws permit them to be singly created and destroyed. Exchanges of these excitations give rise to interactions between braids which are charged under the topological conservation rules."

 

[marcus]

http://arxiv.org/abs/0809.4763
Hamiltonian Analysis of non-chiral Plebanski Theory and its Generalizations
Sergei Alexandrov, Kirill Krasnov
10 pages
(Submitted on 27 Sep 2008)

"We consider non-chiral, full Lorentz group-based Plebanski formulation of general relativity in its version that utilizes the Lagrange multiplier field Phi with "internal" indices. The Hamiltonian analysis of this version of the theory turns out to be simpler than in the previously considered in the literature version with Phi carrying spacetime indices. We then extend the Hamiltonian analysis to a more general class of theories whose action contains scalars invariants constructed from Phi. Such theories have recently been considered in the context of unification of gravity with other forces. We show that these more general theories have six additional propagating degrees of freedom as compared to general relativity, something that has not been appreciated in the literature treating them as being not much different from GR."

 

[MTd2]

http://arxiv.org/abs/0809.5093
Observer Dependent Horizon Temperatures: a Coordinate-Free Formulation of Hawking Radiation as Tunneling
10 pages, 1 2-part figure
Sean Stotyn, Kristin Schleich, Don Witt
(Submitted on 29 Sep 2008)

"We reformulate the Hamilton-Jacobi tunneling method for calculating Hawking radiation in static, spherically-symmetric spacetimes by explicitly incorporating a preferred family of frames. These frames correspond to a family of observers tied to a locally static timelike Killing vector of the spacetime. This formulation separates the role of the coordinates from the choice of vacuum and thus provides a coordinate-independent formulation of the tunneling method. In addition, it clarifies the nature of certain constants and their relation to these preferred observers in the calculation of horizon temperatures. We first use this formalism to obtain the expected temperature for a static observer at finite radius in the Schwarzschild spacetime. We then apply this formalism to the Schwarzschild-de Sitter spacetime, where there is no static observer with 4-velocity equal to the static timelike Killing vector. It is shown that a preferred static observer, one whose trajectory is geodesic, measures the lowest temperature from each horizon. Furthermore, this observer measures horizon temperatures corresponding to the well-known Bousso-Hawking normalization."

-----

I don't think this is a breakthrough, nor is the only article with this point of view, but this is an important analysis nonetheless, and something that urges to be highlitghted: people, either from string or lqg, it seems, ignore the fact that temperature, or even more, perhaps the entropy of black hole is observer dependent. The Hawking temperature is a special and very simplified case.

 

[marcus]

http://arxiv.org/abs/0810.0104
The gravitational wave background from super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
8 pages, 3 figures
(Submitted on 1 Oct 2008)

"We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with $$n_t \approx 2$$. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where $$n_t \approx 0$$."

 

[marcus]

http://arxiv.org/abs/0810.0514
Quantum Space-times: Beyond the Continuum of Minkowski and Einstein
Abhay Ashtekar
31 pages, 6 figures. To appear in the volume Minkowski Space-time: A Hundred Years Later, edited by V. Petkov (Springer-Verlag,Berlin, 2008) commemorating the 100th anniversary of Minkowski's fusion of space and time into a 4 dimensional space-time continuum
(Submitted on 2 Oct 2008)

"In general relativity space-time ends at singularities. The big bang is considered as the Beginning and the big crunch, the End. However these conclusions are arrived at by using general relativity in regimes which lie well beyond its physical domain of validity. Examples where detailed analysis is possible show that these singularities are naturally resolved by quantum geometry effects. Quantum space-times can be vastly larger than what Einstein had us believe. These non-trivial space-time extensions enable us to answer of some long standing questions and resolve of some puzzles in fundamental physics. Thus, a century after Minkowski's revolutionary ideas on the nature of space and time, yet another paradigm shift appears to await us in the wings."

