Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/0907.4887
Possible solution of the cosmological constant problem
Authors: F.R. Klinkhamer, G.E. Volovik
(Submitted on 28 Jul 2009 (v1), last revised 26 Sep 2009 (this version, v3))

Abstract: An extension of the standard model of elementary particle physics and the theory of general relativity is given, which is based on the appropriate introduction of a four-form field strength. The extended theory has, without fine-tuning, a Minkowski-type solution with spacetime-independent fields and provides, therefore, a solution of the main cosmological constant problem.

http://arxiv.org/abs/0909.3084
Topological invariant for superfluid 3He-B and quantum phase transitions
Authors: G.E. Volovik
(Submitted on 16 Sep 2009 (v1), last revised 22 Sep 2009 (this version, v4))

Abstract: We consider topological invariant describing the vacuum states of superfluid 3He-B, which belongs to the special class of time-reversal invariant topological insulators and superfluids. Discrete symmetries important for classification of the topologically distinct vacuum states are discussed. One of them leads to the additional subclasses of 3He-B states and is responsible for the finite density of states of Majorana fermions living on the diffusive wall. Integer valued topological invariant is expressed in terms of the Green's function, which allows us to consider systems with interaction.

 

[MTd2]

http://golem.ph.utexas.edu/category/2009/09/this_weeks_finds_in_mathematic_41.html

This Week’s Finds in Mathematical Physics (Week 280)
Posted by John Baez


In week280 of This Week’s Finds, hear about the courses taught by Ashtekar and Rovelli at the quantum gravity summer school in Corfu. Ashtekar spoke about loop quantum cosmology, and how it could turn the Big Bang into a Big Bounce. Rovelli spoke about spin foam models, and how the new EPRL model cures many problems of the old Barrett-Crane model. The graviton propagator seems to work!

 

[MTd2]

http://arxiv.org/abs/0909.5421

In, Trough and Beyond the Planck Scale

Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 29 Sep 2009)
In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstr\"om-like metric is self-dual in the sense of T-duality: the form of the metric is invariant under the exchange r -> a0/r where a0 is proportional to the minimum area in LQG. Of particular interest, the symmetry imposes that if an observer at infinity sees a black hole of mass m an observer in the other asymptotic infinity beyond the horizon (near r=0) sees a dual mass proportional to m_P^2/m. We then show that small LQBHs are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately 10^(-5) m_P and emit radiation with a typical energy of about 10^(13) - 10^(14) eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.http://arxiv.org/abs/0909.5405
Particle Kinematics in Horava-Lifgarbagez Gravity

Dario Capasso, Alexios P. Polychronakos
(Submitted on 29 Sep 2009)
We study the deformed kinematics of point particles in the Horava theory of gravity. This is achieved by considering particles as the optical limit of fields with a generalized Klein-Gordon action. We derive the deformed geodesic equation and study in detail the cases of flat and spherically symmetric (Schwarzschild-like) spacetimes. As the theory is not invariant under local Lorenz transformations, deviations from standard kinematics become evident even for flat manifolds, supporting superluminal as well as massive luminal particles. These deviations from standard behavior could be used for experimental tests of this modified theory of gravity.http://arxiv.org/abs/0909.4833
Notes on Matter in Horava-Lifgarbagez Gravity

Takao Suyama
(Submitted on 26 Sep 2009)
We investigate the dynamics of a scalar field governed by the Lifgarbagez-type action which should appear naturally in Horava-Lifgarbagez gravity. The wave of the scalar field may propagate with any speed without an upper bound. To preserve the causality, the action cannot have a generic form. Due to the superluminal propagation, a formation of a singularity may cause the breakdown of the predictability of the theory. To check whether such a catastrophe could occur in Horava-Lifgarbagez gravity, we investigate the dynamics of a dust. It turns out that the dust does not collapse completely to form a singularity in a generic situation, but expands again after it attains a maximum energy density.

 

[John86]

http://arxiv.org/abs/0909.5391
Emergent gravitational dynamics in Bose-Einstein condensates
Authors: Lorenzo Sindoni, Florian Girelli, Stefano Liberati
(Submitted on 29 Sep 2009)

Abstract: We discuss a toy model for an emergent non-relativistic gravitational theory. Within a certain class of Bose-Einstein condensates, it is possible to show that, in a suitable regime, a modified version of non-relativistic Newtonian gravity does effectively describes the low energy dynamics of the coupled system condensate/quasi-particles.

 

[marcus]

http://arxiv.org/abs/0909.5435
de Sitter Spaces
Andrew Randono
(Submitted on 29 Sep 2009)
"We exploit an interpretation of gravity as the symmetry broken phase of a de Sitter gauge theory to construct new solutions to the first order field equations. The new solutions are constructed by performing large Spin(4,1) gauge transformations on the ordinary de Sitter solution and extracting first the tetrad, then the induced metric. The class of metrics so obtained is an infinite class labelled by an integer, q. Each solution satisfies the local field equations defining constant positive curvature, and is therefore locally isometric to de Sitter space wherever the metric is non-degenerate. The degeneracy structure of the tetrad and metric reflects the topological differences among the solutions with different q. By topological arguments we show that the solutions are physically distinct with respect to the symmetries of Einstein-Cartan theory. Ultimately, the existence of solutions of this type may be a distinguishing characteristic of gravity as a metric theory versus gravity as a gauge theory."

