Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1010.2426
Lessons for Loop Quantum Gravity from Parametrised Field Theory
Thomas Thiemann
37 pages
(Submitted on 12 Oct 2010)
"In a series of seminal papers, Laddha and Varadarajan have developed in depth the quantisation of Parametrised Field Theory (PFT) in the kind of discontinuous representations that are employed in Loop Quantum Gravity (LQG). In one spatial dimension (circle) PFT is very similar to the closed bosonic string and the constraint algebra is isomorphic to two mutually commuting Witt algebras. Its quantisation is therefore straightforward in LQG like representations which by design lead to non anomalous, unitary, albeit discontinuous representations of the spatial diffeomorphism group. In particular, the complete set of (distributional) solutions to the quantum constraints, a preferred and complete algebra of Dirac observables and the associated physical inner product has been constructed. On the other hand, the two copies of Witt algebras are classically isomorphic to the Dirac or hypersurface deformation algebra of General Relativity (although without structure functions). The question we address in this paper, also raised by Laddha and Varadarajan in their paper, is whether we can quantise the Dirac algebra in such a way that its space of distributional solutions coincides with the one just described. This potentially teaches us something about LQG where a classically equivalent formulation of the Dirac algebra in terms of spatial diffeomorphism Lie algebras is not at our disposal. We find that, in order to achieve this, the Hamiltonian constraint has to be quantised by methods that extend those previously considered. The amount of quantisation ambiguities is somewhat reduced but not eliminated. We also show that the algebra of Hamiltonian constraints closes in a precise sense, with soft anomalies, that is, anomalies that do not cause inconsistencies. We elaborate on the relevance of these findings for full LQG."

http://arxiv.org/abs/1010.2481
Einstein gravity as a 3D conformally invariant theory
Henrique Gomes, Sean Gryb, Tim Koslowski
26 pages
(Submitted on 12 Oct 2010)
"We give an alternative description of the physical content of general relativity that does not require a Lorentz invariant spacetime. Instead, we find that gravity admits a dual description in terms of a theory where local size is irrelevant. The dual theory is invariant under foliation preserving 3-diffeomorphisms and 3D conformal transformations that preserve the 3-volume (for the spatially compact case). Locally, this symmetry is identical to that of Horava-Lifgarbagez gravity in the high energy limit but our theory is equivalent to Einstein gravity. Specifically, we find that the solutions of general relativity, in a gauge where the spatial hypersurfaces have constant mean extrinsic curvature, can be mapped to solutions of a particular gauge fixing of the dual theory. Moreover, this duality is not accidental. We provide a general geometric picture for our procedure that allows us to trade foliation invariance for conformal invariance. The dual theory provides a new proposal for the theory space of quantum gravity."αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔~≈≠≡ ≤≥½∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)

 

[MTd2]

http://arxiv.org/abs/1010.2207

The thermodynamic structure of Einstein tensor
Authors: Dawood Kothawala
(Submitted on 11 Oct 2010)

Abstract: We analyze the generic structure of Einstein tensor projected onto a 2-D spacelike surface S defined by unit timelike and spacelike vectors u_i and n_i respectively, which describe an accelerated observer (see text). Assuming that flow along u_i defines an approximate Killing vector X_i, we then show that near the corresponding Rindler horizon, the flux j_a=G_ab X^b along the ingoing null geodesics k_i normalised to have unit Killing energy, given by j . k, has a natural thermodynamic interpretation. Moreover, change in cross-sectional area of the k_i congruence yields the required change in area of S under virtual displacements \emph{normal} to it. The main aim of this note is to clearly demonstrate how, and why, the content of Einstein equations under such horizon deformations, originally pointed out by Padmanabhan, is essentially different from the result of Jacobson, who employed the so called Clausius relation in an attempt to derive Einstein equations from such a Clausius relation. More specifically, we show how a \emph{very specific geometric term} [reminiscent of Hawking's quasi-local expression for energy of spheres] corresponding to change in \emph{gravitational energy} arises inevitably in the first law: dE_G/d{\lambda} \alpha \int_{H} dA R_(2) (see text) -- the contribution of this purely geometric term would be missed in attempts to obtain area (and hence entropy) change by integrating the Raychaudhuri equation.

 

[marcus]

http://arxiv.org/abs/1010.2650
Newton's constant from a minimal length: Additional models
Hanno Sahlmann
7 pages
(Submitted on 13 Oct 2010)
"We follow arguments of Verlinde and Klinkhamer, and construct two models of the microscopic theory of an entropic screen that allow for the thermodynamical derivation of Newton's law, with Newton's constant expressed in terms of a minimal length scale contained in the area spectrum of the microscopic theory. One of the models is loosely related to the quantum structure of surfaces and isolated horizons in loop quantum gravity. Our investigation shows that the conclusions reached by Klinkhamer regarding the new length scale seem to be generic in all their qualitative aspects."

 

[atyy]

http://arxiv.org/abs/1010.2535
Can Hamiltonians be boundary observables in Parametrized Field Theories?
Tomas Andrade, Donald Marolf, Cederic Deffayet
(Submitted on 12 Oct 2010)
It has been argued that holography in gravitational theories is related to the existence of a particularly useful Gauss Law that allows energy to be measured at the boundary. The present work investigates the extent to which such Gauss Laws follow from diffeomorphism invariance. We study parametrized field theories, which are a class of diffeomorphism-invariant theories without gravity. We find that the Hamiltonian for parametrized field theories vanishes on shell even in the presence of a boundary and under a variety of boundary conditions. We conclude that parametrized theories have no useful Gauss Law, consistent with the absence of holography in these theories.

 

[marcus]

http://arxiv.org/abs/1010.2784
Surprising Connections Between General Relativity and Condensed Matter
Gary T. Horowitz
14 pages; based on talk given at GR19
(Submitted on 13 Oct 2010)
"This brief review is intended to introduce gravitational physicists to recent developments in which general relativity is being used to describe certain aspects of condensed matter systems, e.g., superconductivity."

http://arxiv.org/abs/1010.2799
Exact renormalization group with optimal scale and its application to cosmology
Benjamin Koch, Israel Ramirez
8 pages, 3 figures
(Submitted on 14 Oct 2010)
"Assuming an effective gravitational action with scale dependent coupling constants, a consistency condition for the local form of the cut-off scale is derived. The approach is applied to homogeneous cosmology and running couplings with an ultraviolet fixed point. Within the given approach this allows to derive bounds on the value of the fixed point."

http://arxiv.org/abs/1010.2930
Bianchi I model in terms of nonstandard loop quantum cosmology: Quantum dynamics
Przemyslaw Malkiewicz, Wlodzimierz Piechocki, Piotr Dzierzak
16 pages
(Submitted on 14 Oct 2010)
"We analyze the quantum Bianchi I model in the setting of the nonstandard loop quantum cosmology. Elementary observables are used to quantize the volume operator. The spectrum of the volume operator is bounded from below and discrete. The discreteness may imply a foamy structure of spacetime at semiclassical level. The results are described in terms of a free parameter specifying loop geometry to be determined in astro-cosmo observations. An evolution of the quantum model is described in terms the so-called true Hamiltonian. It is defined on the reduced phase space, and it is free from constraints defining the standard expression for the Hamiltonian. It enables an introduction of a time parameter valued in the set of all real numbers."

Brief mention:
http://arxiv.org/abs/1010.2979
Octonions
Jonathan Hackett, Louis Kauffman
11 pages, 11 figures

 

[atyy]

http://arxiv.org/abs/1008.3392
How to Falsify the GR+LambdaCDM Model with Galaxy Redshift SurveysViviana Acquaviva, Eric Gawiser
(Submitted on 19 Aug 2010 (v1), last revised 13 Oct 2010 (this version, v2))
A wide range of models describing modifications to General Relativity have been proposed, but no fundamental parameter set exists to describe them. Similarly, no fundamental theory exists for dark energy to parameterize its potential deviation from a cosmological constant. This motivates a model-independent search for deviations from the concordance GR+LambdaCDM cosmological model in large galaxy redshift surveys. We describe two model-independent tests of the growth of cosmological structure, in the form of quantities that must equal one if GR+LambdaCDM is correct. The first, epsilon, was introduced previously as a scale-independent consistency check between the expansion history and structure growth. The second, upsilon, is introduced here as a test of scale-dependence in the linear evolution of matter density perturbations. We show that the ongoing and near-future galaxy redshift surveys WiggleZ, BOSS, and HETDEX will constrain these quantities at the 5-10% level, representing a stringent test of concordance cosmology at different redshifts. When redshift space distortions are used to probe the growth of cosmological structure, galaxies at higher redshift with lower bias are found to be most powerful in detecting deviations from the GR+LambdaCDM model.

 

[atyy]

http://arxiv.org/abs/1010.3218
Horava-Lifgarbagez gravity: a status report
Thomas P. Sotiriou
(Submitted on 15 Oct 2010)
This is intended to be a brief introduction and overview of Horava-Lifgarbagez gravity. The motivation and all of the various version of the theory (to date) are presented. The dynamics of the theory are discussed in some detail, with a focus on low energy viability and consistency, as these have been the issues that attracted most of the attention in the literature so far. Other properties of the theory and developments within its framework are also covered, such as: its relation to Einstein-aether theory, cosmology, and future perspectives.

http://arxiv.org/abs/1010.3052
Quantum Singularities in Static Spacetimes
J.P.M. Pitelli, P.S. Letelier
(Submitted on 15 Oct 2010)
We review the mathematical framework necessary to understand the physical content of quantum singularities in static spacetimes. We present many examples of classical singular spacetimes and study their singularities by using wave packets satisfying Klein-Gordon and Dirac equations. We show that in many cases the classical singularities are excluded when tested by quantum particles but unfortunately there are other cases where the singularities remain from the quantum mechanical point of view. When it is possible we also find, for spacetimes where quantum mechanics does not exclude the singularities, the boundary conditions necessary to turn the spatial portion of the wave operator into self-adjoint and emphasize their importance to the interpretation of quantum singularities.

http://arxiv.org/abs/1010.3054
n-Dimensional FRW Quantum Cosmology
J.P.M. Pitelli, P.S. Letelier
(Submitted on 15 Oct 2010)
We introduce the formalism of quantum cosmology in a Friedmann-Robertson-Walker (FRW) universe of arbitrary dimension filled with a perfect fluid with $p=\alpha\rho$ equation of state. First we show that the Schutz formalism, developed in four dimensions, can be extended to a n-dimensional universe. We compute the quantum representant of the scale factor $a(t)$, in the Many-Worlds, as well as, in the de Broglie-Bohm interpretation of quantum mechanics. We show that the singularities, which are still present in the n-dimensional generalization of FRW universe, are excluded with the introduction of quantum theory. We quantize, via the de Broglie-Bohm interpretation of quantum mechanics, the components of the Riemann curvature tensor in a tetrad basis in a n-dimensional FRW universe filled with radiation ($p=\frac{1}{n-1}\rho$). We show that the quantized version of the Ricci scalar are perfectly regular for all time $t$. We also study the behavior of the energy density and pressure and show that the ratio $\left<p\right>_L/\left<\rho\right>_L$ tends to the classical value $1/(n-1)$ only for $n=4$, showing that $n=4$ is somewhat privileged among the other dimensions. Besides that, as $n\to\infty$, $\left<p\right>_L/\left<\rho\right>_L\to 1$.