 

[MTd2]

http://arxiv.org/abs/0810.0613
On the condensed matter scheme for emergent gravity and interferometry
Authors: G. Jannes
11 pages; to appear in: F. Columbus (ed.), "Interferometers: Research, Technology and Applications"
(Submitted on 3 Oct 2008)

"An increasingly popular approach to quantum gravity rests on the idea that gravity (and maybe electromagnetism and the other gauge fields) might be an 'emergent phenomenon', in the sense of representing a collective behaviour resulting from a very different microscopic physics. A prominent example of this approach is the condensed matter scheme for quantum gravity, which considers the possibility that gravity emerges as an effective low-energy phenomenon from the quantum vacuum in a way similar to the emergence of collective excitations in condensed matter systems. This condensed matter view of the quantum vacuum clearly hints that, while the term 'ether' has been discredited for about a century, quantum gravity holds many (if not all) of the characteristics that have led people in the past to label various hypothetical substances with the term 'ether'. Since the last burst of enthusiasm for an ether, at the end of the 19th century, was brought to the grave in part by the performance of a series of important experiments in interferometry, the suggestion then naturally arises that maybe interferometry could also play a role in the current discussion on quantum gravity. We will highlight some aspects of this suggestion in the context of the condensed matter scheme for emergent gravity."

 

[marcus]

Westra is one of Loll's Utrecht bunch:
http://arxiv.org/abs/0810.0771
Topology Change and the Emergence of Geometry in Two Dimensional Causal Quantum Gravity
Willem Westra
110 pages, 30 figures, PhD thesis
(Submitted on 4 Oct 2008)

"In this thesis we analyze a very simple model of two dimensional quantum gravity based on causal dynamical triangulations (CDT). We present an exactly solvable model which indicates that it is possible to incorporate spatial topology changes in the nonperturbative path integral. It is shown that if the change in spatial topology is accompanied by a coupling constant it is possible to evaluate the path integral to all orders in the coupling and that the result can be viewed as a hybrid between causal and Euclidian dynamical triangulation. The second model we describe shows how a classical geometry with constant negative curvature emerges naturally from a path integral over noncompact manifolds. No initial singularity is present, hence the quantum geometry is naturally compatible with the Hartle Hawking boundary condition. Furthermore, we demonstrate that under certain conditions the quantum fluctuations are small! To conclude, we treat the problem of spacetime topology change. Although we are not able to completely solve the path integral over all manifolds with arbitrary topology, we do obtain results that indicate that such a path integral might be consistent, provided suitable causality restrictions are imposed."

 

[marcus]

http://arxiv.org/abs/0810.1599
On the computation of black hole entropy in loop quantum gravity
J. Fernando Barbero G., Eduardo J. S. Villaseñor
25 pages
(Submitted on 9 Oct 2008)

"We discuss some issues related to the computation of black hole entropy in loop quantum gravity from the novel point of view provided by the recent number-theoretical methods introduced by the authors and their collaborators. In particular we give exact expressions, in the form of integral transforms, for the black hole entropy in terms of the area. We do this by following several approaches based both on our combinatorial techniques and also on functional equations similar to those employed by Meissner in his pioneering work on this subject. To put our results in perspective we compare them with those of Meissner. We will show how our methods confirm some of his findings, extend the validity of others, and correct some mistakes. At the end of the paper we will discuss the delicate issue of the asymptotics of black hole entropy."

http://arxiv.org/abs/0810.1653
Evaluation of new spin foam vertex amplitudes with boundary states
Igor Khavkine
18 pages, 4 figures
(Submitted on 9 Oct 2008)

"The numerical evaluation algorithms for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov, recently developed by the author, are extended to efficiently include a large class of boundary states (factored states). A concrete pragmatic proposal is made for a semi-classical state, encompassing to both the Barrett-Crane and new models. Two computations using this boundary state are described together with a uniform comparison methodology for the three different models: semi-classical wave packet propagation and graviton 2-point function evaluation. The new algorithms are applied to the wave packet propagation problem, indicating that the Magliaro, Rovelli and Perini's hypothesis of good semiclassical behavior of the new models may not hold under more general conditions (with unfrozen j-spins)."

http://arxiv.org/abs/0810.1714
Self-energy and vertex radiative corrections in LQG
Claudio Perini, Carlo Rovelli, Simone Speziale
11 pages, 3 figures
(Submitted on 9 Oct 2008)

"We consider the elementary radiative-correction terms in loop quantum gravity. These are a two-vertex 'elementary bubble' and a five-vertex 'ball'; they correspond to the one-loop self-energy and the one-loop vertex correction of ordinary quantum field theory. We compute their naive degree of (infrared) divergence."