http://arxiv.org/abs/0909.5631
Algebraic structures in quantum gravity
Adrian Tanasa
17 pages, 4 figues
(Submitted on 30 Sep 2009)
"Starting from a recently-introduced algebraic structure on spin foam models, we define a Hopf algebra by dividing with an appropriate quotient. The structure, thus defined, naturally allows for a mirror analysis of spin foam models with quantum field theory, from a combinatorial point of view. A grafting operator is introduced allowing for the equivalent of a Dyson-Schwinger equation to be written. Non-trivial examples are explicitly worked out. Finally, the physical significance of the results is discussed."

http://arxiv.org/abs/0909.5688
Breaking and restoring of diffeomorphism symmetry in discrete gravity
B. Bahr, B. Dittrich
To appear in the Proceedings of the XXV Max Born Symposium "The Planck Scale", Wroclaw, 29 June - 3 July, 2009
(Submitted on 30 Sep 2009)
"We discuss the fate of diffeomorphism symmetry in discrete gravity. Diffeomorphism symmetry is typically broken by the discretization. This has repercussions for the observable content and the canonical formulation of the theory. It might however be possible to construct discrete actions, so--called perfect actions, with exact symmetries and we will review first steps towards this end."

http://arxiv.org/abs/0910.0181
Cosmological consequences of the NonCommutative Geometry Spectral Action
Mairi Sakellariadou
7 pages. Invited talk at the XXV Max Bonn Symposium "Physics at the Planck Scale"
(Submitted on 1 Oct 2009)
"Cosmological consequences of the noncommutative geometry spectral action are presented. Neglecting the nonminimal coupling of the Higgs field to the curvature, background cosmology remains unchanged, and only the inhomogeneous perturbations will evolve differently from the equivalent classical system. However, considering the nonminimal coupling, corrections will be obtained even at the level of the background cosmologies. Finally, the Higgs field may act as an inflaton field, due to its nonminimal coupling with geometry."

 

[MTd2]

http://arxiv.org/abs/0910.0282

Quantum Gravity Effects on Unstable Orbits in Schwarzschild Space-time

Arundhati Dasgupta
(Submitted on 1 Oct 2009)
We study semiclassical corrections to the Schwarzschild metric, and their effects on unstable orbits

 

[marcus]

http://arxiv.org/abs/0910.0490
Planck Scale Cosmology and Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
6 pages, 2 figures. To be published in the proceedings of DPF-2009, Detroit, MI, July 2009
(Submitted on 2 Oct 2009)
"We show that, by using amplitude-based resummation techniques for Feynman's formulation of Einstein's theory, we get quantum field theoretic 'first principles' 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."

B. Ward is one of those invited to the Perimeter workshop on AsymSafe to be held next month, in early November. He has other writings on AsymSafe as well.

http://arxiv.org/abs/0910.0623
Preferred foliation effects in Quantum General Relativity
Tim Koslowski, Alexander Schenkel
12 pages
(Submitted on 4 Oct 2009)
"We investigate the infrared (IR) effects of Lorentz violating terms in the gravitational sector using functional renormalization group methods similar to Reuter and collaborators. The model we consider consists of pure quantum gravity coupled to a preferred foliation, described effectively via a scalar field with non-standard dynamics. We find that vanishing Lorentz violation is a UV attractive fixed-point of this model. Thus, this model can only be made phenomenologically acceptable with a certain amount of fine-tuning. We also study effects of additional matter fields on the RG running of the Lorentz violating term and provide a general argument why they are small."

http://arxiv.org/abs/0910.0673
Light, Links and Causal Sets
Rafael D. Sorkin
18 pages
(Submitted on 5 Oct 2009)
"After sketching a context in which to seek observable signals of spatio-temporal discreteness, I briefly review the status of the causal set program for quantum gravity, concluding with a simple model for the field produced by a moving charge in a background causal set."

http://arxiv.org/abs/0910.0834
The Return of the Phoenix Universe
Jean-Luc Lehners, Paul J. Steinhardt, Neil Turok
5 pages, Honorable Mention at the 2009 Gravity Research Foundation essay competition
(Submitted on 5 Oct 2009)
"Georges Lemaitre introduced the term 'phoenix universe' to describe an oscillatory cosmology with alternating periods of gravitational collapse and expansion. This model is ruled out observationally because it requires a supercritical mass density and cannot accommodate dark energy. However, a new cyclic theory of the universe has been proposed that evades these problems. In a recent elaboration of this picture, almost the entire universe observed today is fated to become entrapped inside black holes, but a tiny region will emerge from these ashes like a phoenix to form an even larger smooth, flat universe filled with galaxies, stars, planets, and, presumably, life. Survival depends crucially on dark energy and suggests a reason why its density is small and positive today."

