Loop-and-allied QG bibliography

[marcus]

wow! Is John Baez back to LQG?

No, this is still pure math. One never knows if and for what pure mathematics might prove useful.
In this case, it isn't clear how 2-groups and higher category theory might eventually apply to QG. It might or it might not.
His three co-authors' research has been mostly in QG. He has taken them on a remarkable excursion into higher category theory.

The original rationale was that Freidel and Livine (also with Baratin) did some very interesting work combining 3D spinfoam and matter and Feynman diagrams of usual field theory in the zero gravity limit. That was in 2006. But try as they might, their results would not extend up to 4D. Baez suggested they venture into n-category theory because it offers a general machinery for jacking up dimensions by one step. Today's paper could provide a missing ingredient for Freidel to jack his 2006 work on 3D QG and matter up to 4D, though it seems like long-odds.

As a rule it is better if we don't burden this bibliography thread with discussion. I will fetch a Baez thread that relates to this paper, so we can talk there if you want, instead of here.

Here is the Baez thread
https://www.physicsforums.com/showthread.php?p=2012718#post2012718
https://www.physicsforums.com/showthread.php?t=123902

 

[marcus]

http://arxiv.org/abs/0812.4512
The Problem Of Gauge Theory
Edward Witten
This article is the written form of a lecture presented at the conference "Geometric Analysis: Past and Future" (Harvard University, August 27-September 1, 2008), in honor of the 60th birthday of S.-T. Yau.
(Submitted on 24 Dec 2008)
"I sketch what it is supposed to mean to quantize gauge theory, and how this can be made more concrete in perturbation theory and also by starting with a finite-dimensional lattice approximation. Based on real experiments and computer simulations, quantum gauge theory in four dimensions is believed to have a mass gap. This is one of the most fundamental facts that makes the Universe the way it is."

http://arxiv.org/abs/0812.2696
Emergent Electroweak Gravity
Bob McElrath
4 pages
(Submitted on 15 Dec 2008)
"We show that any massive cosmological relic particle with small self-interactions is a super-fluid today, due to the broadening of its wave packet, and lack of any elastic scattering. The WIMP dark matter picture is only consistent its mass M >> M_Pl in order to maintain classicality. The dynamics of a super-fluid are given by the excitation spectrum of bound state quasi-particles, rather than the center of mass motion of constituent particles. If this relic is a fermion with a repulsive interaction mediated by a heavy boson, such as neutrinos interacting via the Z⁰, the condensate has the same quantum numbers as the vierbein of General Relativity. Because there exists an enhanced global symmetry SO(3,1)_space x SO(3,1)_spin among the fermion's self-interactions broken only by its kinetic term, the long wavelength fluctuation around this condensate is a Goldstone graviton. A gravitational theory exists in the low energy limit of the Standard Model's Electroweak sector below the weak scale, with a strength that is parametrically similar to G_N."
some helpful interpretive comment:
http://blogs.discovermagazine.com/cosmicvariance/2008/12/27/gravity-emergesfrom-neutrinos/
by John Conway:
http://www.physics.ucdavis.edu/~conway/

 

[marcus]

http://arxiv.org/abs/0812.5018
LQG propagator: III. The new vertex
Emanuele Alesci, Eugenio Bianchi, Carlo Rovelli
9 pages
(Submitted on 30 Dec 2008)
"In the first article of this series, we pointed out a difficulty in the attempt to derive the low-energy behavior of the graviton two-point function, from the loop-quantum-gravity dynamics defined by the Barrett-Crane vertex amplitude. Here we show that this difficulty disappears when using the corrected vertex amplitude recently introduced in the literature. In particular, we show that the asymptotic analysis of the new vertex amplitude recently performed by Barrett, Fairbairn and others, implies that the vertex has precisely the asymptotic structure that, in the second article of this series, was indicated as the key necessary condition for overcoming the difficulty."http://arxiv.org/abs/0812.4703
Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
Abhay Ashtekar
12 pages. Addressed to experts in classical general relativity. Based on a talk at the NEB XIII conference held at Thessaloniki. To appear in Journal of Physics Proceedings Series
(Submitted on 26 Dec 2008)
"A brief summary of the singularity resolution in loop quantum cosmology of homogeneous isotropic models is presented. The article is addressed to relativists who do not specialize in quantum gravity. For further details, and answers to more technical asked questions, the reader is directed to the original papers and to more comprehensive recent reviews.
Dedicated to the memory of Basilis Xanthopoulos, a founder of this NEB series of conferences."

http://arxiv.org/abs/0812.4969
Infinite-Dimensional Representations of 2-Groups
John C. Baez, Aristide Baratin, Laurent Freidel, Derek K. Wise
101 pages
(Submitted on 29 Dec 2008)
"A '2-group' is a category equipped with a multiplication satisfying laws like those of a group. Just as groups have representations on vector spaces, 2-groups have representations on '2-vector spaces', which are categories analogous to vector spaces. Unfortunately, Lie 2-groups typically have few representations on the finite-dimensional 2-vector spaces introduced by Kapranov and Voevodsky. For this reason, Crane, Sheppeard and Yetter introduced certain infinite-dimensional 2-vector spaces called 'measurable categories' (since they are closely related to measurable fields of Hilbert spaces), and used these to study infinite-dimensional representations of certain Lie 2-groups. Here we continue this work. We begin with a detailed study of measurable categories. Then we give a geometrical description of the measurable representations, intertwiners and 2-intertwiners for any skeletal measurable 2-group. We study tensor products and direct sums for representations, and various concepts of subrepresentation. We describe direct sums of intertwiners, and sub-intertwiners--features not seen in ordinary group representation theory. We classify irreducible and indecomposable representations and intertwiners. We also classify 'irretractable' representations--another feature not seen in ordinary group representation theory. Finally, we argue that measurable categories equipped with some extra structure deserve to be considered 'separable 2-Hilbert spaces', and compare this idea to a tentative definition of 2-Hilbert spaces as representation categories of commutative von Neumann algebras."

 

[marcus]

http://arxiv.org/abs/0901.0010
The Evolution of Lambda Black Holes in the Mini-Superspace Approximation of Loop Quantum Gravity
J. Brannlund, S. Kloster, A. DeBenedictis
25 pages, 7 figures
(Submitted on 30 Dec 2008)
"Using the improved quantization technique to the mini-superspace approximation of loop quantum gravity, we study the evolution of black holes supported by a cosmological constant. The addition of a cosmological constant allows for classical solutions with planar, cylindrical, toroidal and higher genus black holes. Here we study the quantum analog of these space-times. In all scenarios studied, the singularity present in the classical counter-part is avoided in the quantized version and is replaced by a bounce, and in the late evolution, a series of less severe bounces. Interestingly, although there are differences during the evolution between the various symmetries and topologies, the evolution on the other side of the bounce aymptotes to space-times of Nariai-type. This is independent of the symmetry and topology of the system, indicating that Nariai-type universes are attractors in the quantum evolution, albeit with different parameters. We study here the quantum evolution of each symmetry in detail."

http://arxiv.org/pdf/0901.0041v1
Gamma-ray Bursts: Light on the distant Universe
Jonathan Grindlay
News and Views article for Nature vol. 455, 177 (Sept. 11, 2008)
(Submitted on 31 Dec 2008)
"Observations of a long-lasting Gamma-ray burst, one that has the brightest optical counterpart yet discovered, challenge theoretical understanding of these bursts but may enhance their usefulness as cosmic probes."