 

[marcus]

http://arxiv.org/abs/1010.3420
Experimental Search for Quantum Gravity
Sabine Hossenfelder
This article is partly based on the talks at the workshop on Experimental Search for Quantum Gravity, Stockholm, July 12-16 2010
(Submitted on 17 Oct 2010)
"We offer a brief survey of existent and planned experimental tests for quantum gravity. First, we outline the questions we wish to address, and then introduce some of the phenomenological models that are currently used in quantum gravity, both with and without a lowered Planck scale. After that, we summarize experimental areas where these models can be tested or constrained and discuss the status of the field."

http://pirsa.org/C10020
Invitation to Causal Sets
This is a collection of introductory lectures on Causal Sets QG to be given by Rafael Sorkin and Fay Dowker at Perimeter Institute. The first lecture has been given (18 October) and is online video:
http://pirsa.org/10100038/
Subsequent lectures in the series will be listed at the C10020 link.

http://arxiv.org/abs/1010.3552
Chiral vacuum fluctuations in quantum gravity
Joao Magueijo, Dionigi M. T. Benincasa
(Submitted on 18 Oct 2010)
"We examine tensor perturbations around a deSitter background within the framework of Ashtekar's variables and cousins parameterized by the Immirzi parameter \gamma. At the classical level we recover standard cosmological perturbation theory, with illuminating insights. Quantization leads to real novelties. In the low energy limit we find a second quantized theory of gravitons which displays different vacuum fluctuations for right and left gravitons. Nonetheless right and left gravitons have the same (positive) energies, resolving a number of paradoxes suggested in the literature. The right-left asymmetry of the vacuum fluctuations depends on \gamma and the ordering of the Hamiltonian constraint, and it would leave a distinctive imprint in the polarization of the cosmic microwave background, thus opening quantum gravity to observational test."

 

[MTd2]

http://arxiv.org/abs/1010.3585

Renormalization group scale-setting in astrophysical systems

Silvije Domazet, Hrvoje Stefancic
(Submitted on 18 Oct 2010)
A more general scale-setting procedure for General Relativity with Renormalization Group corrections is proposed. Theoretical aspects of the scale-setting procedure and the interpretation of the renormalization group running scale are discussed. The procedure is elaborated for several highly symmetric systems with matter in the form of an ideal fluid and for two models of running of the Newton coupling and the cosmological term. For a static spherically symmetric system with the matter obeying the polytropic equation of state the running scale-setting is performed analytically. The obtained result for the running scale matches the Ansatz introduced in a recent paper by Rodrigues, Letelier and Shapiro which provides an excellent explanation of rotation curves for a number of galaxies. A systematic explanation of the galaxy rotation curves using the scale-setting procedure introduced in this paper is identified as an important future goal.

 

[atyy]

http://arxiv.org/abs/1010.3395
Rescuing the Born Rule for Quantum Cosmology
Joshua H. Cooperman
(Submitted on 17 Oct 2010)
Page has recently argued that the Born rule does not suffice for computing all probabilities in quantum cosmology. He further asserts that the Born rule's failure gives rise to the cosmological measure problem. Here I contend that Page's result stems from his use of an overly restrictive definition of the Born rule. In particular, I demonstrate that all of the probabilities he wishes to compute follow from the Born rule when generalized measurements are permitted. I also register two comments on Page's theoretical setting, relating respectively to Hilbert space dimensionality and permutation symmetry. These considerations lead me to conclude that the claimed insufficiency of the Born rule is by no means specific to the cosmological context.

 

[atyy]

http://arxiv.org/abs/1010.3700
Holography and Entanglement in Flat Spacetime
Wei Li, Tadashi Takayanagi
(Submitted on 18 Oct 2010)
We propose a holographic correspondence of the flat spacetime based on the behavior of the entanglement entropy and the correlation functions. The holographic dual theory turns out to be highly non-local. We argue that after most part of the space is traced out, the reduced density matrix gives the maximal entropy and the correlation functions become trivial. We present a toy model for this holographic dual using a non-local scalar field theory that reproduces the same property of the entanglement entropy. Our conjecture is consistent with the entropy of Schwarzschild black holes in asymptotically flat spacetimes.

 

[marcus]

http://arxiv.org/abs/1010.4354
Spacetime could be simultaneously continuous and discrete in the same way that information can
Achim Kempf
(Submitted on 21 Oct 2010)
"There are competing schools of thought about the question of whether spacetime is fundamentally either continuous or discrete. Here, we consider the possibility that spacetime could be simultaneously continuous and discrete, in the same mathematical way that information can be simultaneously continuous and discrete. The equivalence of continuous and discrete information, which is of key importance in information theory, is established by Shannon sampling theory: of any bandlimited signal it suffices to record discrete samples to be able to perfectly reconstruct it everywhere, if the samples are taken at a rate of at least twice the bandlimit. It is known that physical fields on generic curved spaces obey a sampling theorem if they possesses an ultraviolet cutoff. Most recently, methods of spectral geometry have been employed to show that also the very shape of a curved space (i.e., of a Riemannian manifold) can be discretely sampled and then reconstructed up to the cutoff scale. Here, we develop these results further, and we here also consider the generalization to curved spacetimes, i.e., to Lorentzian manifolds."
FWIW I really admire Kempf. I watched him give a talk at Perimeter on exactly this topic and it was great. Young, focused, on-his-toes, inspires confidence

http://arxiv.org/abs/1010.4518
Cosmological consequences of the noncommutative spectral geometry as an approach to unification
Mairi Sakellariadou
8 pages, Invited talk at the 14th Conference on recent Developments in gravity (NEB 14), Ioannina, Greece, 8-11 June 2010
(Submitted on 21 Oct 2010)
"Noncommutative spectral geometry succeeds in explaining the physics of the Standard Model of electroweak and strong interactions in all its details as determined by experimental data. Moreover, by construction the theory lives at very high energy scales, offering a natural framework to address early universe cosmological issues. After introducing the main elements of noncommutative spectral geometry, I will summarise some of its cosmological consequences and discuss constraints on the gravitational sector of the theory."

 

[MTd2]

http://arxiv.org/abs/1010.4729

Entropic Gravity, Phase-Space Noncommutativity and the Equivalence Principle

Catarina Bastos, Orfeu Bertolami, Nuno Costa Dias, João Nuno Prata
(Submitted on 22 Oct 2010)
We generalize E. Verlinde's entropic gravity reasoning to a phase-space noncommutativity set-up. This allow us to impose a bound on the product of the noncommutative parameters based on the Equivalence Principle. The key feature of our analysis is an effective Planck's constant that naturally arises when accounting for the noncommutative features of the phase-space.

http://arxiv.org/abs/1010.4787

Operator Spin Foam Models

Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 22 Oct 2010)
The goal of this paper is to introduce a systematic approach to spin foams. We define operator spin foams, that is foams labelled by group representations and operators, as the main tool. An equivalence relation we impose in the set of the operator spin foams allows to split the faces and the edges of the foams. The consistency with that relation requires introduction of the (familiar for the BF theory) face amplitude. The operator spin foam models are defined quite generally. Imposing a maximal symmetry leads to a family we call natural operator spin foam models. This symmetry, combined with demanding consistency with splitting the edges, determines a complete characterization of a general natural model. It can be obtained by applying arbitrary (quantum) constraints on an arbitrary BF spin foam model. In particular, imposing suitable constraints on Spin(4) BF spin foam model is exactly the way we tend to view 4d quantum gravity, starting with the BC model and continuing with the EPRL or FK models. That makes our framework directly applicable to those models. Specifically, our operator spin foam framework can be translated into the language of spin foams and partition functions. We discuss the examples: BF spin foam model, the BC model, and the model obtained by application of our framework to the EPRL intertwiners.

http://arxiv.org/abs/1010.4677

Canonical transformation for stiff matter models in quantum cosmology

C. Neves, G. A. Monerat, G. Oliveira-Neto, E. V. Corrêa Silva, L. G. Ferreira Filho
(Submitted on 22 Oct 2010)
In the present work we consider Friedmann-Robertson-Walker models in the presence of a stiff matter perfect fluid and a cosmological constant. We write the superhamiltonian of these models using the Schutz's variational formalism. We notice that the resulting superhamiltonians have terms that will lead to factor ordering ambiguities when they are written as operators. In order to remove these ambiguities, we introduce appropriate coordinate transformations and prove that these transformations are canonical using the symplectic method.

 

[marcus]

http://arxiv.org/abs/1010.4886
Semiclassical Limit of New Spin Foam Models
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 23 Oct 2010)
"We study the problem of semiclassical limit of Loop Quantum Gravity theory defined by the new spin foam models. This is done by analyzing the large-spin asymptotics of the Hartle-Hawking wavefunction. By using the stationary phase method we determine the wavefunction asymptotics, which then determines the large-distance asymptotics of the corresponding graviton propagator. We show that the graviton propagator behaves as the inverse distance to the fourth power. Our result is a direct consequence of the large-spin asymptotics of the spin foam model vertex amplitude, and it is valid for all new spin foam models, since they all have the same type of the vertex amplitude asymptotics. We also find the type of the vertex amplitude asymptotics which gives the correct graviton propagator."


http://arxiv.org/abs/1010.5149
Towards classical geometrodynamics from Group Field Theory hydrodynamics
Daniele Oriti, Lorenzo Sindoni
32 pages. Contribution submitted to the focus issue of the New Journal of Physics on "Classical and Quantum Analogues for Gravitational Phenomena and Related Effects", R. Schuetzhold, U. Leonhardt and C. Maia, Eds
(Submitted on 25 Oct 2010)
"We take the first steps towards identifying the hydrodynamics of group field theories (GFTs) and relating this hydrodynamic regime to classical geometrodynamics of continuum space. We apply to GFT mean field theory techniques borrowed from the theory of Bose condensates, alongside standard GFT and spin foam techniques. The mean field configuration we study is, in turn, obtained from loop quantum gravity coherent states. We work in the context of 2d and 3d GFT models, in euclidean signature, both ordinary and colored, as examples of a procedure that has a more general validity. We also extract the effective dynamics of the system around the mean field configurations, and discuss the role of GFT symmetries in going from microscopic to effective dynamics. In the process, we obtain additional insights on the GFT formalism itself."

http://arxiv.org/abs/1010.5227
Local spin foams
Elena Magliaro, Claudio Perini
9 pages, 8 figures
(Submitted on 25 Oct 2010)
"The central object of this paper is an holonomy formulation for spin foams. Within this new representation, we analyze three general requirements: locality, composition law, cylindrical consistency. In particular, cylindrical consistency is shown to fix the arbitrary normalization of the vertex amplitude."

