Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1009.5953
An effective approach to the problem of time
Martin Bojowald, Philipp A Hoehn, Artur Tsobanjan
19 pages
(Submitted on 29 Sep 2010)
"A practical way to deal with the problem of time in quantum cosmology and quantum gravity is proposed. The main tool is effective equations, which mainly restrict explicit considerations to semiclassical regimes but have the crucial advantage of allowing the consistent use of local internal times in non-deparameterizable systems. Different local internal times are related merely by gauge transformations, thereby enabling relational evolution through turning points of non-global internal times. The main consequence of the local nature of internal time is the necessity of its complex-valuedness, reminiscent of but more general than non-unitarity of evolution defined for finite ranges of time. By several general arguments, the consistency of this setting is demonstrated. Finally, we attempt an outlook on the nature of time in highly quantum regimes. The focus of this note is on conceptual issues."

 

[atyy]

http://arxiv.org/abs/1009.6032
A New Look At The Path Integral Of Quantum Mechanics
Edward Witten
(Submitted on 30 Sep 2010)
The Feynman path integral of ordinary quantum mechanics is complexified and it is shown that possible integration cycles for this complexified integral are associated with branes in a two-dimensional A-model. This provides a fairly direct explanation of the relationship of the A-model to quantum mechanics; such a relationship has been explored from several points of view in the last few years. These phenomena have an analog for Chern-Simons gauge theory in three dimensions: integration cycles in the path integral of this theory can be derived from N=4 super Yang-Mills theory in four dimensions. Hence, under certain conditions, a Chern-Simons path integral in three dimensions is equivalent to an N=4 path integral in four dimensions.

 

[atyy]

http://arxiv.org/abs/1001.2007
The Holographic Universe
Paul McFadden, Kostas Skenderis
(Submitted on 13 Jan 2010)
We present a holographic description of four-dimensional single-scalar inflationary universes in terms of a three-dimensional quantum field theory. The holographic description correctly reproduces standard inflationary predictions in their regime of applicability. In the opposite case, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative QFT and present models capable of satisfying the current observational constraints while exhibiting a phenomenology distinct from standard inflation. This provides a qualitatively new method for generating a nearly scale-invariant spectrum of primordial cosmological perturbations.

 

[MTd2]

http://arxiv.org/abs/1010.0502

Local spinfoam expansion in loop quantum cosmology

Adam Henderson, Carlo Rovelli, Francesca Vidotto, Edward Wilson-Ewing
(Submitted on 4 Oct 2010)
The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and Bianchi I models defined by loop quantum cosmology have recently been translated into a spinfoam-like formalism. The construction is facilitated by the presence of a massless scalar field which is used as an internal clock. The implicit integration over the matter variable leads to a nonlocal spinfoam amplitude. In this paper we consider a vacuum Bianchi I universe and show that by choosing an appropriate regulator a spinfoam expansion can be obtained without selecting a clock variable and that the resulting spinfoam amplitude is local.

 

[MTd2]

http://arxiv.org/abs/1010.0680

Black holes in an ultraviolet complete quantum gravity

Leonardo Modesto, John W. Moffat, Piero Nicolini
(Submitted on 4 Oct 2010)
In this paper we derive the gravity field equations by varying the action for an ultraviolet complete quantum gravity. Then we consider the case of a static source term and we determine an exact black hole solution. As a result we find a regular spacetime geometry: in place of the conventional curvature singularity extreme energy fluctuations of the gravitational field at small length scales provide an effective cosmological constant in a region locally described in terms of a deSitter space. We show that the new metric coincides with the noncommutative geometry inspired Schwarzschild black hole. Indeed we show that the ultraviolet complete quantum gravity, generated by ordinary matter is the dual theory of ordinary Einstein gravity coupled to a noncommutative smeared matter. In other words we obtain further insights about that quantum gravity mechanism which improves Einstein gravity in the vicinity of curvature singularities. This corroborates all the existing literature in the physics and phenomenology of noncommutative black holes.