 

[marcus]

http://arxiv.org/abs/0901.0640
Horizons and the cosmological constant
Krzysztof A. Meissner
6 pages
(Submitted on 6 Jan 2009)
"A new solution of the Einstein equations for the point mass immersed in the de Sitter Universe is presented. The properties of the metric are very different from both the Schwarzschild black hole and the de Sitter Universe: it is everywhere smooth, light can propagate outward through the horizon, there is an antitrapped surface enclosing the point mass and there is necessarily an initial singularity. The solution for any positive cosmological constant is qualitatively different from the Schwarzschild solution and is not its continuous deformation."

http://arxiv.org/abs/0901.0577
Noncommutative Geometry as the Key to Unlock the Secrets of Space-Time
Ali H. Chamseddine
Contribution to a special volume in honor of Alain Connes in occasion of his 60th birthday. To be published in the Proceedings series of the Clay Mathematics Institute. Editors E. Blanchard et al
(Submitted on 6 Jan 2009)
"I give a summary of the progress made on using the elegant construction of Alain Connes noncommutaive geometry to explore the nature of space-time at very high energies. In particular I show that by making very few natural and weak assumptions about the structure of the noncommutative space, one can deduce the structure of all fundamental interactions at low energies."

http://arxiv.org/abs/0901.0662
Gravity from spontaneous Lorentz violation
Alan Kostelecky, Robertus Potting
51 pages
(Submitted on 6 Jan 2009)
"We investigate a class of theories involving a symmetric two-tensor field in Minkowski spacetime with a potential triggering spontaneous violation of Lorentz symmetry. The resulting massless Nambu-Goldstone modes are shown to obey the linearized Einstein equations in a fixed gauge. Imposing self-consistent coupling to the energy-momentum tensor constrains the potential for the Lorentz violation. The nonlinear theory generated from the self-consistent bootstrap is an alternative theory of gravity, containing kinetic and potential terms along with a matter coupling. At energies small compared to the Planck scale, the theory contains general relativity, with the Riemann-spacetime metric constructed as a combination of the two-tensor field and the Minkowski metric. At high energies, the structure of the theory is qualitatively different from general relativity. Observable effects can arise in suitable gravitational experiments."

http://arxiv.org/abs/0901.0632
Dark Matter
Jaan Einasto
UNESCO EOLSS ENCYCLOPEDIA (accepted) 25 pages, 17 figures
(Submitted on 6 Jan 2009)
"A review of the development of the concept of dark matter is given. I begin the review with the description of the discovery of the mass paradox in our Galaxy and in clusters of galaxies. In mid 1970s the amount of observational data was sufficient to suggest the presence of a massive and invisible population around galaxies and in clusters of galaxies. The nature of the dark population was not clear at that time, but the hypotheses of stellar as well as of gaseous nature of the new population had serious difficulties. These difficulties disappeared when non-baryonic nature of dark matter was suggested in early 1980s. In addition to the presence of Dark Matter, recent observations suggest the presence of Dark Energy, which together with Dark Matter and ordinary baryonic matter makes the total matter/energy density of the Universe equal to the critical cosmological density. There are various hypothesis as for the nature of the dark matter particles, and generally some form of weakly interactive massive particles (WIMPs) are strongly favored. Both Dark Matter and Dark Energy are the greatest challenges for modern physics since their nature is unknown."

 

[marcus]

http://arxiv.org/abs/0901.0933
Quantum field theory on a cosmological, quantum space-time
Abhay Ashtekar, Wojciech Kaminski, Jerzy Lewandowski
19 pages
(Submitted on 7 Jan 2009)
"In loop quantum cosmology, Friedmann-LeMaitre-Robertson-Walker (FLRW) space-times arise as well-defined approximations to specific quantum geometries. We initiate the development of a quantum theory of test scalar fields on these quantum geometries. Emphasis is on the new conceptual ingredients required in the transition from classical space-time backgrounds to quantum space-times. These include a 'relational time' a la Leibnitz, the emergence of the Hamiltonian operator of the test field from the quantum constraint equation, and ramifications of the quantum fluctuations of the background geometry on the resulting dynamics. The familiar quantum field theory on classical FLRW models arises as a well-defined reduction of this more fundamental theory."

http://arxiv.org/abs/0901.0964
Quantum Gravity on the Lattice
Herbert W. Hamber
63 pages, 12 figures
(Submitted on 8 Jan 2009)
"I review the lattice approach to quantum gravity, and how it relates to the non-trivial ultraviolet fixed point scenario of the continuum theory. After a brief introduction covering the general problem of ultraviolet divergences in gravity and other non-renormalizable theories, I cover the general methods and goals of the lattice approach. An underlying theme is an attempt at establishing connections between the continuum renormalization group results, which are mainly based on diagrammatic perturbation theory, and the recent lattice results, which should apply to the strong gravity regime and are inherently non-perturbative. A second theme in this review is the ever-present natural correspondence between infrared methods of strongly coupled non-abelian gauge theories on the one hand, and the low energy approach to quantum gravity based on the renormalization group and universality of critical behavior on the other. Towards the end of the review I discuss possible observational consequences of path integral quantum gravity, as derived from the non-trivial ultraviolet fixed point scenario. I argue that the theoretical framework naturally leads to considering a weakly scale-dependent Newton's costant, with a scaling violation parameter related to the observed scaled cosmological constant (and not, as naively expected, to the Planck length)."

http://arxiv.org/abs/0901.0916
CMB Lensing Constraints on Neutrinos and Dark Energy
Roland de Putter, Oliver Zahn, Eric V. Linder
18 pages, 14 figures
(Submitted on 7 Jan 2009)
"Signatures of lensing of the cosmic microwave background radiation by gravitational potentials along the line of sight carry with them information on the matter distribution, neutrino masses, and dark energy properties. We examine the constraints that Planck, PolarBear, and CMBpol future data, including from the B-mode polarization or the lensing potential, will be able to place on these quantities. We simultaneously fit for neutrino mass and dark energy equation of state including time variation and early dark energy density, and compare the use of polarization power spectra with an optimal quadratic estimator of the lensing. Results are given as a function of systematics level from residual foreground contamination. A realistic CMBpol experiment can effectively constrain the sum of neutrino masses to within 0.05 eV and the fraction of early dark energy to 0.002. We also present a surprisingly simple prescription for calculating dark energy equation of state constraints in combination with supernova distances from JDEM."