 

[John86]

http://arxiv.org/abs/0910.0790
A note on the analogy between superfluids and cosmology
Authors: A. Naddeo, G. Scelza
(Submitted on 5 Oct 2009)
Abstract: A new analogy between superfluid systems and cosmology is here presented, which relies strongly on the following ingredient: the back-reaction of the vacuum to the quanta of sound waves. We show how the presence of thermal phonons, the excitations above the quantum vacuum for $T> 0$, enable us to deduce an hydrodynamical equation formally similar to the one obtained for a perfect fluid in a Universe obeying the Friedmann-Robertson-Walker metric.

http://arxiv.org/abs/0910.0181
Cosmological consequences of the NonCommutative Geometry Spectral Action
Authors: Mairi Sakellariadou
(Submitted on 1 Oct 2009)
Abstract: Cosmological consequences of the noncommutative geometry spectral action are presented. Neglecting the nonminimal coupling of the Higgs field to the curvature, background cosmology remains unchanged, and only the inhomogeneous perturbations will evolve differently from the equivalent classical system. However, considering the nonminimal coupling, corrections will be obtained even at the level of the background cosmologies. Finally, the Higgs field may act as an inflaton field, due to its nonminimal coupling with geometry.

 

[marcus]

http://arxiv.org/abs/0910.0839
A Dialogue on the Nature of Gravity
T. Padmanabhan
32 pages,1 figure; based on the annual Vaidya-Raychaudhuri lecture given at Kolkata, Jan 09, and other lectures--adapted to make a dialogue.
(Submitted on 5 Oct 2009)
"I describe the conceptual and mathematical basis of an approach which describes gravity as an emergent phenomenon. Combining principle of equivalence and principle of general covariance with known properties of local Rindler horizons, perceived by observers accelerated with respect to local inertial frames, one can prove that the field equations describing gravity in any diffeomorphism invariant theory can be given a thermodynamic re-interpretation. This fact, in turn, leads us to the possibility of deriving the field equations of gravity by maximising a suitably defined entropy functional, without using the metric tensor as a dynamical variable. The approach synthesises concepts from quantum theory, thermodynamics and gravity leading to a fresh perspective on the nature of gravity. The description is presented here in the form of a dialogue, thereby addressing several frequently-asked-questions."

http://arxiv.org/abs/0910.0851
One loop beta functions and fixed points in Higher Derivative Sigma Models
R. Percacci, O. Zanusso
26 pages
(Submitted on 6 Oct 2009)
"We calculate the one loop beta functions for nonlinear sigma models in four dimensions containing general two and four derivative terms. In the O(N) model there are four such terms and nontrivial fixed points exist for all N >= 4. In the chiral SU(N) models there are in general six couplings, but only five for N=3 and four for N=2; we find fixed points only for N=2,3. In the approximation considered, the four derivative couplings are asymptotically free but the coupling in the two derivative term has a nonzero limit. These results support the hypothesis that certain sigma models may be asymptotically safe."

Percacci investigates the asymptotic safety of a non-gravity model with the intention of shedding light on asymptotic safety in the case of gravity (about which he has several papers).
He is another of those who will be participating in the Perimeter workshop on asymptotic safety around 3 weeks from now. See post #987 for a paper by Ward, another participant.

 

[marcus]

http://arxiv.org/abs/0910.1278
Loop quantum cosmology of Bianchi type II models
Abhay Ashtekar, Edward Wilson-Ewing
26 pages
(Submitted on 7 Oct 2009)
"The improved dynamics of loop quantum cosmology is extended to include the Bianchi type II model. Because these space-times admit both anisotropies and non-zero spatial curvature, certain technical difficulties arise over and above those encountered in the analysis of the (anisotropic but spatially flat) Bianchi type I space-times, and of the (spatially curved but isotropic) k=+/-1 models. We address these and show that the big-bang singularity is resolved in the same precise sense as in the recent analysis of the Bianchi I model. Bianchi II space-times are of special interest to quantum cosmology because of the expected behavior of the gravitational field near generic space-like singularities in classical general relativity."

 

[marcus]

http://arxiv.org/abs/0910.1542
2-Group Representations for Spin Foams
Aristide Baratin, Derek K. Wise
8 pages; to appear in proceedings of the XXV Max Born Symposium: "The Planck Scale", Wroclaw, Poland
(Submitted on 8 Oct 2009)
"Just as 3d state sum models, including 3d quantum gravity, can be built using categories of group representations, '2-categories of 2-group representations' may provide interesting state sum models for 4d quantum topology, if not quantum gravity. Here we focus on the 'Euclidean 2-group', built from the rotation group SO(4) and its action on the group of translations of 4d Euclidean space. We explain its infinite-dimensional unitary representations, and construct a model based on the resulting representation 2-category. This model, with clear geometric content and explicit 'metric data' on triangulation edges, shows up naturally in an attempt to write the amplitudes of ordinary quantum field theory in a background independent way."

 

[MTd2]

http://arxiv.org/abs/0910.2125

Lifetime of flying particles in canonical Doubly Special Relativity

S. Mignemi
(Submitted on 12 Oct 2009)
We discuss the corrections to the lifetime of unstable elementary particles in some models of doubly special relativity. We assume that the speed of light is invariant and that the position coordinates transform in such a way to ensure the invariance of the deformed symplectic structure of phase space.

 

[marcus]

http://arxiv.org/abs/0910.2117
Can causal dynamical triangulations probe factor-ordering issues?
R L Maitra
12 pages. Presented at the XLIX Cracow School of Theoretical Physics, "Non-perturbative Gravity and Quantum Chromodynamics," Zakopane, Poland, May 31 - June 10, 2009
(Submitted on 12 Oct 2009)
"The causal dynamical triangulations (CDT) program has for the first time allowed for path-integral computation of correlation functions in full general relativity without symmetry reductions and taking into account Lorentzian signature. One of the most exciting recent results in CDT is the strong agreement of these computations with (minisuperspace) path integral calculations in quantum cosmology. Herein I will describe my current project to compute minisuperspace (Friedman-Robertson-Walker) path integrals with a range of different measures corresponding to various factor orderings of the Friedman-Robertson-Walker Hamiltonian. The aim is to compare with CDT results and ask whether CDT can shed light on factor-ordering ambiguities in quantum cosmology models."