 

[marcus]

http://arxiv.org/abs/1010.5384
All 3-edge-connected relativistic BC and EPRL spin-networks are integrable
Wojciech Kaminski
16 pages
(Submitted on 26 Oct 2010)
"We prove statement conjectured in [Baez and Barrett:2001] that every 3-edge-connected SL(2,C) spin-network with invariants of certain class is integrable. It means that the regularized evaluation (defined by a suitable integral) of such a spin-network is finite. Our proof is quite general. It is valid for relativistic spin-networks of Barrett and Crane as well as for spin-networks with the Engle-Pereira-Rovelli-Livine intertwiners and for some generalization of both. The result is interesting from the group representation point of view opens also a possibility of defining vertex amplitudes for Spin-Foam models based on non-simplicial decompositions."

http://arxiv.org/abs/1010.5437
Spinfoams: summing = refining
Carlo Rovelli, Matteo Smerlak
5 pages
(Submitted on 26 Oct 2010)
"In spinfoam quantum gravity, are physical transition amplitudes obtained by summing over foams, or by infinitely refining them? We outline the combinatorial structure of spinfoam models, define their continuum limit, and show that, under general conditions, refining the foams is the same as summing over them. These conditions bear on the cylindrical consistency of the spinfoam amplitudes and on the presence of appropriate combinatorial factors, related to the implementation of diffeomorphisms invariance in the spinfoam sum."

http://arxiv.org/abs/1010.5444
Commuting Simplicity and Closure Constraints for 4D Spin Foam Models
Muxin Han, Thomas Thiemann
41 pages, 4 figures
(Submitted on 26 Oct 2010)
"Spin Foam Models are supposed to be discretised path integrals for quantum gravity constructed from the Plebanski-Holst action. The reason for there being several models currently under consideration is that no consensus has been reached for how to implement the simplicity constraints. Indeed, none of these models strictly follows from the original path integral with commuting B fields, rather, by some non standard manipulations one always ends up with non commuting B fields and the simplicity constraints become in fact anomalous which is the source for there being several inequivalent strategies to circumvent the associated problems. In this article, we construct a new Euclidian Spin Foam Model which is constructed by standard methods from the Plebanski-Holst path integral with commuting B fields discretised on a 4D simplicial complex. The resulting model differs from the current ones in several aspects, one of them being that the closure constraint needs special care. Only when dropping the closure constraint by hand and only in the large spin limit can the vertex amplitudes of this model be related to those of the FK Model but even then the face and edge amplitude differ. Curiously, an ad hoc non-commutative deformation of the B^IJ variables leads from our new model to the Barrett-Crane Model in the case of Barbero-Immirzi parameter goes to infinity."

http://arxiv.org/abs/1010.5451
U(N) tools for Loop Quantum Gravity: The Return of the Spinor
Enrique F. Borja, Laurent Freidel, Iñaki Garay, Etera R. Livine
23 pages
(Submitted on 26 Oct 2010)
"We explore the classical setting for the U(N) framework for SU(2) intertwiners for loop quantum gravity (LQG) and describe the corresponding phase space in terms of spinors with appropriate constraints. We show how its quantization leads back to the standard Hilbert space of intertwiner states defined as holomorphic functionals. We then explain how to glue these intertwiners states in order to construct spin network states as wave-functions on the spinor phase space. In particular, we translate the usual loop gravity holonomy observables to our classical framework. Finally, we propose how to derive our phase space structure from an action principle which induces non-trivial dynamics for the spin network states. We conclude by applying explicitly our framework to states living on the simple 2-vertex graph and discuss the properties of the resulting Hamiltonian."

The next paper has no direct relevance to QG although two of the authors have played a significant role and remain in close touch with the community. I think it worth keeping track of their interests and current work--so make brief mention:
http://arxiv.org/abs/1010.5417
Axions without Peccei-Quinn Symmetry
Adam Latosinski, Krzysztof A. Meissner, Hermann Nicolai
(Submitted on 26 Oct 2010)
"We argue that the axion arising in the solution of the strong CP problem can be identified with the Majoron,...The axionic couplings are then fully computable in terms of known SM parameters and the Majorana mass scale, as we illustrate by computing the effective couplings to photons and quarks at two loops."

 

[MTd2]

This paper is about string theory, but it is an aspect that overlaps with what it should be expected from foamy models.

http://arxiv.org/abs/1010.5399

Stringy Space-Time Foam and High-Energy Cosmic Photons

Nick E. Mavromatos
(Submitted on 26 Oct 2010)
In this review, I discuss briefly stringent tests of Lorentz-violating quantum space-time foam models inspired from String/Brane theories, provided by studies of high energy Photons from intense celestial sources, such as Active Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict modifications to the radiation dispersion relations, which are quadratically suppressed by the string mass scale, and time delays in the arrival times of photons (assumed to be emitted more or less simultaneously from the source), which are proportional to the photon energy, so that the more energetic photons arrive later. Although the astrophysics at the source of these energetic photons is still not understood, and such non simultaneous arrivals, that have been observed recently, might well be due to non simultaneous emission as a result of conventional physics effects, nevertheless, rather surprisingly, the observed time delays can also fit excellently the stringy space-time foam scenarios, provided the space-time defect foam is inhomogeneous. The key features of the model, that allow it to evade a plethora of astrophysical constraints on Lorentz violation, in sharp contrast to other field-theoretic Lorentz-violating models of quantum gravity, are: (i) transparency of the foam to electrons and in general charged matter, (ii) absence of birefringence effects and (iii) a breakdown of the local effective lagrangian formalism.

 

[marcus]

http://arxiv.org/abs/1010.5513
The generalized second law forbids singularity resolution, viable baby universes, traversable wormholes, warp drives, time machines, and negative mass objects
Aron C. Wall
38 pages, 4 figures
(Submitted on 26 Oct 2010)
"The generalized second law can be used to prove a singularity theorem, by generalizing the notion of a trapped surface to quantum situations. Like Penrose's original singularity theorem, it implies that spacetime is null geodesically incomplete inside black holes, and to the past of spatially infinite Friedmann--Robertson--Walker cosmologies. If space is finite instead, the generalized second law requires that there only be a finite amount of entropy producing processes in the past, unless there is a reversal of the arrow of time. In asymptotically flat spacetime, the generalized second law also rules out traversable wormholes, negative masses, and other forms of faster-than-light travel, as well as closed timelike curves. Furthermore it is impossible to form baby universes which eventually become independent of the mother universe, or to restart inflation. Since the semiclassical approximation is used only in regions with low curvature, it is argued that the results may hold in full quantum gravity. An introductory section describes the second law and its time-reverse, in ordinary and generalized thermodynamics, using either the fine-grained or the coarse-grained entropy. A proof of the coarse-grained ordinary second law is given."


http://arxiv.org/abs/1010.5514
Quantum Fields on Causal Sets
Steven Johnston
PhD Thesis, Imperial College London, September 2010
(Submitted on 26 Oct 2010)
"Causal set theory provides a model of discrete spacetime in which spacetime events are represented by elements of a causal set---a locally finite, partially ordered set in which the partial order represents the causal relationships between events. The work presented here describes a model for matter on a causal set, specifically a theory of quantum scalar fields on a causal set spacetime background.
The work starts with a discrete path integral model for particles on a causal set. Here quantum mechanical amplitudes are assigned to trajectories within the causal set. By summing these over all trajectories between two spacetime events we obtain a causal set particle propagator. With a suitable choice of amplitudes this is shown to agree (in an appropriate sense) with the retarded propagator for the Klein-Gordon equation in Minkowski spacetime.
This causal set propagator is then used to define a causal set analogue of the Pauli-Jordan function that appears in continuum quantum field theories. A quantum scalar field is then modeled by an algebra of operators which satisfy three simple conditions (including a bosonic commutation rule). Defining time-ordering through a linear extension of the causal set these field operators are used to define a causal set Feynman propagator. Evidence is presented which shows agreement (in a suitable sense) between the causal set Feynman propagator and the continuum Feynman propagator for the Klein-Gordon equation in Minkowski spacetime. The Feynman propagator is obtained using the eigendecomposition of the Pauli-Jordan function, a method which can also be applied in continuum-based theories.
The free field theory is extended to include interacting scalar fields. This leads to a suggestion for a non-perturbative S-matrix on a causal set. Models for continuum-based phenomenology and spin-half particles on a causal set are also presented."