Some related papers appearing at the same time:
http://arxiv.org/abs/0901.0917
Lensed Cosmic Microwave Background Constraints on Post-General Relativity Parameters
Paolo Serra, Asantha Cooray, Scott F. Daniel, Robert Caldwell, Alessandro Melchiorri

http://arxiv.org/abs/0901.0918
Extending the Gravitational Growth Framework
Eric V. Linder

 

[marcus]

http://arxiv.org/abs/0901.1301
Transcending Big Bang in Loop Quantum Cosmology: Recent Advances
Parampreet Singh
Invited plenary talk at the Sixth International Conference on Gravitation and Cosmology, IUCAA (Pune). 13 pages, 3 figures
(Submitted on 9 Jan 2009)
"We discuss the way non-perturbative quantization of cosmological spacetimes in loop quantum cosmology provides insights on the physics of Planck scale and the resolution of big bang singularity. In recent years, rigorous examination of mathematical and physical aspects of the quantum theory has led to a consistent quantization which is consistent and physically viable and some early ideas have been ruled out. The latter include so called 'physical effects' originating from modifications to inverse scale factors in the flat models. The singularity resolution is understood to originate from the non-local nature of curvature in the quantum theory and the underlying polymer representation. Using an exactly solvable model various insights have been gained. The model predicts a generic occurrence of bounce for states in the physical Hilbert space and a supremum for the spectrum of the energy density operator. It also provides answers to the growth of fluctuations, showing that semi-classicality is preserved to an amazing degree across the bounce."

http://arxiv.org/abs/0901.1302
Black holes and entropy in loop quantum gravity: An overview
Alejandro Corichi
17 pages, 5 figures. Extended version of the contribution for the proceedings of the 6th IGCG Conference and a contribution for Advanced Science Letters
(Submitted on 9 Jan 2009)
"Black holes in equilibrium and the counting of their entropy within Loop Quantum Gravity are reviewed. In particular, we focus on the conceptual setting of the formalism, briefly summarizing the main results of the classical formalism and its quantization. We then focus on recent results for small, Planck scale, black holes, where new structures have been shown to arise, in particular an effective quantization of the entropy. We discuss recent results that employ in a very effective manner results from number theory, providing a complete solution to the counting of black hole entropy. We end with some comments on other approaches that are motivated by loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0901.1757
How loopy is the quantum bounce? A heuristic analysis of higher order holonomy corrections in LQC
Dah-Wei Chiou, Li-Fang Li
5 pages, 1 figure
(Submitted on 13 Jan 2009)
"A well motivated extension of higher order holonomy corrections in loop quantum cosmology (LQC) for the k=0 Friedmann-Robertson-Walker (FRW) model is investigated at the level of heuristic effective dynamics, which reveals that the quantum bounce is generic, regardless of the order of corrections, and the matter density remains finite, bounded by an upper bound in the regime of the Planckian density, even if all orders of corrections are included. This observation provides further evidence that the quantum bounce is essentially a consequence of the loopy nature (i.e. intrinsic discreteness) of LQC and LQC is fundamentally different from the Wheeler-DeWitt (WDW) theory; it also encourages one to construct the quantum theory of LQC with the higher order holonomy corrections, which might be understood as related to the higher j representations in the Hamiltonian operator of loop quantum gravity (LQG)."

brief mention:
http://arxiv.org/abs/0901.1870
Disentangling Forms of Lorentz Violation With Complementary Clock Comparison Experiments
Brett Altschul

 

[marcus]

http://arxiv.org/abs/0901.2414
The weak anthropic principle and the landscape of string theory
George F R Ellis, Lee Smolin
11 pages
(Submitted on 16 Jan 2009)
We note that there is an exception to the general arguments that no falsifiable predictions can be made, on the basis of of presently available data, by applying the weak anthropic principle (WAP) to the landscape of string theory. If there are infinitely more vacua in the landscape for one sign of a parameter than the other, within an anthropically allowed range, then under very weak assumptions about the probability measure one gets a firm prediction favoring that sign of that parameter. It is interesting to note that while the understanding of the string landscape is evolving, present evidence on the nature of the landscape allows such an argument to be made, leading to the conclusion that the WAP favors a negative value for the cosmological constant, Lambda, in contradiction to the result of astronomical observations. The viability of applying the WAP to string theory then requires that either there are found an infinite discretum of anthropically allowed vacua for Lambda >0, or the recently found infinite discretum of solutions for Lambda <0 be reduced to a finite value.

http://arxiv.org/abs/0901.2750
Are loop quantum cosmos never singular?
Parampreet Singh
17 pages, 5 figures
(Submitted on 19 Jan 2009)
"A unified treatment of all known types of singularities for flat, isotropic and homogeneous spacetimes in the framework of loop quantum cosmology (LQC) is presented. These include bangs, crunches and all future singularities. Using effective spacetime description we perform a model independent general analysis of the properties of curvature, behavior of geodesics and strength of singularities. For illustration purposes a phenomenological model based analysis is also performed. We show that all values of the scale factor at which a strong singularity may occur are excluded from the effective loop quantum spacetime. Further, if the evolution leads to either a vanishing or divergent scale factor then the loop quantum universe is asymptotically deSitter in that regime. We also show that there exist a class of sudden extremal events, which includes a recently discussed possibility, for which the curvature or its derivatives will always diverge. Such events however turn out to be harmless weak curvature singularities beyond which geodesics can be extended. Our results point towards a generic resolution of physical singularities in LQC."

 

[marcus]

http://arxiv.org/abs/0901.3156
Conservative solutions to the black hole information problem
Sabine Hossenfelder, Lee Smolin
25 pages, 7 figures
(Submitted on 20 Jan 2009)
"We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence."

http://arxiv.org/abs/0901.2984
Asymptotic safety in higher-derivative gravity
Dario Benedetti, Pedro F. Machado, Frank Saueressig
4 pages
(Submitted on 20 Jan 2009)
"We study the non-perturbative renormalization group flow of higher-derivative gravity employing functional renormalization group techniques. The beta-functions give rise to a non-trivial ultraviolet fixed point with three UV-attractive and one UV-repulsive eigendirections, substantiating previous evidence that gravity is asymptotically safe. We argue that asymptotic safety also resolves the unitarity problem typically haunting higher-derivative gravity theories."

http://arxiv.org/abs/0901.2963
Running of Newton's Constant and Quantum Gravitational Effects
David Reeb
11 pages, 5 figures, latex; contribution to the proceedings of the 46th course of the International School of Subnuclear Physics held in Erice, Sicily/Italy, August 29 - September 7, 2008
(Submitted on 20 Jan 2009)
"Newton's gravitational constant is shown to be a running coupling constant, much like the familiar running gauge couplings of the Standard Model. This implies that, in models with appropriate particle content, the true Planck scale, i.e. the scale at which quantum gravity effects become important, can have a value different from 10^19 GeV, which would be expected from naive dimensional analysis. Then, two scenarios involving this running effect are presented. The first one is a model which employs huge particle content to realize quantum gravity at the TeV scale in 4 dimensions, thereby solving the hierarchy problem of the Standard Model. Secondly, effects of the running of Newton's constant in grand unified theories are examined and shown to introduce new significant uncertainties in their predictions, but possibly also to provide better gauge coupling unification results in some cases."