Since Rachel Lash Maitra (née Rachel Lash) is a comparatively new postdoc on Loll's team, I'll give background. MS Yale. PhD Yale 2007. As undergrad she majored in Math Physics and graduated first in her class, summa cum laude. I think her research direction as shown here is interesting and potentially valuable. A notable fact is that she is a professional-grade writer. You can see this by reading the excellent clear prose style of the paper and also her CV shows that she can support herself by moonlighting as a technical writing instructor while also doing physics. It's an asset. A research line like CDT needs to grow. The way a startup line of research grows is by attracting exceptional people like this and getting them on board. It's a hopeful sign that Loll's group appears to be doing this.
http://www1.phys.uu.nl/wwwitf/Foto's/Rachel.JPG
http://www.facebook.com/people/Rachel-Lash-Maitra/649117033
http://staff.science.uva.nl/%7Erlash/academic_resume_amsterdam.pdf
http://trostomaten.blogspot.com/ (surprise blog with remarkable nature photos shows yet another side)

 

[MTd2]

http://arxiv.org/abs/0910.2425

The 6j-symbol: Recursion, Correlations and Asymptotics

Maite Dupuis, Etera R. Livine
(Submitted on 13 Oct 2009)
We study the asymptotic expansion of the 6j-symbol using the Schulten-Gordon recursion relations. We focus on the particular case of the isosceles tetrahedron and we provide explicit formulas for up to the third order corrections beyond the leading order. Moreover, in the framework of spinfoam models for 3d quantum gravity, we show how these recursion relations can be used to derive Ward-Takahashi-like identities between the expectation values of graviton-like spinfoam correlations.

 

[MTd2]

http://arxiv.org/abs/0910.2498

Causal set as a discretized phase spacetime

Roman Sverdlov
(Submitted on 13 Oct 2009)
Re-interpretting causal set as a phase spacetime as opposed to the ordinary spacetime allows to restore locality. It also gives one hope to perform analytic calculations of QFT on a causal set.

http://arxiv.org/abs/0910.2574

Matter from Space

Domenico Giulini
(Submitted on 14 Oct 2009)

General Relativity offers the possibility to model attributes of matter, like mass, momentum, angular momentum, spin, chirality etc. from pure space, endowed only with a single field that represents its Riemannian geometry. I review this picture of `Geometrodynamics' and comment on various developments after Einstein.

 

[John86]

http://arxiv.org/abs/0910.2499
Reduction and Emergence in Bose-Einstein Condensates
Authors: Richard Healey
(Submitted on 14 Oct 2009)
Abstract: A closer look at some proposed Gedanken-experiments on BECs promises to shed light on several aspects of reduction and emergence in physics. These include the relations between classical descriptions and different quantum treatments of macroscopic systems, and the emergence of new properties and even new objects as a result of spontaneous symmetry breaking.

 

[marcus]

http://arxiv.org/abs/0910.2936
Quantum geometry and quantum dynamics at the Planck scale
Martin Bojowald
10 pages, 2 figures, Proceedings of "The Planck Scale"; XXV Max Born Symposium
(Submitted on 15 Oct 2009)
"Canonical quantum gravity provides insights into the quantum dynamics as well as quantum geometry of space-time by its implications for constraints. Loop quantum gravity in particular requires specific corrections due to its quantization procedure, which also results in a discrete picture of space. The corresponding changes compared to the classical behavior can most easily be analyzed in isotropic models, but perturbations around them are more involved. For one type of corrections, consistent equations have been found which shed light on the underlying space-time structure at the Planck scale: not just quantum dynamics but also the concept of space-time manifolds changes in quantum gravity. Effective line elements provide indications for possible relationships to other frameworks, such as non-commutative geometry."

http://arxiv.org/abs/0910.2892
Fully LQC-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
9 pages
(Submitted on 15 Oct 2009)
"The cosmological primordial power spectrum is known to be one of the most promising observable to probe quantum gravity effects. In this article, we investigate how the tensor power spectrum is modified by Loop Quantum Gravity corrections. The two most important quantum terms, holonomy and inverse volume, are explicitly taken into account in a unified framework. The equation of propagation of gravitational waves is derived and solved for one set of parameters."

 

[marcus]

http://arxiv.org/abs/0910.3200
A Momentous Arrow of Time
Martin Bojowald
23 pages, 3 figures, Chapter contributed to "The Arrow of Time" Ed. L. Mersini-Houghton and R. Vaas (Springer-Verlag)
(Submitted on 16 Oct 2009)
"Quantum cosmology offers a unique stage to address questions of time related to its underlying (and perhaps truly quantum dynamical) meaning as well as its origin. Some of these issues can be analyzed with a general scheme of quantum cosmology, others are best seen in loop quantum cosmology. The latter's status is still incomplete, and so no full scenario has yet emerged. Nevertheless, using properties that have a potential of pervading more complicated and realistic models, a vague picture shall be sketched here. It suggests the possibility of deriving a beginning within a beginningless theory, by applying cosmic forgetfulness to an early history of the universe."