 

[marcus]

http://arxiv.org/abs/1010.5822
Gauge Gravity: a forward-looking introduction
Andrew Randono
62 pages, 8 figures
(Submitted on 27 Oct 2010)
"This article is a review of modern approaches to gravity that treat the gravitational interaction as a type of gauge theory. The purpose of the article is twofold. First, it is written in a colloquial style and is intended to be a pedagogical introduction to the gauge approach to gravity. I begin with a review of the Einstein-Cartan formulation of gravity, move on to the Macdowell-Mansouri approach, then show how gravity can be viewed as the symmetry broken phase of an (A)dS-gauge theory. This covers roughly the first half of the article. Armed with these tools, the remainder of the article is geared toward new insights and new lines of research that can be gained by viewing gravity from this perspective. Drawing from familiar concepts from the symmetry broken gauge theories of the standard model, we show how the topological structure of the gauge group allows for an infinite class of new solutions to the Einstein-Cartan field equations that can be thought of as degenerate ground states of the theory. We argue that quantum mechanical tunneling allows for transitions between the degenerate vacua. Generalizing the tunneling process from a topological phase of the gauge theory to an arbitrary geometry leads to a modern reformulation of the Hartle-Hawking 'no boundary' proposal."


http://arxiv.org/abs/1010.5826
The Poincaré Gauge Theory of Gravty and the Immirzi parameter
Marcin Kaźmierczak
5 pages, extended version of proc. of 12 Marcel Grossman meeting
(Submitted on 27 Oct 2010)
"The minimal coupling method proved to yield definite and correct physical predictions when applied to fundamental fermions within the framework of Yang--Mills theories of Standard Model. Similarly, the possibility of formulating gravity as the Poincaré gauge theory gives the opportunity to produce definite predictions for fermions in the presence of gravitational field. The minimal coupling procedure, however, cannot be applied naively but rather needs to be modified slightly such that it is unambiguous. Application of the corrected coupling method to fermions, together with the inclusion of the Holst term in the gravitational part of the action, leads to the conclusion that the Immirzi parameter is in principle classically measurable, in agreement with the result of Perez and Rovelli."

http://arxiv.org/abs/1010.5837
Gravity and Nonequilibrium Thermodynamics of Classical Matter
B. L. Hu
25 pages essay. Invited Talk at Mariofest, March 2010, Rosario, Argentina. Festschrift to appear as an issue of IJMPD
(Submitted on 28 Oct 2010)
"Renewed interest in deriving gravity (more precisely, the Einstein equations) from thermodynamics considerations [1, 2] is stirred up by a recent proposal that 'gravity is an entropic force' [3] (see also [4]). Even though I find the arguments justifying such a claim in this latest proposal rather ad hoc and simplistic compared to the original one I would unreservedly support the call to explore deeper the relation between gravity and thermodynamics, this having the same spirit as my long-held view that general relativity is the hydrodynamic limit [5, 6] of some underlying theories for the microscopic structure of spacetime - all these proposals, together with that of [7, 8], attest to the emergent nature of gravity [9]. In this first paper of two we set the modest goal of studying the nonequilibrium thermodynamics of classical matter only, bringing afore some interesting prior results, without invoking any quantum considerations such as Bekenstein-Hawking entropy, holography or Unruh effect. This is for the sake of understanding the nonequilibrium nature of classical gravity which is at the root of many salient features of black hole physics. One important property of gravitational systems, from self-gravitating gas to black holes, is their negative heat capacity, which is the source of many out-of-the ordinary dynamical and thermodynamic features such as the non-existence in isolated systems of thermodynamically stable configurations, which actually provides the condition for gravitational stability. A related property is that, being systems with long range interaction, they are nonextensive and relax extremely slowly towards equilibrium. Here we explore how much of the known features of black hole thermodynamics can be derived from this classical nonequilibrium perspective. A sequel paper will address gravity and nonequilibrium thermodynamics of quantum fields [10]."

 

[marcus]

http://arxiv.org/abs/1010.6262
Translation invariance and doubly special relativity
S. Mignemi
3 pages; Talk presented at the 12th Marcel Grossmann Meeting
(Submitted on 29 Oct 2010)
We propose a new interpretation of doubly special relativity based on the distinction between the momenta and the translation generators in its phase space realization. We also argue that the implementation of the theory does not necessarily require a deformation of the Lorentz symmetry, but only of the translation invariance."

 

[marcus]

http://arxiv.org/abs/1011.0543
Bottom up approach to quantum gravity
Enrique Alvarez
Contribution to ERE2010 (Granada, Spain)
(Submitted on 2 Nov 2010)
"A general introduction is given to what can be predicated about quantum gravity once the lessons from the standard model of particle physics are taken into account.
In particular, the effective lagrangian point of view is briefly commented upon."

 

[John86]

http://arxiv.org/abs/1011.0061
The Conformal Constraint in Canonical Quantum Gravity
Authors: Gerard 't Hooft
(Submitted on 30 Oct 2010)
Abstract: Perturbative canonical quantum gravity is considered, when coupled to a renormalizable model for matter fields. It is proposed that the functional integral over the dilaton field should be disentangled from the other integrations over the metric fields. This should generate a conformally invariant theory as an intermediate result, where the conformal anomalies must be constrained to cancel out. When the residual metric is treated as a background, and if this background is taken to be flat, this leads to a novel constraint: in combination with the dilaton contributions, the matter lagrangian should have a vanishing beta function. The zeros of this beta function are isolated points in the landscape of quantum field theories, and so we arrive at a denumerable, or perhaps even finite, set of quantum theories for matter, where not only the coupling constants, but also the masses and the cosmological constant are all fixed, and computable, in terms of the Planck units.

 

[atyy]

http://arxiv.org/abs/1008.1062
Global aspects of the space of 6D N = 1 supergravities
Vijay Kumar, David R. Morrison, Washington Taylor
"If these models cannot be realized through some novel string construction, it will indicate that string theory imposes strong constraints on 6D N = 1 supergravity theories beyond the known stringent anomaly cancellation and gauge kinetic term sign constraints. If these additional constraints can be understood in terms of new quantum consistency conditions on the set of low-energy eective theories, it will provide a new window on general theories of quantum gravity; if not, it will indicate the existence of stringy constraints which may distinguish string theory from other possible UV-complete quantum gravity theories."

http://arxiv.org/abs/1011.0726
6D supergravity without tensor multiplets
Vijay Kumar, Daniel Park, Washington Taylor
We systematically investigate the finite set of possible gauge groups and matter content for N = 1 supergravity theories in six dimensions with no tensor multiplets, focusing on nonabelian gauge groups which are a product of SU(N) factors. We identify a number of models which obey all known low-energy consistency conditions, but which have no known string theory realization. Many of these models contain novel matter representations, suggesting possible new string theory constructions. Many of the most exotic matter structures arise in models which precisely saturate the gravitational anomaly bound on the number of hypermultiplets. Such models have a rigid symmetry structure, in the sense that there are no moduli which leave the full gauge group unbroken.

 

[John86]

http://arxiv.org/abs/1011.0746
Entropic Time
Authors: Ariel Caticha
(Submitted on 2 Nov 2010)
Abstract: The formulation of quantum mechanics within the framework of entropic dynamics includes several new elements. In this paper we concentrate on one of them: the implications for the theory of time. Entropic time is introduced as a book-keeping device to keep track of the accumulation of changes. One new feature is that, unlike other concepts of time appearing in the so-called fundamental laws of physics, entropic time incorporates a natural distinction between past and future.

 

[MTd2]

Pointed out by Peter Woit:

http://media.physics.harvard.edu/video/index.php?id=COLLOQ_VERLINDE_102510.flv&width=640&height=360

Emergency of Gravity Colloquium at harvard

Erik Verlinde 25 Oct 2010

 

[MTd2]

http://arxiv.org/abs/1011.1024

Path Integrals and the WKB approximation in Loop Quantum Cosmology

Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 3 Nov 2010)
We follow the Feynman procedure to obtain a path integral formulation of loop quantum cosmology starting from the Hilbert space framework. Quantum geometry effects modify the weight associated with each path so that the effective measure on the space of paths is different from that used in the Wheeler-DeWitt theory. These differences introduce some conceptual subtleties in arriving at the WKB approximation. But the approximation is well defined and provides intuition for the differences between loop quantum cosmology and the Wheeler-DeWitt theory from a path integral perspective.

 

[MTd2]

http://arxiv.org/abs/1011.1456

Asymptotic safety: a simple example

Jens Braun, Holger Gies, Daniel D. Scherer
(Submitted on 5 Nov 2010)
We use the Gross-Neveu model in 2<d<4 as a simple fermionic example for Weinberg's asymptotic safety scenario: despite being perturbatively nonrenormalizable, the model defines an interacting quantum field theory being valid to arbitrarily high momentum scales owing to the existence of a non-Gaussian fixed point. Using the functional renormalization group, we study the UV behavior of the model in both the purely fermionic as well as a partially bosonized language. We show that asymptotic safety is realized at non-Gaussian fixed points in both formulations, the universal critical exponents of which we determine quantitatively. The partially bosonized formulation allows to make contact to the large-Nf expansion where the model is known to be renormalizable to all-orders. In this limit, the fixed-point action as well as all universal critical exponents can be computed analytically. As asymptotic safety has become an important scenario for quantizing gravity, our description of a well-understood model is meant to provide for an easily accessible and controllable example of modern nonperturbative quantum field theory.

 

[marcus]

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
12 pages, 5 figures
(Submitted on 8 Nov 2010)
"Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory."

brief mention:
http://arxiv.org/abs/1011.1630
Ultraviolet Divergences in Cosmological Correlations
Steven Weinberg
26 pages, 1 figure
(Submitted on 7 Nov 2010)
"A method is developed for dealing with ultraviolet divergences in calculations of cosmological correlations, which does not depend on dimensional regularization. An extended version of the WKB approximation is used to analyze the divergences in these calculations, and these divergences are controlled by the introduction of Pauli--Villars regulator fields... Explicit formulas are given for the counterterms needed to cancel all dependence on the regulator properties, and an explicit prescription is given for calculating finite regulator-independent correlation functions..."

 

[John86]

http://arxiv.org/abs/1011.1657
Physics from information
Authors: Jae-Weon Lee
(Submitted on 7 Nov 2010)
Abstract: This is an ongoing review on my conjecture that information processing at causal horizons is the key ingredient of all physics.
Assuming that information is fundamental and the information propagates with finite velocity, one can find that main physical laws such as Newton's second law and Einstein equation simply describe the energy-information relation (dE=TdS) for matter or space time crossing a causal horizon with temperature T for observers. Quantum mechanics arises from ignorance of the observers about matter crossing the horizon, which explains why superluminal communication is impossible even with quantum entanglement. This approach also explains the origin of Jacobson's thermodynamic formalism of Einstein gravity and Verlinde's entropic gravity. When applied to a cosmic causal horizon, the conjecture reproduces the observed dark energy and demands the zero cosmological constant.

http://arxiv.org/abs/1011.1604
Noncommutative (generalized) sine-Gordon/massive Thirring correspondence, integrability and solitons
Authors: H. Blas, H. L. Carrion
(Submitted on 7 Nov 2010)
Abstract: Some properties of the correspondence between the non-commutative versions of the (generalized) sine-Gordon (NCGSG$_{1,2}$) and the massive Thirring (NCGMT$_{1,2}$) models are studied. Our method relies on the master Lagrangian approach to deal with dual theories. The master Lagrangians turn out to be the NC versions of the so-called affine Toda model coupled to matter fields (NCATM$_{1,2}$), in which the Toda field $g$ belongs to certain subgroups of $ GL(3)$, and the matter fields lie in the higher grading directions of an affine Lie algebra. Depending on the form of $g$ one arrives at two different NC versions of the NCGSG$_{1,2}$/NCGMT$_{1,2}$ correspondence. In the NCGSG$_{1,2}$ sectors, through consistent reduction procedures, we find NC versions of some well-known models, such as the NC sine-Gordon (NCSG$_{1,2}$) (Lechtenfeld et al. and Grisaru-Penati proposals, respectively), NC (bosonized) Bukhvostov-Lipatov (NCbBL$_{1,2}$) and NC double sine-Gordon (NCDSG$_{1,2}$) models. The NCGMT$_{1,2}$ models correspond to Moyal product extension of the generalized massive Thirring model. The NCGMT$_{1,2}$ models posses constrained versions with relevant Lax pair formulations, and other sub-models such as the NC massive Thirring (NCMT$_{1,2}$), the NC Bukhvostov-Lipatov (NCBL$_{1,2}$) and constrained versions of the last models with Lax pair formulations. We have established that, except for the well known NCMT$_{1,2}$ zero-curvature formulations, generalizations ($n_{F} \ge 2$, $n_F=$number of flavors) of the massive Thirring model allow zero-curvature formulations only for constrained versions of the models and for each one of the various constrained sub-models defined for less than $n_F$ flavors, in the both NCGMT$_{1,2}$ and ordinary space-time descriptions (GMT), respectively. The non-commutative solitons and kinks of the $ GL(3)$ NCGSG$_{1,2}$ models are investigated.