http://arxiv.org/abs/0901.2932
Exploring Quantum Gravity with Very-High-Energy Gamma-Ray Instruments - Prospects and Limitations
Robert Wagner (MPI für Physik)
11 pages, 4 figures, proceedings of "Science with the New Generation of High Energy Gamma-Ray Experiments", Euganean Spa Area, Padova: October 8-10, 2008
(Submitted on 20 Jan 2009)
"Some models for quantum gravity (QG) violate Lorentz invariance and predict an energy dependence of the speed of light, leading to a dispersion of high-energy gamma-ray signals that travel over cosmological distances. Limits on the dispersion from short-duration substructures observed in gamma-rays emitted by gamma-ray bursts (GRBs) at cosmological distances have provided interesting bounds on Lorentz invariance violation (LIV). Recent observations of unprecedentedly fast flares in the very-high energy gamma-ray emission of the active galactic nuclei (AGNs) Mkn 501 in 2005 and PKS 2155-304 in 2006 resulted in the most constraining limits on LIV from light-travel observations, approaching the Planck mass scale, at which QG effects are assumed to become important. I review the current status of LIV searches using GRBs and AGN flare events, and discuss limitations of light-travel time analyses and prospects for future instruments in the gamma-ray domain."

 

[John86]

http://arxiv.org/abs/0901.3156
Conservative solutions to the black hole information problem
Authors: Sabine Hossenfelder, Lee Smolin
(Submitted on 20 Jan 2009)
Abstract: We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence.

 

[MTd2]

http://arxiv.org/abs/0901.3775v1

Quantum Gravity at a Lifgarbagez Point
Authors: Petr Horava
(Submitted on 26 Jan 2009)

Abstract: We present a candidate quantum field theory of gravity with dynamical critical exponent equal to z=3 in the UV. (As in condensed matter systems, z measures the degree of anisotropy between space and time.) This theory, which at short distances describes interacting nonrelativistic gravitons, is power-counting renormalizable in 3+1 dimensions. When restricted to satisfy the condition of detailed balance, this theory is intimately related to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor. At long distances, this theory flows naturally to the relativistic value z=1, and could therefore serve as a possible candidate for a UV completion of Einstein's general relativity or an infrared modification thereof. The effective speed of light, the Newton constant and the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic z=3 theory at short distances.

http://arxiv.org/abs/0901.3879v1

Emergent Einstein Universe under Deconstruction
Authors: Nahomi Kan (Yamaguchi Junior College), Kiyoshi Shiraishi (Yamaguchi University)
(Submitted on 25 Jan 2009)

Abstract: We study self-consistent static solutions for an Einstein universe in a graph-based induced gravity. The one-loop quantum action is computed at finite temperature. In particular, we demonstrate specific results for the models based on cycle graphs.

 

[marcus]

http://arxiv.org/abs/0901.4009
Quantum Histories and Quantum Gravity
Joe Henson
35 pages, 2 figures
(Submitted on 26 Jan 2009)
"This paper reviews the histories approach to quantum mechanics. This discussion is then applied to theories of quantum gravity. It is argued that some of the quantum histories must approximate (in a suitable sense) to classical histories, if the correct classical regime is to be recovered. This observation has significance for the formulation of new theories (such as quantum gravity theories) as it puts a constraint on the kinematics, if the quantum/classical correspondence principle is to be preserved. Consequences for quantum gravity, particularly for Lorentz symmetry and the idea of 'emergent geometry', are discussed."

 

[marcus]

http://arxiv.org/abs/0902.0145
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Accepted by Phys. Rev. Lett., 7 pages, 2 figures
(Submitted on 2 Feb 2009)
"The primordial spectrum of cosmological tensor perturbations is considered as a possible probe of quantum gravity effects. Together with string theory, loop quantum gravity is one of the most promising frameworks to study quantum effects in the early universe. We show that the associated holonomy correction should modify the potential seen by gravitational waves during the inflationary amplification. The resulting power spectrum should exhibit a characteristic tilt. This opens a new window for cosmological tests of quantum gravity."

http://arxiv.org/abs/0902.0351
Quantum geometry from phase space reduction
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
31 pages, 1 figure
(Submitted on 2 Feb 2009)
"In this work we give an explicit isomorphism between the usual spin network basis and the direct quantization of the reduced phase space of tetrahedra. The main outcome is a formula that describes the space of SU(2) invariant states by an integral over coherent states satisfying the closure constraint exactly, or equivalently, as an integral over the space of classical tetrahedra. This provides an explicit realization of theorems by Guillemin--Sternberg and Hall that describe the commutation of quantization and reduction. In the final part of the paper, we use our result to express the FK spin foam model as an integral over classical tetrahedra and the asymptotics of the vertex amplitude is determined."

 

[marcus]

http://arxiv.org/pdf/0902.0590v1
Lorentz symmetry breaking as a quantum field theory regulator
Matt Visser (Victoria University of Wellington)
(Submitted on 3 Feb 2009)
"Perturbative expansions of relativistic quantum field theories typically contain ultraviolet divergences requiring regularization and renormalization. Many different regularization techniques have been developed over the years, but most regularizations require severe mutilation of the logical foundations of the theory. In contrast, breaking Lorentz invariance, while it is certainly a radical step, at least does not damage the logical foundations of the theory. We shall explore the features of a Lorentz symmetry breaking regulator in a simple polynomial scalar field theory, and discuss its implications. We shall quantify just "how much" Lorentz symmetry breaking is required to fully regulate the theory and render it finite. This scalar field theory provides a simple way of understanding many of the key features of Horava's recent article [arXiv:0901.3775 [hep-th]] on 3+1 dimensional quantum gravity."

(thx to CarlB)

http://arxiv.org/abs/0902.0697
Big Bounce and inhomogeneities
David Brizuela, Guillermo A. Mena Marugan, Tomasz Pawlowski
4 pages, 2 figures
(Submitted on 4 Feb 2009)
"The dynamics of an inhomogeneous universe is studied with the methods of Loop Quantum Cosmology as an example of the quantization of vacuum cosmological spacetimes containing gravitational waves (Gowdy spacetimes). The analysis performed at the effective level shows that: (i) The initial Big Bang singularity is replaced (as in the case of homogeneous cosmological models) by a Big Bounce, joining deterministically two large universes, (ii) the universe size at the bounce is at least of the same order of magnitude as that of the background homogeneous universe, (iii) for each gravitational wave mode, the difference in amplitude at very early and very late times has a vanishing statistical average when the bounce dynamics is strongly dominated by the inhomogeneities, whereas this average is positive when the dynamics is in a near-vacuum regime, so that statistically the inhomogeneities are amplified."