 

[MTd2]

This Week’s Finds in Mathematical Physics (Week 281)

http://golem.ph.utexas.edu/category/2009/10/this_weeks_finds_in_mathematic_42.html

[...]Finally, hear about strings in 4d BF theory, and spin foam models based on the representation theory of 2-groups.

 

[marcus]

http://arxiv.org/abs/0910.4028
Metric Lagrangians with two propagating degrees of freedom
Kirill Krasnov
4 pages
(Submitted on 21 Oct 2009)
"There exists a large class of generally covariant metric Lagrangians that contain only local terms and describe two propagating degrees of freedom. Trivial examples can be be obtained by applying a local field redefinition to the Lagrangian of general relativity, but we show that the class of two propagating degrees of freedom Lagrangians is much larger. Thus, we exhibit a large family of non-local field redefinitions that map the Einstein-Hilbert Lagrangian into ones containing only local terms. These redefinitions have origin in the topological shift symmetry of BF theory, to which GR is related in Plebanski formulation, and can be computed order by order as expansions in powers of the Riemann curvature. At its lowest non-trivial order such a field redefinition produces the (Riemann)^3 invariant that arises as the two-loop quantum gravity counterterm. Possible implications for quantum gravity are discussed."

 

[John86]

http://arxiv.org/abs/0910.4289
Aspects of nonrelativistic quantum gravity
Authors: Johan Hansson
(Submitted on 22 Oct 2009)
Abstract: A nonrelativistic approach to quantum gravity is studied. At least for weak gravitational fields it should be a valid approximation. Such an approach can be used to point out problems and prospects inherent in a more exact theory of quantum gravity, yet to be discovered. Nonrelativistic quantum gravity, e.g., shows promise for prohibiting black holes altogether (which would eliminate singularities and also solve the black hole information paradox), gives gravitational radiation even in the spherically symmetric case, and supports non-locality (quantum entanglement). Its predictions should also be testable at length scales well above the "Planck scale", by high-precision experiments feasible with existing technology.

http://arxiv.org/abs/0910.4017
Wormhole Spaces: the Common Cause for the Black Hole Entropy-Area Law,the Holographic Principle and Quantum Entanglement
Authors: Manfred Requardt
(Submitted on 21 Oct 2009)
Abstract: We present strong arguments that the deep structure of the quantum vacuum contains a web of microscopic wormholes or short-cuts. We develop the concept of wormhole spaces and show that this web of wormholes generate a peculiar array of long-range correlations in the patterns of vacuum fluctuations on the Planck scale. We conclude that this translocal structure represents the common cause for both the BH-entropy-area law, the more general holographic principle and the entanglement phenomena in quantum theory. In so far our approach exhibits a common structure which underlies both gravity and quantum theory on a microscopic scale. A central place in our analysis is occupied by a quantitative derivation of the distribution laws of microscopic wormholes in the quantum vacuum. This makes it possible to address a number of open questions and controversial topics in the field of quantum gravity.

http://arxiv.org/abs/0910.2600
Reformulating the Schrodinger equation as a Shabat-Zakharov system
Authors: Petarpa Boonserm (Chulalongkorn University), Matt Visser (Victoria University of Wellington)
(Submitted on 14 Oct 2009)
Abstract: We reformulate the second-order Schrodinger equation as a set of two coupled first order differential equations, a so-called "Shabat-Zakharov system", (sometimes called a "Zakharov-Shabat" system). There is considerable flexibility in this approach, and we emphasise the utility of introducing an "auxiliary condition" or "gauge condition" that is used to cut down the degrees of freedom. Using this formalism, we derive the explicit (but formal) general solution to the Schrodinger equation. The general solution depends on three arbitrarily chosen functions, and a path-ordered exponential matrix. If one considers path ordering to be an "elementary" process, then this represents complete quadrature, albeit formal, of the second-order linear

 

[marcus]

http://arxiv.org/abs/0910.4951
Asymptotic safety in gravity and sigma models
R. Percacci
Talk given at International Workshop on Continuum and Lattice Approaches to Quantum Gravity, Brighton, United Kingdom, 17-19 Sep 08. To appear in PoS
(Submitted on 26 Oct 2009)
"There are deep analogies between Einstein's theory of gravity and the nonlinear sigma models. It is suggested that these similarities may extend also to the ultraviolet behaviour, in the sense that both theories could turn out to be asymptotically safe."

http://arxiv.org/abs/0910.4837
Asymmetric cyclic evolution in polymerised cosmology
Orest Hrycyna, Jakub Mielczarek, Marek Szydlowski
10 pages, 4 figs
(Submitted on 26 Oct 2009)
"The dynamical systems methods are used to study evolution of the polymerised scalar field cosmologies with the cosmological constant. We have found all evolutional paths admissible for all initial conditions on the two-dimensional phase space. We have shown that the cyclic solutions are generic. The exact solution for polymerised cosmology is also obtained. Two basic cases are investigated, the polymerised scalar field and the polymerised gravitational and scalar field part. In the former the division on the cyclic and non-cyclic behaviour is established following the sign of the cosmological constant. The value of the cosmological constant is upper bounded purely from the dynamical setting."

