Loop-and-allied QG bibliography

[member 11137]

http://arxiv.org/abs/1008.0283
Unknown system boundaries cannot be determined within quantum Darwinism
Authors: Chris Fields
(Submitted on 2 Aug 2010)
Abstract: Observers restricted to interactions with environmental degrees of freedom that nondestructively encode pointer states of a system of interest S cannot determine from such interactions which degrees of freedom of S interact directly or indirectly with the environment E. Without a specification of the S-E boundary, such observers cannot use einselection and quantum Darwinism to calculate the pointer states of S or their environmental encodings. Quantum Darwinism requires S-E boundary specifications assumed or stipulated on the basis of classical-scale observations, and therefore cannot be regarded as providing a predictive, purely quantum-mechanical explanation of the "emergence" of classicality.

http://arxiv.org/abs/1008.0373
Little Boxes: The Simplest Demonstration of the Failure of Einstein's Attempt to Show the Incompleteness of Quantum Theory
Authors: John D. Norton
(Submitted on 2 Aug 2010)
Abstract: The failure of Einstein's co-authored "EPR" attempt to show the incompleteness of quantum theory is demonstrated directly for spatial degrees of freedom using only elementary notions. A GHZ construction is realized in the position properties of three particles whose quantum waves are distributed over three two-chambered boxes. The same system is modeled more realistically using three spatially separated, singly ionized hydrogen molecules.

http://arxiv.org/abs/1008.0033
Emergent Photons and Gravitons:The Problem of Vacuum Structure
Authors: J. D. Bjorken
(Submitted on 30 Jul 2010)
Abstract: We discuss vacuum condensates associated with emergent QED and with torsion, as well as the possible role of the Kodama wave function in quantum cosmology

http://arxiv.org/abs/1008.0154
Gauge Theories under Incorporation of a Generalized Uncertainty Principle
Authors: Martin Kober
(Submitted on 1 Aug 2010)
Abstract: It 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 it 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.

Thanks Marcus for all these very interesting references (I like the Bjorken's article on emergent photons and gravitons) and the introduction of considerations concerning the torsion into the theory. Respect for Martin Kober.

 

[marcus]

Blackforest,
Thanks go to John86 for spotting the papers you mentioned! However we don't usually include comment/discussion in this bibliography thread. Including more than an occasional very brief comment would risk overloading the thread. Please feel free to start a separate thread for discussion of any paper(s) you find especially interesting, or to send us PMs (private messages) related to this collection of links to articles.

 

[marcus]

http://arxiv.org/abs/1008.1196
The gravitational effect of the vacuum
George F. R. Ellis, Jeff Murugan, Henk van Elst
(Submitted on 6 Aug 2010)
"The quantum field theoretic prediction for the vacuum energy density leads to a value for the effective cosmological constant that is incorrect by between 60 to 120 orders of magnitude. We review an old proposal of replacing Einstein's Field Equations by their trace-free part (the Trace-Free Einstein Equations), together with an independent assumption of energy--momentum conservation by matter fields. We confirm that while this does not solve the fundamental issue of why the cosmological constant has the value it has, it is indeed a viable theory that resolves the problem of the discrepancy between the vacuum energy density and the observed value of the cosmological constant. We also point out that this proposal may have a valid quantum field theory basis in terms of a spin-2 field theory for the graviton interaction with matter."

Briefly noted (partly because of the authors' prominence)
http://arxiv.org/abs/1008.1066
Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics
Anthony Aguirre, Max Tegmark, David Layzer
17 pages, 2 figures
(Submitted on 5 Aug 2010)
"We study the quantum measurement problem in the context of an infinite, statistically uniform space, as could be generated by eternal inflation. It has recently been argued that when identical copies of a quantum measurement system exist, the standard projection operators and Born rule method for calculating probabilities must be supplemented by estimates of relative frequencies of observers. We argue that an infinite space actually renders the Born rule redundant, by physically realizing all outcomes of a quantum measurement in different regions..."