 

[marcus]

http://arxiv.org/abs/0902.0928
Chaplygin inflation in loop quantum cosmology
Xin Zhang, Jingfei Zhang, Jinglei Cui, Li Zhang
6 pages, accepted for publication in Mod. Phys. Lett. A
(Submitted on 5 Feb 2009)
"In this paper we discuss the inflationary universe in the context of a Chaplygin gas equation of state within the framework of the effective theory of loop quantum cosmology. Under the slow-roll approximation, we calculate the primordial perturbations for this model. We give the general expressions of the scalar spectral index, its running, and the tensor-to-scalar ratio, etc. For the chaotic inflation with a quadratic potential, using the WMAP 5-year results, we determine the parameters of the Chaplygin inflation model in loop quantum cosmology. The results are consistent with the WMAP observations."


http://arxiv.org/abs/0902.0957
The Barbero-Immirzi field in canonical formalism of pure gravity
Gianluca Calcagni, Simone Mercuri
9 pages
(Submitted on 5 Feb 2009)
"The Barbero-Immirzi (BI) parameter is promoted to a field and a canonical analysis is performed when it is coupled with a Nieh-Yan topological invariant. It is shown that, in the effective theory, the BI field is a canonical pseudo-scalar minimally coupled with gravity. This framework is argued to be more natural than the one of the usual Holst action. Potential consequences in relation with inflation and the quantum theory are briefly discussed."

 

[MTd2]

http://arxiv.org/abs/0902.1170
Asymptotic analysis of the EPRL four-simplex amplitude
Authors: John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Henrique Gomes, Frank Hellmann
(Submitted on 6 Feb 2009)

Abstract: An asymptotic formula for a certain 4d Euclidean spin foam 4-simplex amplitude is given for the limit of large spins. The analysis covers the model with Immirzi parameter less than one defined separately by Engle, Livine, Pereira and Rovelli (EPRL) and Freidel and Krasnov (FK). We are also able to analyse the EPRL model with Immirzi parameter greater than one. The asymptotic formula has one term which is proportional to the cosine of the Regge action for gravity, and it is shown that this term is present whenever the boundary data determines a non-degenerate Euclidean geometry for the 4-simplex. A scheme for resolving the phase ambiguity of the boundary data in these cases is also presented.

 

[MTd2]

http://arxiv.org/abs/0902.2001v1
Deformed General Relativity and Torsion
Authors: Gary W. Gibbons, Steffen Gielen
(Submitted on 11 Feb 2009)

Abstract: We argue that the natural framework for embedding the ideas of deformed, or doubly, special relativity (DSR) into a curved spacetime is a generalisation of Einstein-Cartan theory, considered by Stelle and West. Instead of interpreting the noncommuting "spacetime coordinates" of the Snyder algebra as endowing spacetime with a fundamentally noncommutative structure, we are led to consider a connection with torsion in this framework. This may lead to the usual ambiguities in minimal coupling. We note that observable violations of charge conservation induced by torsion should happen on a time scale of 10^3 s, which seems to rule out these modifications as a serious theory. Our considerations show, however, that the noncommutativity of translations in the Snyder algebra need not correspond to noncommutative spacetime in the usual sense.

LoL Gary Gibbons on DSL bandwagon!

 

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http://arxiv.org/abs/0902.1551
Solving the Problem of Time in Mini-superspace: Measurement of Dirac Observables
Donald Marolf
12 pages
(Submitted on 9 Feb 2009)
"One solution to the so-called problem of time is to construct certain Dirac observables, sometimes called evolving constants of motion. There has been some discussion in the literature about the interpretation of such observables, and in particular whether single Dirac observables can be measured. Here we clarify the situation by describing a class of interactions that can be said to implement measurements of such observables. Along the way, we describe a useful notion of perturbation theory for the rigging map eta of group averaging (sometimes loosely called the physical state 'projector'), which maps states from the auxiliary Hilbert space to the physical Hilbert space."

http://arxiv.org/abs/0902.1913
Alternative quantization of the Hamiltonian in isotropic loop quantum cosmology
Jinsong Yang, You Ding, Yongge Ma
6 pages, 2 figures
(Submitted on 11 Feb 2009)
"Since there are quantization ambiguities in constructing the Hamiltonian constraint operator in isotropic loop quantum cosmology, it is crucial to check whether the key features of loop quantum cosmology, such as the quantum bounce and effective scenario, are robust against the ambiguities. In this paper, we consider a typical quantization ambiguity arising from the quantization of the field strength of the gravitational connection. An alternative Hamiltonian constraint operator is constructed, which is shown to have the correct classical limit by the semiclassical analysis. The effective Hamiltonian incorporating higher order quantum corrections is also obtained. In the spatially flat FRW model with a massless scalar field, the classical big bang is again replaced by a quantum bounce. Moreover, there are still great possibilities for the expanding universe to recollapse due to the quantum gravity effect. Thus, these key features are robust against this quantization ambiguity."

http://arxiv.org/abs/0902.1313
What the small angle CMB really tells us about the curvature of the Universe
Timothy Clifton, Pedro G. Ferreira, Joe Zuntz
4 pages, 4 figures
(Submitted on 8 Feb 2009)
"It is well known that observations of the cosmic microwave background (CMB) are highly sensitive to the spatial curvature of the Universe, k. Here we find that what is in fact being tightly constrained by small angle fluctuations is spatial curvature near the surface of last scattering, and that if we allow k to be a function of position, rather than taking a constant value everywhere, then considerable spatial curvature is permissible within our own locale. This result is of interest for the giant void models that attempt to explain the supernovae observations without Dark Energy. We find such voids to be compatible with the observed small angle CMB, but they must be either very deep (and unnaturally empty) or exist in a positively curved Universe."

 

[MTd2]

http://arxiv.org/abs/0902.2301v1

A note on discrete Holonomy through directed edges, with no lengths
Authors: Stuart Armstrong, Jussi Westergren
(Submitted on 13 Feb 2009)

Abstract: This note demonstrates how both the concept of distance and the concept of holonomy can be constructed from a suitable network with directed edges (and no lengths). The number of different edge types depends on the signature of the metric and the dimension of the holonomy group. If the holonomy group is of dimension one and the metric is positive-definite, a single type of directed edges is needed.

 

[MTd2]

http://arxiv.org/abs/0902.2247The Science Case for STEP
Authors: James Overduin, Francis Everitt, John Mester, Paul Worden

STEP (the Satellite Test of the Equivalence Principle) will advance experimental limits on violations of Einstein's equivalence principle (EP) from their present sensitivity of 2 parts in 10^13 to 1 part in 10^18 through multiple comparison of the motions of four pairs of test masses of different compositions in an earth-orbiting drag-free satellite. Dimensional arguments suggest that violations, if they exist, should be found in this range, and they are also suggested by leading attempts at unified theories of fundamental interactions (e.g. string theory) and cosmological theories involving dynamical dark energy. Discovery of a violation would constitute the discovery of a new force of nature and provide a critical signpost toward unification. A null result would be just as profound, because it would close off any possibility of a natural-strength coupling between standard-model fields and the new light degrees of freedom that such theories generically predict (e.g., dilatons, moduli, quintessence). STEP should thus be seen as the intermediate-scale component of an integrated strategy for fundamental physics experiments that already includes particle acclerators (at the smallest scales) and supernova probes (at the largest). The former may find indirect evidence for new fields via their missing-energy signatures, and the latter may produce direct evidence through changes in cosmological equation of state--but only a gravitational experiment like STEP can go further and reveal how or whether such a field couples to the rest of the standard model. It is at once complementary to the other two kinds of tests, and a uniquely powerful probe of fundamental physics in its own right.