 

[MTd2]

http://arxiv.org/abs/0910.4938

Running Gauge Coupling in Asymptotically Safe Quantum Gravity

Jan-Eric Daum, Ulrich Harst, Martin Reuter
(Submitted on 26 Oct 2009)
We investigate the non-perturbative renormalization group behavior of the gauge coupling constant using a truncated form of the functional flow equation for the effective average action of the Yang-Mills-gravity system. We find a non-zero quantum gravity correction to the standard Yang-Mills beta function which has the same sign as the gauge boson contribution. Our results fit into the picture according to which Quantum Einstein Gravity (QEG) is asymptotically safe, with a vanishing gauge coupling constant at the non-trivial fixed point.

 

[marcus]

http://arxiv.org/abs/0910.5167
Gravity from a Particle Physicists' perspective
R. Percacci
Lectures given at the Fifth International School on Field Theory and Gravitation, Cuiaba, Brazil April 20-24 2009. To appear in PoS
(Submitted on 27 Oct 2009)
"In these lectures I review the status of gravity from the point of view of the gauge principle and renormalization, the main tools in the toolbox of theoretical particle physics. In the first lecture I start from the old question "in what sense is gravity a gauge theory?" I will reformulate the theory of gravity in a general kinematical setting which highlights the presence of two Goldstone boson-like fields, and the occurrence of a gravitational Higgs phenomenon. The fact that in General Relativity the connection is a derived quantity appears to be a low energy consequence of this Higgs phenomenon. From here it is simple to see how to embed the group of local frame transformations and a Yang Mills group into a larger unifying group, and how the distinction between these groups, and the corresponding interactions, derives from the VEV of an order parameter. I will describe in some detail the fermionic sector of a realistic "GraviGUT" with SO(3,1)xSO(10) \subset SO(3,11). In the second lecture I will discuss the possibility that the renormalization group flow of gravity has a fixed point with a finite number of attractive directions. This would make the theory well behaved in the ultraviolet, and predictive, in spite of being perturbatively nonrenormalizable. There is by now a significant amount of evidence that this may be the case. There are thus reasons to believe that quantum field theory may eventually prove sufficient to explain the mysteries of gravity."

[my note: PoS is Proceedings of Science, an online resource maintained by SISSA, the Italian Institute for Advanced Studies at Trieste]

 

[MTd2]

http://arxiv.org/abs/0910.5401

The Effective Potential of the Conformal Factor in Asymptotically Safe Quantum Gravity

Jan-Eric Daum, Martin Reuter
(Submitted on 28 Oct 2009)
The effective potential of the conformal factor in the effective average action approach to Quantum Einstein Gravity is discussed. It is shown, without invoking any truncation or other approximations, that if the theory has has a non-Gaussian ultraviolet fixed point and is asymptotically safe the potential has a characteristic behavior near the origin. This behavior might be observable in numerical simulations.

 

[John86]

http://arxiv.org/abs/0910.5377
Bose-Einstein condensed supermassive black holes: a case of renormalized quantum field theory in curved space-time
Authors: Theo M. Nieuwenhuizen, Vaclav Spicka
(Submitted on 28 Oct 2009)
Abstract: This paper investigates the question whether a realistic black hole can be in principal similar to a star, having a large but finite redshift at its horizon. If matter spreads throughout the interior of a supermassive black hole with mass $M\sim10^9M_\odot$, it has an average density comparable to air and it may arise from a Bose-Einstein condensate of densely packed H-atoms. Within the Relativistic Theory of Gravitation with a positive cosmological constant, a bosonic quantum field describing H atoms is coupled to the curvature scalar with dimensionless coupling $\xi$. In the Bose-Einstein condensed groundstate an exact, self-consistent solution for the metric occurs for a certain large value of $\xi$, quadratic in the black hole mass. It is put forward that $\xi$ is set by proper choice of the background metric as a first step of a renormalization approach, while otherwise the non-linearities are small. The black hole has a hair, the binding energy. Fluctuations about the ground state are considered.

 

[atyy]

http://physics.aps.org/articles/v2/90
Tensor networks—a new tool for old problems
Subir Sachdev
A new renormalization group approach that maps lattice problems to tensor networks may hold the key to solving seemingly intractable models of strongly correlated systems in any dimension.

Wen's talk is also helpful http://dao.mit.edu/~wen/talks/09Dresden.pdf

 

[marcus]

http://arxiv.org/abs/0910.5836
Can the Arrow of Time be understood from Quantum Cosmology?
Claus Kiefer
(Submitted on 30 Oct 2009)
I address the question whether the origin of the observed arrow of time can be derived from quantum cosmology. After a general discussion of entropy in cosmology and some numerical estimates, I give a brief introduction into quantum geometrodynamics and argue that this may provide a sufficient framework for studying this question. I then show that a natural boundary condition of low initial entropy can be imposed on the universal wave function. The arrow of time is then correlated with the size of the Universe and emerges from an increasing amount of decoherence due to entanglement with unobserved degrees of freedom. Remarks are also made concerning the arrow of time in multiverse pictures and scenarios motivated by dark energy.
Comments: 14 pages, to appear in "The Arrow of Time", ed. by L. Mersini-Houghton and R. Vaas

Brief mention--possible tangential relevance to 4D QG:
http://arxiv.org/abs/0910.5891
Quantum Knots and Lattices, or a Blueprint for Quantum Systems that Do Rope Tricks
Samuel J. Lomonaco, Louis H. Kauffman

http://arxiv.org/pdf/0910.5733
Decoherence in an Interacting Quantum Field Theory: The Vacuum Case
Jurjen F. Koksma (Utrecht), Tomislav Prokopec (Utrecht), Michael G. Schmidt (Heidelberg)

 

[MTd2]

"Quantum Knots and Lattices, or a Blueprint for Quantum Systems that Do Rope Tricks" is broken. The pictures of the paper are messed up. Is is not available at the time of this post. Look for v2, when it shows up.