 

[MTd2]

This paper will be published on Nature!

http://arxiv.org/abs/1010.0793

Quantum gravitational contributions to quantum electrodynamics

David J. Toms
(Submitted on 5 Oct 2010)
Quantum electrodynamics describes the interactions of electrons and photons. Electric charge (the gauge coupling constant) is energy dependent, and there is a previous claim that charge is affected by gravity (described by general relativity) with the implication that the charge is reduced at high energies. But that claim has been very controversial with the situation inconclusive. Here I report an analysis (free from earlier controversies) demonstrating that that quantum gravity corrections to quantum electrodynamics have a quadratic energy dependence that result in the reduction of the electric charge at high energies, a result known as asymptotic freedom.

 

[atyy]

http://arxiv.org/abs/1010.1148
Thermodynamics of photons on fractals
Eric Akkermans, Gerald V. Dunne, Alexander Teplyaev
(Submitted on 6 Oct 2010)
A thermodynamical treatment of a massless scalar field (a "photon") confined to a fractal spatial manifold leads to an equation of state relating pressure to internal energy, $P V_s=U/d_s$, where $d_s$ is the spectral dimension and $V_s$ defines the "spectral volume". For regular manifolds, $V_s$ coincides with the usual geometric spatial volume, but on a fractal this is not necessarily the case. This is further evidence that on a fractal, momentum space can have a different dimension than position space. Our analysis also provides a natural definition of the vacuum (Casimir) energy of a fractal. We suggest ways that these unusual properties might be probed experimentally.

 

[marcus]

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
5 pages
(Submitted on 6 Oct 2010)
"We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. We show that a non-singular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semi-classical proprieties of an initial state are preserved on the other side of the bounce. This model thus enhances the relation between Loop Quantum Cosmology and the full theory."

 

[atyy]

http://arxiv.org/abs/1010.1264
Holographic and Wilsonian Renormalization Groups
Idse Heemskerk, Joseph Polchinski
(Submitted on 6 Oct 2010)
Abstract: We develop parallels between the holographic renormalization group in the bulk and the Wilsonian renormalization group in the dual field theory. Our philosophy differs from much of the previous work on the holographic RG; notable features are the key role of multi-trace operators, and the treatment of the constraints from coordinate invariance. We work out the forms of various single- and double-trace flows. The key question, `what cutoff on the field theory corresponds to a radial cutoff in the bulk?' is left unanswered, but by sharpening the analogy between the two sides we identify possible directions.

http://arxiv.org/abs/1010.1290
Holographic Non-Abelian Charged Hydrodynamics from the Dynamics of Null Horizons
Christopher Eling, Yasha Neiman, Yaron Oz
(Submitted on 6 Oct 2010)
Abstract: We analyze the dynamics of a four-dimensional null hypersurface in a five-dimensional bulk spacetime with Einstein-Yang-Mills fields. In an appropriate ansatz, the projection of the field equations onto the hypersurface takes the form of conservation laws for relativistic hydrodynamics with global non-abelian charges. A Chern-Simons term in the bulk action corresponds to anomalies in the global charges, with a vorticity term arising in the hydrodynamics. We derive the entropy current and obtain unique expressions for some of the leading-order transport coefficients (in the abelian case, all of them) for arbitrary equations of state. As a special case and a concrete example, we discuss the event horizon of a boosted Einstein-Yang-Mills black brane in an asymptotically Anti-de-Sitter spacetime. The evolution equations in that case describe the hydrodynamic limit of a conformal field theory with anomalous global non-abelian charges on the Anti-de-Sitter boundary.

 

[atyy]

http://arxiv.org/abs/1009.1771
Awaking the vacuum in relativistic stars
William C. C. Lima, George E. A. Matsas, Daniel A. T. Vanzella
(Submitted on 9 Sep 2010)
Void of any inherent structure in classical physics, the vacuum has revealed to be incredibly crowded with all sorts of processes in relativistic quantum physics. Yet, its direct effects are usually so subtle that its structure remains almost as evasive as in classical physics. Here, in contrast, we report on the discovery of a novel effect according to which the vacuum is compelled to play an unexpected central role in an astrophysical context. We show that the formation of relativistic stars may lead the vacuum energy density of a quantum field to an exponential growth. The vacuum-driven evolution which would then follow may lead to unexpected implications for astrophysics, while the observation of stable neutron-star configurations may teach us much on the field content of our Universe.