 

[MTd2]

http://arxiv.org/abs/0902.2764v1

Peccei--Quinn mechanism in gravity and the nature of the Barbero--Immirzi parameter
Authors: Simone Mercuri
(Submitted on 16 Feb 2009)

Abstract: A general argument provides the motivation to consider the Barbero--Immirzi parameter as a field. The specific form of the geometrical effective action allows to relate the value of the Barbero--Immirzi parameter to other quantum ambiguities through the analog of the Peccei--Quinn mechanism.

 

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http://pirsa.org/09020023/
Graviton propagator from EPRL spinfoam model
Claudio Perini - CPT
"We derive geometric correlation functions in the new spinfoam model with coherent states techniques, making connection with quantum Regge calculus and perturbative quantum gravity. In particular we recover the expected scaling with distance for all components of the propagator. We expect the same technique to be well-suited for other spinfoam models."
Date: 11/02/2009 - 4:00 pm

http://arxiv.org/abs/0902.2490
Tensor power spectrum with holonomy corrections in LQC
Jakub Mielczarek
13 pages, 13 figures
(Submitted on 14 Feb 2009)
"In this paper we consider tensor perturbations produced at a bounce phase in presence of the holonomy corrections. Here bounce phase and holonomy corrections originate from Loop Quantum Cosmology. We re-derive formulas for the of the corrections for the model with a scalar field content. Background dynamics with a free scalar field and multi-fluid potential are considered. Both analytical approximations as well as numerical investigations were performed. We have found analytical solutions on super-horizontal and sub-horizontal regimes and derived corresponding power spectra. Also occupation number $$n_{\bf k}$$ and parameter $$\Omega_{\text{gw}}$$ were derived in sub-horizontal limit, leading to its extremely low present value. Final results are numerical power spectra of the gravitational waves produced in the presence of quantum holonomy corrections. In the super-horizontal limit the obtained spectrum behaves like $$\mathcal{P}_T \propto k^3(C_1+\log^2(k))$$ while on sub-horizontal scales it exhibits oscillations around $$\mathcal{P}_T \propto k^2$$. These results can be directly applied as initial conditions for the inflationary modes. We mention possible resulting observational features of the CMB in particular BB spectrum of polarization."

http://arxiv.org/abs/0902.2764
Peccei--Quinn mechanism in gravity and the nature of the Barbero--Immirzi parameter
Simone Mercuri
5 pages
(Submitted on 16 Feb 2009)
"A general argument provides the motivation to consider the Barbero--Immirzi parameter as a field. The specific form of the geometrical effective action allows to relate the value of the Barbero--Immirzi parameter to other quantum ambiguities through the analog of the Peccei--Quinn mechanism."

 

[MTd2]

http://arxiv.org/abs/0902.3447
Connection between Fermionic Strings and Quantum Gravity States -- A loop space approach
Authors: Luiz C.L. Botelho
(Submitted on 19 Feb 2009)

Abstract: We present physical arguments based on loop space representations for Dirac/Klein gordon determinants that some suitable Fermionic String Ising models at the critical point and defined on the space-time base manifold are formal quantum states of the gravitational field when quantized in the Ashtekar-Sen connection canonical formalism .These results complements our previous Loop Space studies on the subject (Random surface representation for Einstein quantum gravity, Phys.Rev.D52,6941 (1995))

Hmm, I knew that Ashok Sen proposed at least 5 years ago that LQG was on of the limits of M Theory. But many people didnt give credit to that.

 

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http://arxiv.org/abs/0902.3416
4d Lorentzian Holst action with topological terms
Danilo Jimenez Rezende, Alejandro Perez
(Submitted on 19 Feb 2009)
"We study the Hamiltonian formulation of the general first order action of general relativity compatible with local Lorentz invariance and background independence. The most general simplectic structure (compatible with diffeomorphism invariance and local Lorentz transformations) is obtained by adding to the Holst action the Pontryagin, Euler and Nieh-Yan invariants with independent coupling constants. We perform a detailed canonical analysis of this general formulation (in the time gauge) exploring the structure of the phase space in terms of connection variables. We explain the relationship of these topological terms, and the effect of large SU(2) gauge transformations in quantum theories of gravity defined in terms of the Ashtekar-Barbero connection."

 

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http://arxiv.org/abs/0902.3903
Group field theory and simplicial quantum gravity
Daniele Oriti
11 pages
(Submitted on 23 Feb 2009)
"We present a new Group Field Theory for 4d quantum gravity. It incorporates the constraints that give gravity from BF theory, and has quantum amplitudes with the explicit form of simplicial path integrals for 1st order gravity. The geometric interpretation of the variables and of the contributions to the quantum amplitudes is manifest. This allows a direct link with other simplicial gravity approaches, like quantum Regge calculus, in the form of the amplitudes of the model, and dynamical triangulations, which we show to correspond to a simple restriction of the same."

http://arxiv.org/abs/0902.3657
Spectral Dimension of Spacetime in Quantum Gravity at a Lifgarbagez Point
Petr Horava
11 pages, 1 figure
(Submitted on 23 Feb 2009)
"We extend the definition of "spectral dimension" (usually defined for fractal and lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that in quantum gravity dominated by a Lifgarbagez point with dynamical critical exponent z in D+1 spacetime dimensions, the spectral dimension of spacetime is equal to d_s=1+D/z. In the case of gravity in 3+1 dimensions presented in arXiv:0901.3775, which is dominated by z=3 in the UV and flows to z=1 in the IR, the spectral dimension of spacetime flows from d_s=4 at large scales, to d_s=2 at short distances. Remarkably, this is the qualitative behavior of d_s found numerically by Ambjorn, Jurkiewicz and Loll in their causal dynamical triangulations approach to quantum gravity."

Brief mention:
http://arxiv.org/abs/0902.3817
Noncommutative D=4 gravity coupled to fermions
Paolo Aschieri, Leonardo Castellani
19 pages
(Submitted on 23 Feb 2009)

http://arxiv.org/abs/0902.3823
Noncommutative supergravity in D=3 and D=4
Paolo Aschieri, Leonardo Castellani
22 pages
(Submitted on 23 Feb 2009)

 

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http://arxiv.org/abs/0902.4191
Holonomy Loops, Spectral Triples & Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
24 pages, 7 figures, based on talk given by J.M.G. at the QG2 conference, Nottingham, juli 2008; at the QSTNG conference in Rome in sept/oct 2008; at the AONCG conference, Canberra, dec. 2008
(Submitted on 24 Feb 2009)
"We review the motivation, construction and physical interpretation of a semi-finite spectral triple obtained through a rearrangement of central elements of loop quantum gravity. The triple is based on a countable set of oriented graphs and the algebra consists of generalized holonomy loops in this set. The Dirac type operator resembles a global functional derivation operator and the interaction between the algebra of holonomy loops and the Dirac type operator reproduces the structure of a quantized Poisson bracket of general relativity. Finally we give a heuristic argument as to how a natural candidate for a quantized Hamiltonian might emerge from this spectral triple construction."