 

[marcus]

http://arxiv.org/abs/0911.0386
Renormalization Group Flow in Scalar-Tensor Theories. I
Gaurav Narain, Roberto Percacci
18 pages, 10 figures
(Submitted on 2 Nov 2009)
"We study the renormalization group flow in a class of scalar-tensor theories involving at most two derivatives of the fields. We show in general that minimal coupling is self consistent, in the sense that when the scalar self couplings are switched off, their beta functions also vanish. Complete, explicit beta functions that could be applied to a variety of cosmological models are given in a five parameter truncation of the theory in d=4. In any dimension d>2 we find that the flow has only a 'Gaussian Matter' fixed point, where all scalar self interactions vanish but Newton's constant and the cosmological constant are nontrivial. The properties of these fixed points can be studied algebraically to some extent. In d=3 we also find a gravitationally dressed version of the Wilson-Fisher fixed point, but it seems to have unphysical properties. These findings are in accordance with the hypothesis that these theories are asymptotically safe."

http://arxiv.org/abs/0911.0394
Renormalization Group Flow in Scalar-Tensor Theories. II
Gaurav Narain, Christoph Rahmede
14 pages
(Submitted on 2 Nov 2009)
"We study the UV behaviour of actions including integer powers of scalar curvature and even powers of scalar fields with Functional Renormalization Group techniques. We find UV fixed points where the gravitational couplings have non-trivial values while the matter ones are Gaussian. We prove several properties of the linearized flow at such a fixed point in arbitrary dimensions in the one-loop approximation and find recursive relations among the critical exponents. We illustrate these results in explicit calculations in d=4 for actions including up to four powers of scalar curvature and two powers of the scalar field. In this setting we notice that the same recursive properties among the critical exponents, which were proven at one-loop order, still hold, in such a way that the UV critical surface is found to be five dimensional. We then search for the same type of fixed point in a scalar theory with minimal coupling to gravity in d=4 including up to eight powers of scalar curvature. Assuming that the recursive properties of the critical exponents still hold, one would conclude that the UV critical surface of these theories is five dimensional."

http://arxiv.org/abs/0911.0401
Spectral geometry as a probe of quantum spacetime
Dario Benedetti, Joe Henson
25 pages, 6 figures
(Submitted on 2 Nov 2009)
"Employing standard results from spectral geometry, we provide strong evidence that in the classical limit the ground state of three-dimensional causal dynamical triangulations is de Sitter spacetime. This result is obtained by measuring the expectation value of the spectral dimension on the ensemble of geometries defined by these models, and comparing its large scale behaviour to that of a sphere (Euclidean de Sitter). From the same measurement we are also able to confirm the phenomenon of dynamical dimensional reduction observed in this and other approaches to quantum gravity -- the first time this has been done for three-dimensional causal dynamical triangulations. In this case, the value for the short-scale limit of the spectral dimension that we find is approximately 2. We comment on the relevance of these results for the comparison to asymptotic safety and Horava-Lifgarbagez gravity, among other approaches to quantum gravity."

 

[marcus]

http://arxiv.org/abs/0911.0437
Fractal Space-Time from Spin-Foams
Elena Magliaro, Claudio Perini, Leonardo Modesto
5 pages, 2 figures
(Submitted on 2 Nov 2009)
"In this paper we perform the calculation of the spectral dimension of spacetime in 4d quantum gravity using the Barrett-Crane (BC) spinfoam model. We realize this considering a very simple decomposition of the 4d spacetime already used in the graviton propagator calculation and we introduce a boundary state which selects a classical geometry on the boundary. We obtain that the spectral dimension of the spacetime runs from ~ 2 to 4, across a ~1.5 phase, when the energy of a probe scalar field decreases from high $$E \lesssim E_P/25$$ to low energy. The spectral dimension at the Planck scale $$E \approx E_P$$ depends on the areas spectrum used in the calculation. For three different spectra $$l_P^2 \sqrt{j(j+1)}$$, $$l_P^2 (2 j+1)$$ and $$l_P^2 j$$ we find respectively dimension ~ 2.31, 2.45 and 2.08."

http://arxiv.org/abs/0911.0543
The volume operator in covariant quantum gravity
You Ding, Carlo Rovelli
9 pages
(Submitted on 3 Nov 2009)
"A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In particular, the geometrical observable giving the area of a surface has been shown to be the same as the one in loop quantum gravity. Here we discuss the volume observable. We derive the volume operator in the covariant theory, and show that it matches the one of loop quantum gravity, as does the area. We also reconsider the implementation of the constraints that defines the model: we derive in a simple way the boundary Hilbert space of the theory from a suitable form of the classical constraints, and show directly that all constraints vanish weakly on this space."