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Authors: Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
(Submitted on 8 Nov 2010)
Abstract: Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory.

 

[atyy]

http://arxiv.org/abs/1011.2149
Generalized Spinfoams
You Ding, Muxin Han, Carlo Rovelli
(Submitted on 9 Nov 2010)
We reconsider the spinfoam dynamics that has been recently introduced, in the generalized Kaminski-Kisielowski-Lewandowski (KKL) version where the foam is not dual to a triangulation. We study the Euclidean as well as the Lorentzian case. We show that this theory can still be obtained as a constrained BF theory satisfying the simplicity constraint, now discretized on a general oriented 2-cell complex. This constraint implies that boundary states admit a (quantum) geometrical interpretation in terms of polyhedra, generalizing the tetrahedral geometry of the simplicial case. We also point out that the general solution to this constraint (imposed weakly) depends on a quantum number r_f in addition to those of loop quantum gravity. We compute the vertex amplitude and recover the KKL amplitude in the Euclidean theory when r_f=0. We comment on the eventual physical relevance of r_f, and the formal way to eliminate it.

 

[MTd2]

http://arxiv.org/abs/1011.2041

Inducing Barbero-Immirzi Connections along SU(2)-reductions of Bundles on Spacetime

L. Fatibene, M. Ferraris, M. Francaviglia
(Submitted on 9 Nov 2010)
We shall present here a general apt technique to induce connections along bundle reductions which is different from the standard restriction. The technique is a generalization of the mechanism presented in [1] to define at spacetime level the Barbero-Immirzi (BI) connection used in LQG. The general prescription to define such a reduced connection is interesting from a mathematical viewpoint and it allows a general and direct control on transformation laws of the induced object. Moreover, unlike what happens by using standard restriction, we shall show that once a bundle reduction is given, then any connection induces a reduced connection with no constraint on the original holonomy as it happens when connections are simply restricted.

 

[marcus]

http://arxiv.org/abs/1011.2463
The Hamiltonian constraint in Polymer Parametrized Field Theory
Alok Laddha, Madhavan Varadarajan
53 pages
(Submitted on 10 Nov 2010)
"Recently, a generally covariant reformulation of 2 dimensional flat spacetime free scalar field theory known as Parameterised Field Theory was quantized using Loop Quantum Gravity (LQG) type 'polymer' representations. Physical states were constructed, without intermediate regularization structures, by averaging over the group of gauge transformations generated by the constraints, the constraint algebra being a Lie algebra. We consider classically equivalent combinations of these constraints corresponding to a diffeomorphism and a Hamiltonian constraint, which, as in gravity, define a Dirac algebra. Our treatment of the quantum constraints parallels that of LQG and obtains the following results, expected to be of use in the construction of the quantum dynamics of LQG:
(i) the (triangulated) Hamiltonian constraint acts only on vertices, its construction involves some of the same ambiguities as in LQG and its action on diffeomorphism invariant states admits a continuum limit
(ii) if the regulating holonomies are in representations tailored to the edge labels of the state, all previously obtained physical states lie in the kernel of the Hamiltonian constraint,
(iii) the commutator of two (density weight 1) Hamiltonian constraints as well as the operator correspondent of their classical Poisson bracket converge to zero in the continuum limit defined by diffeomorphism invariant states, and vanish on the Lewandowski- Marolf (LM) habitat
(iv) the rescaled density 2 Hamiltonian constraints and their commutator are ill defined on the LM habitat despite the well defined-ness of the operator correspondent of their classical Poisson bracket there
(v) there is a new habitat which supports a non-trivial representation of the Poisson-Lie algebra of density 2 constraints."

http://arxiv.org/abs/1011.2726
The 1/N expansion of colored tensor models
Razvan Gurau
(Submitted on 11 Nov 2010)
"In this paper we perform the 1/N expansion of the colored three dimensional Boulatov tensor model. As in matrix models, we obtain a systematic topological expansion, with more and more complicated topologies suppressed by higher and higher powers of N. We compute the first orders of the expansion and prove that only graphs corresponding to three spheres S³ contribute to the leading order in the large N limit."

Gurau's work is relevant to GFT, and cites a lot of LQG papers

brief mention:
http://arxiv.org/abs/1011.2545
Solitonic Models Based on Quantum Groups and the Standard Model
Robert J. Finkelstein
59 pages, review of the author's way of representing particles as knots.

 

[marcus]

http://arxiv.org/abs/1011.2779
Inflationary observables in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni
40 pages
(Submitted on 11 Nov 2010)
"The full set of cosmological observables coming from linear scalar and tensor perturbations of loop quantum cosmology is computed in the presence of inverse-volume corrections. Background inflationary solutions are found at linear order in the quantum corrections; depending on the values of quantization parameters, they obey an exact or perturbed power-law expansion in conformal time. The comoving curvature perturbation is shown to be conserved at large scales, just as in the classical case. Its associated Mukhanov equation is obtained and solved. Combined with the results for tensor modes, this yields the scalar and tensor indices, their running, and the tensor-to-scalar ratio, which are all first order in the quantum correction. The latter could be sizable in phenomenological scenarios. Contrary to a pure minisuperspace parametrization, the lattice refinement parametrization is in agreement with both anomaly cancellation and our results on background solutions and linear perturbations. The issue of the choice of parametrization is also discussed in relation with a possible superluminal propagation of perturbative modes, and conclusions for quantum spacetime structure are drawn."

http://arxiv.org/abs/1011.2961
Static isolated horizons: SU(2) invariant phase space, quantization, and black hole entropy
Alejandro Perez, Daniele Pranzetti
22 pages, 1 figure
(Submitted on 12 Nov 2010)
"We study the classical field theoretical formulation of static generic isolated horizons in a manifestly SU(2) invariant formulation. We show that the usual classical description requires revision in the non-static case due to the breaking of diffeomorphism invariance at the horizon leading to the non conservation of the usual pre-symplectic structure. We argue how this difficulty could be avoided by a simple enlargement of the field content at the horizon that restores diffeomorphism invariance. Restricting our attention to static isolated horizons we study the effective theories describing the boundary degrees of freedom. A quantization of the horizon degrees of freedom is proposed. By defining a statistical mechanical ensemble where only the area A of the horizon is fixed macroscopically-states with fluctuations away from spherical symmetry are allowed-we show that it is possible to obtain agreement with the Hawking's area law---S = A/4 (in Planck Units)---without fixing the Immirzi parameter to any particular value: consistency with the area law only imposes a relationship between the Immirzi parameter and the level of the Chern-Simons theory involved in the effective description of the horizon degrees of freedom."

http://arxiv.org/abs/1011.3022
High-order quantum back-reaction and quantum cosmology with a positive cosmological constant
Martin Bojowald, David Brizuela, Hector H. Hernandez, Michael J. Koop, Hugo A. Morales-Tecotl
33 pages, 9 figures
(Submitted on 12 Nov 2010)
"When quantum back-reaction by fluctuations, correlations and higher moments of a state becomes strong, semiclassical quantum mechanics resembles a dynamical system with a high-dimensional phase space. Here, systematic numerical methods to derive the dynamical equations including all quantum corrections to high order in the moments are introduced, together with a quantum cosmological example to illustrate some implications. The results show, for instance, that the initial Gaussian form of an initial state is maintained only briefly, but that the evolving state settles down to a new characteristic shape afterwards. At some point during the evolution all moments considered become of equal size and no truncation to finite order is possible. But until that time is reached, numerical evaluations provide a large amount of information about dynamical quantum states."

http://arxiv.org/abs/1011.3040
An effective approach to the problem of time: general features and examples
Martin Bojowald, Philipp A Hoehn, Artur Tsobanjan
59 pages, 9 figures
(Submitted on 12 Nov 2010)
"The effective approach to quantum dynamics allows a reformulation of the Dirac quantization procedure for constrained systems in terms of an infinite-dimensional constrained system of classical type. For semiclassical approximations, the quantum constrained system can be truncated to finite size and solved by the reduced phase space or gauge-fixing methods. In particular, the classical feasibility of local internal times is directly generalized to quantum systems, overcoming the main difficulties associated with the general problem of time in the semiclassical realm. The key features of local internal times and the procedure of patching global solutions using overlapping intervals of local internal times are described and illustrated by two quantum mechanical examples. The choice of time is tantamount to a choice of gauge at the effective level and changing the clock is, therefore, equivalent to a gauge transformation. This article complements the conceptual discussion in arXiv:1009.5953."

http://arxiv.org/abs/1011.2794
Entropy Production during Asymptotically Safe Inflation
Alfio Bonanno, Martin Reuter
17 pages, 4 figures, Invited contribution to the special issue of Entropy on "Entropy in Quantum Gravity"
(Submitted on 11 Nov 2010)
"The Asymptotic Safety scenario predicts that the deep ultraviolet of Quantum Einstein Gravity is governed by a nontrivial renormalization group fixed point. Analyzing its implications for cosmology using renormalization group improved Einstein equations we find that it can give rise to a phase of inflationary expansion in the early Universe. Inflation is a pure quantum effect here and requires no inflaton field. It is driven by the cosmological constant and ends automatically when the renormalization group evolution has reduced the vacuum energy to the level of the matter energy density. The quantum gravity effects also provide a natural mechanism for the generation of entropy. It could easily account for the entire entropy of the present Universe in the massless sector."