 

[MTd2]

http://arxiv.org/abs/0902.4630

Taming perturbative divergences in asymptotically safe gravity
Authors: Dario Benedetti, Pedro F. Machado, Frank Saueressig

Abstract: We use functional renormalization group methods to study gravity minimally coupled to a free scalar field. This setup provides the prototype of a gravitational theory which is perturbatively non-renormalizable at one-loop level, but may possesses a non-trivial renormalization group fixed point controlling its UV behavior. We show that such a fixed point indeed exists within the truncations considered, lending strong support to the conjectured asymptotic safety of the theory. In particular, we demonstrate that the counterterms responsible for its perturbative non-renormalizability have no qualitative effect on this feature.

 

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http://arxiv.org/abs/0902.4731
Alternatives to Cosmological Inflation
Robert H. Brandenberger (McGill and IHEP)
14 pages, 5 figures, to be published in the proceedings of CosPA08
(Submitted on 27 Feb 2009)
"The inflationary paradigm, although very successful phenomenologically, suffers from several conceptual problems which motivate the search for alternative scenarios of early universe cosmology. Here, two possible alternatives will be reviewed. - "string gas cosmology" and the "matter bounce". Their successes and problems will be pointed out."

 

[MTd2]

http://arxiv.org/abs/0903.0267

From lattice BF gauge theory to area-angle Regge calculus
Authors: Valentin Bonzom

Abstract: We consider Riemannian 4d BF lattice gauge theory, on a triangulation of spacetime. Introducing the simplicity constraints which turn BF theory into simplicial gravity, some geometric quantities of Regge calculus, areas, and 3d and 4d dihedral angles, are identified. The parallel transport conditions are taken care of to ensure a consistent gluing of simplices. We show that these gluing relations, together with the simplicity constraints, contain the constraints of area-angle Regge calculus in a simple way, via the group structure of the underlying BF gauge theory. This provides a precise road from constrained BF theory to area-angle Regge calculus. Doing so, a framework combining variables of lattice BF theory and Regge calculus is built. The action takes a form {\it \`a la Regge} and includes the contribution of the Immirzi parameter. In the absence of simplicity constraints, the standard spin foam model for BF theory is recovered. Insertions of local observables are investigated, leading to Casimir insertions for areas and 6j-symbols for 3d angles. The present formulation is argued to be suitable for deriving spin foam models from discrete path integrals.

 

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http://arxiv.org/abs/0903.0986
Cosmological solutions of emergent noncommutative gravity
Daniela Klammer, Harold Steinacker
4 pages, 2 figures
(Submitted on 5 Mar 2009)
"We find 2-parameter families of (approximate) cosmological solutions of Friedmann-Robertson-Walker type in matrix models of Yang-Mills type. Gravity arises through an intrinsically noncommutative mechanism on these branes, distinct from general relativity. No fine-tuning of a cosmological constant is required, in fact the solutions are stabilized through vacuum fluctuations. They generically have a big bounce, and an early inflation-like phase with graceful exit. The mechanism is purely geometrical, no ad-hoc scalar fields are introduced. The case k=-1 leads to a Milne-like universe after inflation, which appears to be in remarkably good agreement with observation."

http://arxiv.org/abs/0903.1015
Matrix Models, Emergent Gravity, and Gauge Theory
Harold Steinacker
8 pages. Based on invited talks given at the Conferences "Quantum Spacetime and Noncommutative Geometry", Rome, 2008 and at "Workshop on quantum gravity and nocommutative geometry", Lisbon, 2008 and at "Emergent Gravity", Boston, 2008 and at DICE2008, Italy, 2008 and at "QG2 2008 Quantum Geometry and Quantum Gravity", Nottingham, 2008
(Submitted on 5 Mar 2009)
"Matrix models of Yang-Mills type induce an effective gravity theory on 4-dimensional branes, which are considered as models for dynamical space-time. We review recent progress in the understanding of this emergent gravity. The metric is not fundamental but arises effectively in the semi-classical limit, along with nonabelian gauge fields. This leads to a mechanism for protecting certain geometries from corrections due to the vacuum energy."

http://arxiv.org/abs/0903.0902
Observing the Evolution of the Universe
James Aguirre, Alexandre Amblard, Amjad Ashoorioon, Carlo Baccigalupi, Amedeo Balbi, James Bartlett, Nicola Bartolo, Dominic Benford, Mark Birkinshaw, Jamie Bock, Dick Bond, Julian Borrill, Franois Bouchet, Michael Bridges, Emory Bunn, Erminia Calabrese, Christopher Cantalupo, Ana Caramete, Carmelita Carbone, Suchetana Chatterjee, Sarah Church, David Chuss, Carlo Contaldi, Asantha Cooray, Sudeep Das, Francesco De Bernardis, Paolo De Bernardis, Gianfranco De Zotti, Jacques Delabrouille, F.-Xavier Dsert, Mark Devlin, Clive Dickinson, Simon Dicker, Matt Dobbs, Scott Dodelson, Olivier Dore, Jessie Dotson, Joanna Dunkley, Maria Cristina Falvella, Dale Fixsen, Pablo Fosalba, Joseph Fowler, Evalyn Gates, Walter Gear, Sunil Golwala, Krzysztof Gorski, Alessandro Gruppuso, Josh Gundersen, Mark Halpern, et al. (126 additional authors not shown)
Science White Paper submitted to the US Astro2010 Decadal Survey. Full list of 177 author available at this http URL
(Submitted on 5 Mar 2009)
"How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass.
We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages."

Brief mention:
http://arxiv.org/abs/0903.0878
Emergent Quantum Gravity: Macro/Micro Structures of Spacetime
B. L. Hu
15 pages. Essay for invited talks at the Symposium on "Foundations of Physics", University of Maryland, April 24-27, 2008, Workshop on "Emergent Gravity", MIT, August 25-29, 2008. In Proceedings of DICE 08 Conference, J. Phys. Conf. Ser. (2009)
(Submitted on 4 Mar 2009)

 

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http://arxiv.org/abs/0903.1471
Dynamics and entanglement in spherically symmetric quantum gravity
Viqar Husain, Daniel R. Terno
12 pages, 1 figure
(Submitted on 9 Mar 2009)
"The gravity-scalar field system in spherical symmetry provides a natural setting for exploring gravitational collapse and its aftermath in quantum gravity. In a canonical approach, we give constructions of the Hamiltonian operator, and of semiclassical states peaked on constraint free data. Such states provide explicit examples of physical states. We also show that matter-gravity entanglement is an inherent feature of physical states, whether or not there is a black hole."

http://arxiv.org/abs/0903.1520
Natural inflation mechanism in asymptotic noncommutative geometry
William Nelson, Mairi Sakellariadou
3 pages
(Submitted on 9 Mar 2009)
"The possibility of having an inflationary epoch within a noncommutative geometry approach to unifying gravity and the standard model is demonstrated. This inflationary phase occurs without the need to introduce 'ad hoc' additional fields or potentials, rather it is a consequence of a nonminimal coupling between the geometry and the Higgs field."