 

[atyy]

http://arxiv.org/abs/1010.0912
Asymptotic safety and the gauged SU(N) nonlinear sigma-model
M. Fabbrichesi, R. Percacci, A. Tonero, O. Zanusso
(Submitted on 5 Oct 2010)
We study the beta functions of the leading, two-derivative terms of the left-gauged SU(N) nonlinear sigma-model in d dimensions. In d>2, we find the usual Gaussian ultraviolet fixed point for the gauge coupling and an attractive non-Gaussian fixed point for the Goldstone boson coupling. The position of the latter fixed point controls the chiral expansion, unitarity and the strength of the tree-level Goldstone boson scattering amplitudes. For large N the model is weakly coupled, unitary at all energies and well described by the lowest order of chiral perturbation theory. Attention is payed to the gauge- and scheme-dependence of the results.

 

[marcus]

http://arxiv.org/abs/1010.1637
Quantization of Midisuperspace Models
J. Fernando Barbero G., Eduardo J. S. Villaseñor
To appear in Living Reviews in Relativity
(Submitted on 8 Oct 2010)
"We give a comprehensive review of the quantization of midisuperspace models. Though the main focus of the paper is on quantum aspects, we also provide an introduction to several classical points related to the definition of these models. We cover some important issues, in particular, the use of the principle of symmetric criticality as a very useful tool to obtain the required Hamiltonian formulations. Two main types of reductions are discussed: those involving metrics with two Killing vector fields and spherically symmetric models. We also review the more general models obtained by coupling matter fields to these systems. Throughout the paper we give separate discussions for standard quantizations using geometrodynamical variables and those relying on loop quantum gravity inspired methods."

 

[marcus]

http://arxiv.org/abs/1010.1939
Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
8 pages, 3 figures
(Submitted on 10 Oct 2010)
"New progress in loop gravity has lead to a simple model of 'general-covariant quantum field theory'. I sum up the definition of the model in self-contained form, in terms accessible to those outside the subfield. I emphasize its formulation as a generalized topological quantum field theory with an infinite number of degrees of freedom, and its relation to lattice theory. I list the indications supporting the conjecture that the model is related to general relativity and UV finite."

http://arxiv.org/abs/1010.2067
Algorithmic Thermodynamics
John C. Baez, Mike Stay
20 pages, 1 figure
(Submitted on 11 Oct 2010)
"Algorithmic entropy can be seen as a special case of entropy as studied in statistical mechanics. This viewpoint allows us to apply many techniques developed for use in thermodynamics to the subject of algorithmic information theory. In particular, suppose we fix a universal prefix-free Turing machine and let X be the set of programs that halt for this machine. Then we can regard X as a set of 'microstates', and treat any function on X as an 'observable'. For any collection of observables, we can study the Gibbs ensemble that maximizes entropy subject to constraints on expected values of these observables. We illustrate this by taking the log runtime, length, and output of a program as observables analogous to the energy E, volume V and number of molecules N in a container of gas. The conjugate variables of these observables allow us to define quantities which we call the 'algorithmic temperature' T, 'algorithmic pressure' P and algorithmic potential' mu, since they are analogous to the temperature, pressure and chemical potential. We derive an analogue of the fundamental thermodynamic relation dE = T dS - P d V + mu dN, and use it to study thermodynamic cycles analogous to those for heat engines. We also investigate the values of T, P and mu for which the partition function converges. At some points on the boundary of this domain of convergence, the partition function becomes uncomputable. Indeed, at these points the partition function itself has nontrivial algorithmic entropy."

 

[atyy]

http://arxiv.org/abs/1004.2952
Lectures on LQG/LQC
Ghanashyam Date
(Submitted on 17 Apr 2010)
A School on Loop Quantum Gravity was held at the IMSc during Sept 8 -- 18, 2009. In the first week a basic introduction to LQG was provided while in the second week the focus was on the two main application, to cosmology (LQC) and to the black hole entropy. These notes are an expanded written account of the lectures that I gave. These are primarily meant for beginning researchers.

http://arxiv.org/abs/1010.2062
Lectures on Constrained Systems
Ghanashyam Date
(Submitted on 11 Oct 2010)
These lecture notes were prepared as a basic introduction to the theory of constrained systems which is how the fundamental forces of nature appear in their Hamiltonian formulation. Only a working knowledge of Lagrangian and Hamiltonian formulation of mechanics is assumed. These notes are based on the set of eight lectures given at the Refresher Course for College Teachers held at IMSc during May-June, 2005. These are submitted to the arxiv for an easy access to a wider body of students.

 

[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.