 

[MTd2]

http://arxiv.org/abs/1011.2794

Entropy Production during Asymptotically Safe Inflation

Alfio Bonanno, Martin Reuter
(Submitted on 11 Nov 2010)
The Asymptotic Safety scenario predicts that the deep ultraviolet of Quantum Einstein Gravity is governed by a nontrivial renormalization group fixed point. Analyzing its implications for cosmology using renormalization group improved Einstein equations we find that it can give rise to a phase of inflationary expansion in the early Universe. Inflation is a pure quantum effect here and requires no inflaton field. It is driven by the cosmological constant and ends automatically when the renormalization group evolution has reduced the vacuum energy to the level of the matter energy density. The quantum gravity effects also provide a natural mechanism for the generation of entropy. It could easily account for the entire entropy of the present Universe in the massless sector.

http://arxiv.org/abs/1011.3022

High-order quantum back-reaction and quantum cosmology with a positive cosmological constant

Martin Bojowald, David Brizuela, Hector H. Hernandez, Michael J. Koop, Hugo A. Morales-Tecotl
(Submitted on 12 Nov 2010)
When quantum back-reaction by fluctuations, correlations and higher moments of a state becomes strong, semiclassical quantum mechanics resembles a dynamical system with a high-dimensional phase space. Here, systematic numerical methods to derive the dynamical equations including all quantum corrections to high order in the moments are introduced, together with a quantum cosmological example to illustrate some implications. The results show, for instance, that the initial Gaussian form of an initial state is maintained only briefly, but that the evolving state settles down to a new characteristic shape afterwards. At some point during the evolution all moments considered become of equal size and no truncation to finite order is possible. But until that time is reached, numerical evaluations provide a large amount of information about dynamical quantum states.

http://arxiv.org/abs/1011.3040

An effective approach to the problem of time: general features and examples

Martin Bojowald, Philipp A Hoehn, Artur Tsobanjan
(Submitted on 12 Nov 2010)
The effective approach to quantum dynamics allows a reformulation of the Dirac quantization procedure for constrained systems in terms of an infinite-dimensional constrained system of classical type. For semiclassical approximations, the quantum constrained system can be truncated to finite size and solved by the reduced phase space or gauge-fixing methods. In particular, the classical feasibility of local internal times is directly generalized to quantum systems, overcoming the main difficulties associated with the general problem of time in the semiclassical realm. The key features of local internal times and the procedure of patching global solutions using overlapping intervals of local internal times are described and illustrated by two quantum mechanical examples. The choice of time is tantamount to a choice of gauge at the effective level and changing the clock is, therefore, equivalent to a gauge transformation. This article complements the conceptual discussion in arXiv:1009.5953.

http://arxiv.org/abs/1011.2961

Static isolated horizons: SU(2) invariant phase space, quantization, and black hole entropy

Alejandro Perez, Daniele Pranzetti
(Submitted on 12 Nov 2010)
We study the classical field theoretical formulation of static generic isolated horizons in a manifestly SU(2) invariant formulation. We show that the usual classical description requires revision in the non-static case due to the breaking of diffeomorphism invariance at the horizon leading to the non conservation of the usual pre-symplectic structure. We argue how this difficulty could be avoided by a simple enlargement of the field content at the horizon that restores diffeomorphism invariance. Restricting our attention to static isolated horizons we study the effective theories describing the boundary degrees of freedom. A quantization of the horizon degrees of freedom is proposed. By defining a statistical mechanical ensemble where only the area A of the horizon is fixed macroscopically-states with fluctuations away from spherical symmetry are allowed-we show that it is possible to obtain agreement with the Hawking's area law---S = A/4 (in Planck Units)---without fixing the Immirzi parameter to any particular value: consistency with the area law only imposes a relationship between the Immirzi parameter and the level of the Chern-Simons theory involved in the effective description of the horizon degrees of freedom.

 

[marcus]

http://arxiv.org/abs/1011.3335
Effective Theory in Spinfoam Cosmology: A First Order LQG-corrected FRW Cosmology and the Stiff Fluid
Christian Röken
(Submitted on 15 Nov 2010)
"We present an effective theory of a basic holomorphic spinfoam cosmology peaked on homogeneous isotropic metrics. The quantum-corrected Hamiltonian constraint of this effective theory is motivated by an operator equation H W = 0, satisfied by the LQG transition amplitude W, reduced to a classical phase space symplectic structure. The analysis of this quantity shows that this effective model gives first order corrections to the classical FRW dynamical expressions in h resembling a universe with an ultralight irrotational stiff perfect fluid as matter-energy content. Such an exotic fluid can also be regarded as a massless real scalar field."

http://arxiv.org/abs/1011.3418
Quantum of volume in de Sitter space
Jakub Mielczarek, Wlodzimierz Piechocki
12 pages, 3 figures
(Submitted on 15 Nov 2010)
"We apply the nonstandard loop quantum cosmology method to quantize a flat FRW cosmological model with a free scalar field and the cosmological constant Λ >0. Modification of the Hamiltonian in terms of loop geometry parametrized by a length λ introduces a scale dependence of the model. The spectrum of the volume operator is discrete and depends on Λ. Relating quantum of the volume with an elementary lattice cell leads to an explicit dependence of Λ on λ. Based on this assumption, we investigate the possibility of interpreting Λ as a running constant."

Copied from Arivero:
αβγδεζηθικλμνξοπρσςτυφχψω...ΓΔΘΛΞΠΣΦΨΩ...∏∑∫∂√ ...± ÷...←↓→↑↔~≈≠≡≤≥...½...∞...(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)
___________

 

[marcus]

http://arxiv.org/abs/1011.3641
A Unified Theory of Non-Linear Electrodynamics and Gravity
Alexander Torres-Gomez, Kirill Krasnov, Carlos Scarinci
31 pages
(Submitted on 16 Nov 2010)
"We describe a class of unified theories of electromagnetism and gravity. The Lagrangian is of the BF type, with a potential for the B-field, the gauge group is U(2) (complexified). Given a choice of the potential function the theory is a deformation of (complex) general relativity and electromagnetism, and describes just two propagating polarisations of the graviton and two of the photon. When gravity is switched off the theory becomes the usual non-linear electrodynamics with a general structure function. The Einstein-Maxwell theory can be recovered by sending some of the parameters of the defining potential to zero, but for any generic choice of the potential the theory is indistinguishable from Einstein-Maxwell at low energies. A real theory is obtained by imposing suitable reality conditions. We also study the spherically-symmetric solution and show how the usual Reissner-Nordstrom solution is recovered."

http://arxiv.org/abs/1011.3667
Coarse graining theories with gauge symmetries
Benjamin Bahr, Bianca Dittrich, Song He
31 pages, 3 pictures
(Submitted on 16 Nov 2010)
"Discretizations of continuum theories often do not preserve the gauge symmetry content. This occurs in particular for diffeomorphism symmetry in general relativity, which leads to severe difficulties both in canonical and covariant quantization approaches. We discuss here the method of perfect actions, which attempts to restore gauge symmetries by mirroring exactly continuum physics on a lattice via a coarse graining process. Analytical results can only be obtained via a perturbative approach, for which we consider the first steps, namely the coarse graining of the linearized theory. The linearized gauge symmetries are exact also in the discretized theory, hence we develop a formalism to deal with gauge systems. Finally we provide a discretization of linearized gravity as well as a coarse graining map and show that with this choice the 3D linearized gravity action is invariant under coarse graining."

Brief mention (only indirect connection with QG):
http://arxiv.org/abs/1011.3532
The moment of truth for WIMP Dark Matter
Gianfranco Bertone
To appear in Nature (Nov 18, 2010)
(Submitted on 15 Nov 2010)
We know that dark matter constitutes 85% of all the matter in the Universe, but we do not know of what it is made. Amongst the many Dark Matter candidates proposed, WIMPs (weakly interacting massive particles) occupy a special place, as they arise naturally from well motivated extensions of the standard model of particle physics. With the advent of the Large Hadron Collider at CERN, and a new generation of astroparticle experiments, the moment of truth has come for WIMPs: either we will discover them in the next five to ten years, or we will witness the inevitable decline of WIMP paradigm.

 

[MTd2]

http://arxiv.org/abs/1011.3888

Dual DSR

Jose A. Magpantay
(Submitted on 17 Nov 2010)
We develop the physics of dual kappa Poincare algebra, which we will call dual DSR. First, we show that the dual kappa Poincare algebra is isomorphic to de Sitter algebra and its spactime is essentially de Sitter spacetime. Second, we show how to derive the coproduct rules for Beltrami and conformal coordinates of de Sitter spacetime. It follows from the current literature on de Sitter relativity that the speed of light c and the de Sitter length are the two invariant scales of the physics of dual kappa Poincare algebra. Third, we derive the Casimir invariant of the dual kappa Popincare algebra and use this to derive an expression for the speed of light, our fourth result. Fifth, the field equation for the scalar field is derived from the Casimir invariant. The results for the coordinate speed of light and the scalar field theory are the same as in de Sitter theory in the planar coordinate basis. Thus, we have shown that the physics of dual kappa Poincare algebra (in the dual bicrossproduct basis), which can be apprpriately called dual DSR, is essentially de Sitter relativity. Finally, we note that dual DSR is not a quantum theory of spacetime but a quantum theory of momenta.

 

[marcus]

http://arxiv.org/abs/1011.4249
On the measure problem in slow roll inflation and loop quantum cosmology
Alejandro Corichi, Asieh Karami
12 pages, 3 figures
(Submitted on 18 Nov 2010)
"We consider the measure problem in standard slow-roll inflationary models from the perspective of loop quantum cosmology (LQC). Following recent results by Ashtekar and Sloan, we study the probability of having enough e-foldings and focus on the transition of the theory to the 'continuum limit', where general relativity (GR) is recovered. Contrary to the standard expectation, the probability of having enough inflation, that is close to one in LQC, grows and tends to 1 as one approaches the classical limit. We study the origin of the tension between these results with those by Gibbons and Turok, and offer an explanation that brings these apparent contradictory results into a coherent picture. As we show, the conflicting results stem from different choices of initial conditions. The singularity free scenario of loop quantum cosmology offers a natural choice of initial conditions, and suggests that enough inflation is generic."

http://arxiv.org/abs/1011.4290
Two-point functions in (loop) quantum cosmology
Gianluca Calcagni, Steffen Gielen, Daniele Oriti
28 pages
(Submitted on 18 Nov 2010)
"The path-integral formulation of quantum cosmology with a massless scalar field as a sum-over-histories of volume transitions is discussed, with particular but non-exclusive reference to loop quantum cosmology. Exploiting the analogy with the relativistic particle, we give a complete overview of the possible two-point functions, pointing out the choices involved in their definitions, deriving their vertex expansions and the composition laws they satisfy. We clarify the origin and relations of different quantities previously defined in the literature, in particular the tie between definitions using a group averaging procedure and those in a deparametrized framework. Finally, we draw some conclusions about the physics of a single quantum universe (where there exist superselection rules on positive- and negative-frequency sectors and different choices of inner product are physically equivalent) and multiverse field theories where the role of these sectors and the inner product are reinterpreted."