http://arxiv.org/abs/0903.1105
The Event Horizon of Sagittarius A*
Avery E. Broderick (1), Abraham Loeb (2), Ramesh Narayan (2) ((1) CITA,(2) Harvard CfA)
11 pages, 6 figures, submitted to ApJ
(Submitted on 5 Mar 2009)
"Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*'s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories."

http://arxiv.org/abs/0903.1376
Particle Topology, Braids, and Braided Belts
Sundance Bilson-Thompson, Jonathan Hackett, Louis H. Kauffman
21 pages, 16 figures
(Submitted on 7 Mar 2009)
"Recent work suggests that topological features of certain quantum gravity theories can be interpreted as particles, matching the known fermions and bosons of the first generation in the Standard Model. This is achieved by identifying topological structures with elements of the framed Artin braid group on three strands, and demonstrating a correspondence between the invariants used to characterise these braids (a braid is a set of non-intersecting curves, that connect one set of N points with another set of N points), and quantities like electric charge, colour charge, and so on. In this paper we show how to manipulate a modified form of framed braids to yield an invariant standard form for sets of isomorphic braids, characterised by a vector of real numbers. This will serve as a basis for more complete discussions of quantum numbers in future work."

 

[MTd2]

http://arxiv.org/abs/0903.1667v1

Computing Black Hole entropy in Loop Quantum Gravity from a Conformal Field Theory perspective

Authors: Ivan Agullo, Enrique F. Borja, Jacobo Diaz-Polo
(Submitted on 10 Mar 2009)

Abstract: Motivated by the analogy proposed by Witten between Chern-Simons and Conformal Field Theories, we explore an alternative way of computing the entropy of a black hole starting from the isolated horizon framework in Loop Quantum Gravity. The consistency of the result opens a window for the interplay between Conformal Field Theory and the description of black holes in Loop Quantum Gravity.

 

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http://arxiv.org/abs/0903.1859
Free will, undecidability, and the problem of time in quantum gravity
Rodolfo Gambini, Jorge Pullin
10 pages. This essay received the Second Community Prize in the Foundational Questions Institute (fqxi.org) essay contest
(Submitted on 10 Mar 2009)
"In quantum gravity there is no notion of absolute time. Like all other quantities in the theory, the notion of time has to be introduced 'relationally', by studying the behavior of some physical quantities in terms of others chosen as a 'clock'. We have recently introduced a consistent way of defining time relationally in general relativity. When quantum mechanics is formulated in terms of this new notion of time the resolution of the measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same experimental results of ordinary quantum mechanics, but every time an event is produced or a measurement happens two alternatives are possible: a) the state collapses; b) the system evolves without changing the state. One therefore has two possible behaviors of the quantum mechanical system and physical observations cannot decide between them, not just as a matter of experimental limitations but as an issue of principle. This first-ever example of fundamental undecidability in physics suggests that nature may behave sometimes as described by one alternative and sometimes as described by another. This in particular may give new vistas on the issue of free will."

http://arxiv.org/abs/0903.2065
The Trans-Planckian Problem for Inflationary Cosmology Revisited
Robert Brandenberger, Xinmin Zhang
8 pages, 2 figures
(Submitted on 12 Mar 2009)
"We consider an inflationary universe model in which the phase of accelerated expansion was preceded by a non-singular bounce and a period of contraction which involves a phase of deceleration. We follow fluctuations which exit the Hubble radius in the radiation-dominated contracting phase as quantum vacuum fluctuations, re-enter the Hubble radius in the deflationary period and re-cross during the phase of inflationary expansion. Evolving the fluctuations using the general relativistic linear perturbation equations, we find that they exit the Hubble radius during inflation not with a scale-invariant spectrum, but with a highly red spectrum with index n_s = -3. We also show that the back-reaction of fluctuations limits the time interval of deflation. Our toy model demonstrates the importance for inflationary cosmology both of the trans-Planckian problem for cosmological perturbations and of back-reaction effects . Firstly, without understanding both Planck-scale physics and the phase which preceded inflation, it is a non-trivial assumption to take the perturbations to be in their local vacuum state when they exit the Hubble radius at late times. Secondly, the back-reaction effects of fluctuations can influence the background in an important way."

 

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http://arxiv.org/abs/0903.2270
A possible topological interpretation of the Barbero--Immirzi parameter
Simone Mercuri
6 pages, based on a talk given at the III Stueckelberg workshop on Relativistic Field Theories, July 8-18, 2008, Pescara (Italy)
(Submitted on 12 Mar 2009)
A possible topological interpretation of the Barbero--Immirzi parameter is proposed. Classically, by generalizing the Holst action to torsional spacetimes, we construct a precise analogy between the Barbero--Immirzi parameter and the $\theta$-angle of Yang--Mills gauge theories, where the role of the Pontryagin class is played by a well known topological term containing the Holst modification, the Nieh--Yan density. Quantum mechanically, the same analogy requires the study of the large gauge sector of the theory. In particular it is argued that a specific component of the large sector of the gauge group can be correlated with the Nieh--Yan density, where the Barbero--Immirzi parameter plays the role of the free angular parameter of the large gauge transformations.

http://arxiv.org/abs/0903.2438
Quantum cosmic censor: gravitation makes reality undecidable
Rodolfo Gambini, Jorge Pullin
3 Pages, This essay received an honorable mention from the Gravity Research Foundation essay competition in 2008
(Submitted on 13 Mar 2009)
"When one takes into account gravitation, the measurement of space and time cannot be carried out with infinite accuracy. When quantum mechanics is reformulated taking into account this lack of accuracy, the resolution of the measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same physical predictions of quantum mechanics with a reduction postulate, but is radically different, with the quantum states evolving unitarily in terms of the underlying variables. Gravitation therefore makes this worrisome situation, potentially leading to two completely different views of reality, irrelevant from an empirical point of view. It may however be highly relevant from a philosophical point of view."

Brief mention:
http://arxiv.org/abs/0801.4415
Huge entropy production inside black holes
Colin S. Wallace, Andrew J. S. Hamilton, Gavin Polhemus (JILA)
Paper has been split into two. This paper now confines itself to presenting the general relativistic model. A companion paper, arXiv:0903.2290, discusses the quantum gravity implications.
(Submitted on 29 Jan 2008)
"... may exceed the Bekenstein-Hawking entropy by a large factor. If the black hole subsequently evaporates, radiating only the Bekenstein-Hawking entropy, then the black hole appears to destroy entropy, violating the second law of thermodynamics. A companion paper discusses the implications of this startling result..."
http://arxiv.org/abs/0903.2290
Entropy creation inside black holes points to observer complementarity
Gavin Polhemus, Andrew J. S. Hamilton, Colin S. Wallace
(Submitted on 12 Mar 2009)
Conclusions here seem bizarre, it may be interesting to see if the authors can get the paper published.