 

[MTd2]

http://arxiv.org/abs/0911.0437

The volume operator in covariant quantum gravity

You Ding, Carlo Rovelli
(Submitted on 3 Nov 2009)
A covariant spin-foam formulation of quantum gravity has been recently developed, characterized by a kinematics which appears to match well the one of canonical loop quantum gravity. In particular, the geometrical observable giving the area of a surface has been shown to be the same as the one in loop quantum gravity. Here we discuss the volume observable. We derive the volume operator in the covariant theory, and show that it matches the one of loop quantum gravity, as does the area. We also reconsider the implementation of the constraints that defines the model: we derive in a simple way the boundary Hilbert space of the theory from a suitable form of the classical constraints, and show directly that all constraints vanish weakly on this space.

 

[John86]

http://arxiv.org/abs/0909.5421
In, Through and Beyond the Planck Scale
Authors: Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 29 Sep 2009 (v1), last revised 31 Oct 2009 (this version, v2))
Abstract: In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstr\"om-like metric is self-dual in the sense of T-duality: the form of the metric is invariant under the exchange r -> a0/r where a0 is proportional to the minimum area in LQG. Of particular interest, the symmetry imposes that if an observer at infinity sees a black hole of mass m an observer in the other asymptotic infinity beyond the horizon (near r=0) sees a dual mass proportional to m_P^2/m. We then show that small LQBHs are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately 10^(-5) m_P and emit radiation with a typical energy of about 10^(13) - 10^(14) eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.

http://arxiv.org/abs/0908.4224
Stochastic quantization and the role of time in quantum gravity
Authors: J. Ambjorn, R. Loll, W. Westra, S. Zohren
(Submitted on 28 Aug 2009 (v1), last revised 1 Nov 2009 (this version, v2))
Abstract: We show that the noncritical string field theory developed from two-dimensional quantum gravity in the framework of causal dynamical triangulations can be viewed as arising through a stochastic quantization. This requires that the proper time appearing in the string field theory be identified with the stochastic time of the stochastic formulation. The framework of stochastic quantization gives rise to a natural nonperturbative quantum Hamiltonian, which incorporates a sum over all spacetime topologies. We point out that the external character of stochastic time is a feature that pertains more generally to the proper time or distance appearing in nonperturbative correlation functions in quantum gravity.

http://arxiv.org/abs/0910.5733
Decoherence in an Interacting Quantum Field Theory: The Vacuum Case
Authors: Jurjen F. Koksma (Utrecht University), Tomislav Prokopec (Utrecht University), Michael G. Schmidt (Heidelberg University)
(Submitted on 29 Oct 2009)
Abstract: We apply the decoherence formalism to an interacting scalar field theory. In the spirit of the decoherence literature, we consider a "system field" and an "environment field" that interact via a cubic coupling. We solve for the propagator of the system field, where we include the self-energy corrections due to the interaction with the environment field. In this paper, we consider an environment in the vacuum state (T=0). We show that neglecting inaccessible non-Gaussian correlators increases the entropy of the system as perceived by the observer. Moreover, we consider the effect of a changing mass of the system field in the adiabatic regime, and we find that at late times no additional entropy has been generated.

 

[MTd2]

http://arxiv.org/abs/0911.1020

Constraining the energy-momentum dispersion relation with Planck-scale sensitivity using cold atoms

Giovanni Amelino-Camelia, Claus Laemmerzahl, Flavio Mercati, Guglielmo M. Tino
(Submitted on 5 Nov 2009)
We use the results of ultra-precise cold-atom-recoil experiments to constrain the form of the energy-momentum dispersion relation, a structure that is expected to be modified in several quantum-gravity approaches. Our strategy of analysis applies to the nonrelativistic (small speeds) limit of the dispersion relation, and is therefore complementary to an analogous ongoing effort of investigation of the dispersion relation in the ultrarelativistic regime using observations in astrophysics. For the leading correction in the nonrelativistic limit the exceptional sensitivity of cold-atom-recoil experiments remarkably allows us to set a limit within a single order of magnitude of the desired Planck-scale level, thereby providing the first example of Planck-scale sensitivity in the study of the dispersion relation in controlled laboratory experiments. For the next-to-leading term we obtain a limit which is a few orders of magnitude away from the Planck scale, but still amounts to the best limit on a class of Lorentz-symmetry test theories that has been extensively used to investigate the hypothesis of "deformation" (rather than breakdown) of spacetime symmetries.

 

[John86]

http://arxiv.org/abs/0911.1063
The cosmological constant and higher dimensional dilatation symmetry
Authors: C. Wetterich
(Submitted on 5 Nov 2009)
Abstract: We discuss the hypothesis of a fixed point for quantum gravity coupled to a scalar, in the limit where the scalar field goes to infinity, accompanied by a suitable scaling of the metric. We propose that no scalar potential is present for the dilatation symmetric quantum effective action at the fixed point. Dimensional reduction of such a higher dimensional effective action leads to solutions with a vanishing effective four-dimensional constant. Under rather general circumstances these are the only quasistatic stable solutions with finite four-dimensional gravitational constant. If cosmological runaway solutions approach the fixed point as time goes to infinity, the cosmological constant vanishes asymptotically. For our old Universe the fixed point is not yet reached completely, resulting in a tiny amount of dark energy, comparable to dark matter. We discuss explicitly higher dimensional geometries which realize such asymptotic solutions for $t\to\infty$. They include Ricci-flat spaces as well as warped spaces, potentially with singularities.