 

[marcus]

http://arxiv.org/abs/1011.3706
Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity
V.G.Gurzadyan, R.Penrose
8 pages, 6 figs
(Submitted on 16 Nov 2010)
"Conformal cyclic cosmology (CCC) posits the existence of an aeon preceding our Big Bang 'B', whose conformal infinity 'I' is identified, conformally, with 'B', now regarded as a spacelike 3-surface. Black-hole encounters, within bound galactic clusters in that previous aeon, would have the observable effect, in our CMB sky, of families of concentric circles over which the temperature variance is anomalously low, the centre of each such family representing the point of 'I' at which the cluster converges. These centres appear as fairly randomly distributed fixed points in our CMB sky. The analysis of Wilkinson Microwave Background Probe's (WMAP) cosmic microwave background 7-year maps does indeed reveal such concentric circles, of up to 6 sigma significance. This is confirmed when the same analysis is applied to BOOMERanG98 data, eliminating the possibility of an instrumental cause for the effects. These observational predictions of CCC would not be easily explained within standard inflationary cosmology."

 

[marcus]

http://pirsa.org/10110071/
Is temperature the speed of time? Thermal time and the Tolman effect
Matteo Smerlak
Perimeter video lecture, 16 November 2010
"Why is a vertical column of gas at thermal equilibrium slighly hotter at the bottom than a the top? My answer in this talk will be that time runs slower in a deeper gravitational potential, and temperature is nothing but the (inverse) speed of time. Specifically, I will (i) introduce Rovelli's notion of thermal time, (ii) use it to provide a 'principle' characterization of thermal equilibrium in stationary spacetimes, and (iii) effortlessly derive the Tolman-Ehrenfest relation. This approach contrasts with the 'constructive' accounts of thermal equilibrium in curved spacetimes given in the literature, and vindicates the time-temperature relationship cropping up in the Hawking-Unruh and Kubo-Martin-Schwinger relations."

Seminar talk based on this paper:
http://arxiv.org/abs/1005.2985
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time"
Carlo Rovelli, Matteo Smerlak

 

[MTd2]

http://arxiv.org/abs/1011.4705

Spinfoam Cosmology: quantum cosmology from the full theory

Francesca Vidotto
(Submitted on 21 Nov 2010)
Quantum cosmology is usually studied quantizing symmetry-reduced variables. Is it possible, instead, to define quantum cosmology starting from the full quantum gravity theory? In Loop Quantum Gravity (LQG), it is possible to cut the degrees of freedom in a suitable way, in order to define a cosmological model. Such a model provides a tool for describing general fluctuations of the quantum geometry at the bounce that replaces the initial singularity. I focus on its simplest version, a "dipole" formed by two tetrahedra. This has been shown to describe a universe with anisotropic and inhomogenous degrees of freedom. Its dynamics can be given using the spinfoam formalism. I briefly review the present state of this approach.

 

[MTd2]

http://arxiv.org/abs/1011.4618

Evolution of cosmological perturbations in an RG-driven inflationary scenario

Adriano Contillo
(Submitted on 20 Nov 2010)
A gauge-invariant, linear cosmological perturbation theory of an almost homogeneous and isotropic universe with dynamically evolving Newton constant G and cosmological constant $\Lambda$ is presented. The equations governing the evolution of the comoving fractional spatial gradients of the matter density, G and $\Lambda$ are thus obtained. Explicit solutions are discussed in cosmologies, featuring an accelerated expansion, where both G and $\Lambda$ vary according to renormalization group equations in the vicinity of an ultraviolet fixed point. Finally, a similar analysis is carried out in the late universe regime described by the part of the renormalization group trajectory close to the gaussian fixed point.

 

[marcus]

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
5 pages, Proceedings of the 35th International Conference on High Energy Physics, Paris, 2010 (ICHEP 2010)
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

http://arxiv.org/abs/1011.5628
Black Hole Entropy, Loop Gravity, and Polymer Physics
Eugenio Bianchi
13 pages, 2 figures
(Submitted on 25 Nov 2010)
"Loop Gravity provides a microscopic derivation of Black Hole entropy. In this paper, I show that the microstates counted admit a semiclassical description in terms of shapes of a tessellated horizon. The counting of microstates and the computation of the entropy can be done via a mapping to an equivalent statistical mechanical problem: the counting of conformations of a closed polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of Black Holes and the physics of polymers."

http://arxiv.org/abs/1011.5676
Coherent states for FLRW space-times
Elena Magliaro, Antonino Marciano, Claudio Perini
10 pages, 1 figure
(Submitted on 25 Nov 2010)
"We construct a class of coherent spin-network states that capture proprieties of curved space-times of the Friedmann-Lamaiître-Robertson-Walker type on which they are peaked. The data coded by a coherent state are associated to a cellular decomposition of a spatial (t=const.) section with dual graph given by the complete five-vertex graph, though the construction can be easily generalized to other graphs. The labels of coherent states are complex SL(2,C) variables, one for each link of the graph and are computed through a smearing process starting from a continuum extrinsic and intrinsic geometry of the canonical surface. The construction covers both Euclidean and Lorentzian signatures; in the Euclidean case and in the limit of flat space we reproduce the simplicial 4-simplex semiclassical states used in Spin Foams."

http://arxiv.org/abs/1011.5754
Background independent condensed matter models for quantum gravity
Alioscia Hamma, Fotini Markopoulou
Contribution submitted to the focus issue of the New Journal of Physics on "Classical and Quantum Analogues for Gravitational Phenomena and Related Effects", R. Schuetzhold, U. Leonhardt and C. Maia, Eds.
(Submitted on 26 Nov 2010)
"A number of recent proposals for a quantum theory of gravity are based on the idea that spacetime geometry and gravity are derivative concepts and only apply at an approximate level. There are two fundamental challenges to any such approach. At the conceptual level, there is a clash between the 'timelessness' of general relativity and emergence. Second, the lack of a fundamental spacetime makes difficult the straightforward application of well-known methods of statistical physics to the problem. We recently initiated a study of such problems using spin systems based on evolution of quantum networks with no a priori geometric notions as models for emergent geometry and gravity. In this article we review two such models. The first is a model of emergent (flat) space and matter and we show how to use methods from quantum information theory to derive features such as speed of light from a non-geometric quantum system. The second model exhibits interacting matter and geometry, with the geometry defined by the behavior of matter. This model has primitive notions of gravitational attraction which we illustrate with a toy black hole, and exhibits entanglement between matter and geometry and thermalization of the quantum geometry."

 

[John86]

http://arxiv.org/abs/1011.5593
Minimal conditions for the existence of a Hawking-like flux
Authors: Carlos Barcelo (IAA-CSIC, Granada), Stefano Liberati (SISSA, Trieste), Sebastiano Sonego (Universita di Udine), Matt Visser (Victoria University of Wellington)
(Submitted on 25 Nov 2010)
Abstract: We investigate the minimal conditions that an asymptotically flat general relativistic spacetime must satisfy in order for a Hawking-like Planckian flux of particles to arrive at future null infinity. We demonstrate that there is no requirement that any sort of horizon form anywhere in the spacetime. We find that the irreducible core requirement is encoded in an approximately exponential "peeling" relationship between affine coordinates on past and future null infinity. As long as a suitable adiabaticity condition holds, then a Planck-distributed Hawking-like flux will arrive at future null infinity with temperature determined by the e-folding properties of the outgoing null geodesics. The temperature of the Hawking-like flux can slowly evolve as a function of time. We also show that the notion of "peeling" of null geodesics is distinct, and in general different, from the usual notion of "inaffinity" used in Hawking's definition of surface gravity.

http://arxiv.org/abs/1011.4634
Quasi-normal frequencies: Semi-analytic results for highly damped modes
Authors: Jozef Skakala, Matt Visser
(Submitted on 21 Nov 2010)
Abstract: Black hole highly-damped quasi-normal frequencies (QNFs) are very often of the form (offset)} + i n (gap). We have investigated the genericity of this phenomenon for the Schwarzschild--deSitter (SdS) black hole by considering a model potential that is piecewise Eckart (piecewise Poeschl-Teller), and developing an analytic ``quantization condition'' for the highly-damped quasi-normal frequencies. We find that the (offset) + i n(gap) behaviour is common but not universal, with the controlling feature being whether or not the ratio of the surface gravities is a rational number. We furthermore observed that the relation between rational ratios of surface gravities and periodicity of QNFs is very generic, and also occurs within different analytic approaches applied to various types of black hole spacetimes. These observations are of direct relevance to any physical situation where highly-damped quasi-normal modes are important.

http://arxiv.org/abs/1011.4538
Entropy bounds for uncollapsed matter
Authors: Gabriel Abreu (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 19 Nov 2010)
Abstract: In any static spacetime the quasilocal Tolman mass contained within a volume can be reduced to a Gauss-like surface integral involving the flux of a suitably defined generalized surface gravity. By introducing some basic thermodynamics, and invoking the Unruh effect, one can then develop elementary bounds on the quasilocal entropy that are very similar in spirit to the holographic bound, and closely related to entanglement entropy.

http://arxiv.org/abs/1009.0080
Generic master equations for quasi-normal frequencies
Authors: Jozef Skakala (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 1 Sep 2010 (v1), last revised 23 Nov 2010 (this version, v2))
Abstract: Generic master equations governing the highly-damped quasi-normal frequencies [QNFs] of one-horizon, two-horizon, and even three-horizon spacetimes can be obtained through either semi-analytic or monodromy techniques. While many technical details differ, both between the semi-analytic and monodromy approaches, and quite often among various authors seeking to apply the monodromy technique, there is nevertheless widespread agreement regarding the the general form of the QNF master equations. Within this class of generic master equations we can establish some rather general results, relating the existence of "families" of QNFs of the form omega_{a,n} = (offset)_a + i n (gap) to the question of whether or not certain ratios of parameters are rational or irrational.

 

[marcus]

http://arxiv.org/abs/1011.6257
One-loop quantum gravity repulsion in the early Universe
Bogusław Broda
7 pages
(Submitted on 29 Nov 2010)
"Perturbative quantum gravity is used to compute the lowest order corrections to the classical, spatially flat cosmological FLRW solution (for the radiation). The presented approach is analogous to the approach used to compute quantum corrections to the Coulomb potential in electrodynamics, or rather to the approach used to compute quantum corrections to the Schwarzschild solution in gravity. In the framework of the standard perturbative quantum gravity, it is shown that the corrections to the classical deceleration, coming from the one-loop graviton vacuum polarization (self-energy), have (UV cutoff free) opposite to the classical repulsive properties which are not negligible in the very early Universe. The repulsive "quantum forces" are akin to those known from loop quantum cosmology."

 

[murray92]

http://arxiv.org/abs/1008.1046
An Estimate of Λin Resummed Quantum Gravity in the Context of Asymptotic Safety
B.F.L. Ward
(Submitted on 5 Aug 2010 (v1), last revised 18 Oct 2010 (this version, v3))
We show that, by using recently developed exact resummation techniques based on the extension of the methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's theory, we get quantum field theoretic descriptions for the UV fixed-point behaviors of the dimensionless gravitational and cosmological constants postulated by Weinberg. Connecting our work to the attendant phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter, we predict the value of the cosmological constant \Lambda. We find the encouraging estimate \rho_\Lambda\equiv \frac{\Lambda}{8\pi G_N} \simeq (2.400\times 10^{-3}eV)^4.

I was bit suprised this wasn't mentioned. But - if true, and his methods hold- this is one of the BIG papers, ...
(just thought I'd chip it in)
... If anybody has anything interesting to say about the paper please start a thread.

 

[marcus]

http://arxiv.org/abs/1011.6442
Surprises in the Evaporation of 2-Dimensional Black Holes
Abhay Ashtekar, Frans Pretorius, Fethi M. Ramazanoğlu
4 pages, 3 figures
(Submitted on 30 Nov 2010)
"Quantum evaporation of Callen-Giddings-Harvey-Strominger (CGHS) black holes is analyzed in the mean field approximation. The resulting semi-classical theory incorporates back reaction. Detailed analytical and numerical calculations show that, while some of the assumptions underlying the standard evaporation paradigm are borne out, several are not. Furthermore, if the black hole is initially macroscopic, the evaporation process exhibits remarkable universal properties. Although the literature on CGHS black holes is quite rich, these features had escaped previous analyses, in part because of lack of required numerical precision, and in part because certain properties and symmetries of the model were not recognized. Finally, our results provide support for the full quantum scenario recently developed by Ashtekar, Taveras and Varadarajan."
To help interpret this abstract I will add that of the earlier ATV paper, published in Physical Review Letters:
http://arxiv.org/abs/0801.1811
Information is Not Lost in the Evaporation of 2-dimensional Black Holes
Abhay Ashtekar, Victor Taveras, Madhavan Varadarajan
4 pages, 2 figures
(Submitted on 11 Jan 2008)
"We analyze Hawking evaporation of the Callen-Giddings-Harvey-Strominger (CGHS) black holes from a quantum geometry perspective and show that information is not lost, primarily because the quantum space-time is sufficiently larger than the classical. Using suitable approximations to extract physics from quantum space-times we establish that: i)future null infinity of the quantum space-time is sufficiently long for the the past vacuum to evolve to a pure state in the future; ii) this state has a finite norm in the future Fock space; and iii) all the information comes out at future infinity; there are no remnants."

A LQG blog (new) associated with the International LQG Seminar (ILQGS, teleconference with slides, some Tuesdays)
http://ilqgs.blogspot.com/

 

[John86]

http://arxiv.org/abs/1011.6466
Cosmic alignment of the aether
Authors: Isaac Carruthers, Ted Jacobson
(Submitted on 30 Nov 2010)
Abstract: In Einstein-aether theory and Horava gravity, a timelike unit vector is coupled to the spacetime metric. It has previously been shown that in an exponentially expanding homogeneous, isotropic background, small perturbations of the vector relax back to the isotropic frame. Here we investigate large deviations from isotropy, maintaining homogeneity. We find that, for generic values of the coupling constants, the aether and metric relax to the isotropic configuration if the initial aether hyperbolic boost angle and its time derivative in units of the cosmological constant are less than something of order unity. For larger angles or angle derivatives, the behavior is strongly dependent on the values of the coupling constants. Generally there is runaway behavior, in which the anisotropy increases with time, and/or singularities occur.

http://arxiv.org/abs/1005.3293
Minimal Scales from an Extended Hilbert Space
Authors: Martin Kober, Piero Nicolini
(Submitted on 18 May 2010 (v1), last revised 30 Nov 2010 (this version, v2))
Abstract: We consider an extension of the conventional quantum Heisenberg algebra, assuming that coordinates as well as momenta fulfil nontrivial commutation relations. As a consequence, a minimal length and a minimal mass scale are implemented. Our commutators do not depend on positions and momenta and we provide an extension of the coordinate coherent state approach to Noncommutative Geometry. We explore, as toy model, the corresponding quantum field theory in a (2+1)-dimensional spacetime. Then we investigate the more realistic case of a (3+1)-dimensional spacetime, foliated into noncommutative planes. As a result, we obtain propagators, which are finite in the ultraviolet as well as the infrared regime

http://arxiv.org/abs/1008.0154
Gauge Theories under Incorporation of a Generalized Uncertainty Principle
Authors: Martin Kober
(Submitted on 1 Aug 2010 (v1), last revised 23 Oct 2010 (this version, v2))
Abstract: There is considered an extension of gauge theories according to the assumption of a generalized uncertainty principle which implies a minimal length scale. A modification of the usual uncertainty principle implies an extended shape of matter field equations like the Dirac equation. If there is postulated invariance of such a generalized field equation under local gauge transformations, the usual covariant derivative containing the gauge potential has to be replaced by a generalized covariant derivative. This leads to a generalized interaction between the matter field and the gauge field as well as to an additional self interaction of the gauge field. Since the existence of a minimal length scale seems to be a necessary assumption of any consistent quantum theory of gravity, the gauge principle is a constitutive ingredient of the standard model and even gravity can be described as gauge theory of local translations or Lorentz transformations, the presented extension of gauge theories appears as a very important consideration.

 

[marcus]

http://arxiv.org/abs/1012.0053
Symmetry Reduction of Loop Quantum Gravity
Johannes Brunnemann, Tim A. Koslowski
34 pages, 2 figures
(Submitted on 30 Nov 2010)
"We reexamine the non-embedability of the configuration space of standard Loop Quantum Cosmology into the configuration space of full Loop Quantum Gravity. For this purpose we first consider a perturbation series around piecewise linear Loop Quantum Gravity into which standard Loop Quantum Cosmology is embeddable, which hints to use a Liouville Green expansion. Using the Liouville Green expansion and results from asymptotic analysis, we find that an arbitrary cylindrical function has asymptotically almost periodic dependence on the minisuperspace parameter in agreement with arXiv:1010.0449v1 . This allows us to construct an explicit extension of standard Loop Quantum Cosmology that permits embedability."

http://arxiv.org/abs/1012.0077
Evaporation of 2-Dimensional Black Holes
Abhay Ashtekar, Frans Pretorius, Fethi M. Ramazanoğlu
18 pages
(Submitted on 1 Dec 2010)
"We present a detailed analysis of results from a new study of the quantum evaporation of Callan-Giddings-Harvey-Strominger (CGHS) black holes within the mean field approximation. This semi-classical theory incorporates back reaction. Our analytical and numerical calculations show that, while some of the assumptions underlying the standard evaporation paradigm are borne out, several are not. One of the anticipated properties we confirm is that the semi-classical space-time is asymptotically flat at right future null infinity, I_R⁺, yet incomplete in the sense that null observers reach a future Cauchy horizon in finite affine time. Unexpected behavior includes that the Bondi mass traditionally used in the literature can become negative even when the area of the horizon is macroscopic; an improved Bondi mass remains positive until the end of semi-classical evaporation, yet the final value can be arbitrarily large relative to the Planck mass; and the flux of the quantum radiation at I_R⁺ is non-thermal even when the horizon area is large compared to the Planck scale. Furthermore, if the black hole is initially macroscopic, the evaporation process exhibits remarkable universal properties. Although the literature on CGHS black holes is quite rich, these features had escaped previous analyses, in part because of lack of required numerical precision, and in part due to misinterpretation of certain properties and symmetries of the model. Finally, our results provide support for the full quantum scenario recently developed by Ashtekar, Taveras and Varadarajan, and also offer a number of interesting problems to the mathematical relativity and geometric analysis communities."

 

[MTd2]

http://arxiv.org/abs/1012.0119
Entropy density of spacetime and the Navier-Stokes fluid dynamics of null surfaces

T. Padmanabhan
(Submitted on 1 Dec 2010)
It has been known for several decades that Einstein's field equations, when projected onto a null surface, exhibits a structure very similar to non-relativistic Navier-Stokes equation. I show that this result arises quite naturally when gravitational dynamics is viewed as an emergent phenomenon. Extremising the spacetime entropy density associated with the null surfaces leads to a set of equations which, when viewed in the local inertial frame, becomes identical to the Navier-Stokes (NS) equation. This is in contrast with the usual description of Damour-Navier-Stokes (DNS) equation in a general coordinate system, in which there appears a Lie derivative rather than convective derivative. I discuss this difference, its importance and why it is more appropriate to view the equation in a local inertial frame. The viscous force on fluid, arising from the gradient of the viscous stress-tensor, involves the second derivatives of the metric and does not vanish in the local inertial frame while the viscous stress-tensor itself vanishes so that inertial observers detect no dissipation. We thus provide an entropy extremisation principle that leads to the DNS equation, which makes the hydrodynamical analogy with gravity completely natural and obvious. Several implications of these results are discussed.

 

[MTd2]

http://xxx.lanl.gov/abs/1012.0013

An alternative to the gauge theoretic setting

Bert Schroer
(Submitted on 30 Nov 2010)
The standard formulation of gauge theories results from the Lagrangian (functional integral) quantization of classical gauge theories. A more intrinsic qunantum theoretical access in the spirit of Wigner's representation theory shows that there is a fundamental clash between the pointlike localization of zero mass (vector, tensor) potentials and the Hilbert space (positivity, unitarity) structure of QT. The quantization approach has no other way than to stay with pointlike localization and sacrifice the Hilbert space whereas the approach build on the intrinsic quantum concept of modular localization keeps the Hilbert space and trades the conflict creating pointlike generation with the tightest consistent localization:: semiinfinite spacelike string localization. Whereas these potentials in the presence of interactions stay quite close to associated pointlike field strength, the interacting matter fields to which they are coupled bear the brunt of the nonlocal aspect in that they are string.generated in a way which cannot be undone by any differentiation. The new stringlike approach to gauge theory also revives the idea of a Schwinger-Higgs screening mechanism as a deeper and less metaphoric description of the Higgs spontaneous symmetry breaking and its accompanying tale about "God's particle" and its mass generation for all other particles.