 

[marcus]

http://arxiv.org/abs/1008.1312
Comment on arXiv:1007.0718 by Lee Smolin
Sabine Hossenfelder
(Submitted on 7 Aug 2010)
"In a recent paper it was suggested a novel interpretation of deformed special relativity. In that new approach, nonlocal effects that had previously been shown to occur and be incompatible with experiment to high precision, are interpreted as coordinate artifacts that do not lead to real physical consequences. It is argued here that if one follows through the consequences of this thought, one finds that the theory one is dealing with needs to be ordinary special relativity to precision even better than the bound on nonlocal effect already requires. Consequently, the new approach cannot be understood as a version of deformed special relativity that circumvents the bound."

http://arxiv.org/abs/1008.1340
Properties of Quantum Graphity at Low Temperature
Francesco Caravelli, Fotini Markopoulou
19 pages, 4 figures
(Submitted on 7 Aug 2010)
"We present a mapping of dynamical graphs, and, in particular, the graphs used in the Quantum Graphity models for emergent geometry, into an Ising hamiltonian on the line graph of a complete graph with a fixed number of vertices. We use this method to study the properties of Quantum Graphity models at low temperature in the limit in which the valence coupling constant of the model is much greater than the coupling constants of the loop terms. Using mean field theory we find that an order parameter for the model is the average valence of the graph. We calculate the equilibrium distribution for the valence as an implicit function of the temperature. In the approximation in which the temperature is low, we find the first two Taylor coefficients of the valence in the temperature expansion. A discussion of the susceptibility function and a generalization of the model are given in the end."

 

[marcus]

http://arxiv.org/abs/1008.1759
Unimodular loop quantum gravity and the problems of time
Lee Smolin
14 pages
(Submitted on 10 Aug 2010)
"We develop the quantization of unimodular gravity in the Plebanski and Ashtekar formulations and show that the quantum effective action defined by a formal path integral is unimodular. This means that the effective quantum geometry does not couple to terms in the expectation value of the energy-momentum tensor proportional to the metric tensor. The path integral takes the same form as is used to define spin foam models, with the additional constraint that the determinant of the four metric is constrained to be a constant by a gauge fixing term. We also review the proposal of Unruh, Wald and Sorkin--that the hamiltonian quantization yields quantum evolution in a physical time variable equal to elapsed four volume--and discuss how this may be carried out in loop quantum gravity. This then extends the results of arXiv:0904.4841 to the context of loop quantum gravity."

 

[ensabah6]

Still searching for SUSY -- the discovery of SUSY is direct relevance to QG

http://arxiv.org/abs/1008.0407

High Energy Physics - Phenomenology
Title: It's On: Early Interpretations of ATLAS Results in Jets and Missing Energy Searches
Authors: Daniele S. M. Alves, Eder Izaguirre, Jay G. Wacker
(Submitted on 2 Aug 2010)

Abstract: The first search for supersymmetry from ATLAS with 70/nb of integrated luminosity sets new limits on colored particles that decay into jets plus missing transverse energy. For gluinos that decay directly or through a one step cascade into the LSP and two jets, these limits translate into a bound of m_g > 205 GeV, regardless of the mass of the LSP. In some cases the limits extend up to m_g ~= 295 GeV, already surpassing the Tevatron's reach for compressed supersymmetry spectra.

Comments: 5 pages, 3 figures, 1 table
Subjects: High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Experiment (hep-ex)
Cite as: arXiv:1008.0407v1 [hep-ph]

 

[marcus]

Actual experimental aparatus, measurements and all that, see figure 2 on page 6
http://arxiv.org/abs/1008.1911
Measurement of stimulated Hawking emission in an analogue system
Silke Weinfurtner, Edmund W. Tedford, Matthew C. J. Penrice, William G. Unruh, Gregory A. Lawrence
(Submitted on 11 Aug 2010)
"There is a mathematical analogy between the propagation of fields in a general relativistic space-time and long (shallow water) surface waves on moving water. Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. Similar arguments predict the same effect near wave horizons in fluid flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process."

The next is by an author who comes to Quantum Field Theory from the direction of condensed matter, which is his main specialty at ETH Zürich. Posted in case others are especially interested in that approach--I personally cannot judge the merit.

http://arxiv.org/abs/1008.1867
Dynamic coarse-graining approach to quantum field theory
Hans Christian Öttinger
4 pages
(Submitted on 11 Aug 2010)
"We build quantum field theory on the thermodynamic master equation for dissipative quantum systems. The vacuum is represented by a thermodynamic equilibrium state; even in the low-temperature limit, the population and evolution of excited states matter. All regularization is consistently provided by a friction mechanism; with decreasing friction parameter, only shorter and shorter scales are damped out of a quantum field theory. No divergent integrals need to be manipulated, no counterterms need to be invented. Relativistic covariance is recovered in the final results. We illustrate the proposed thermodynamic approach to quantum fields for the phi⁴ theory."

 

[marcus]

http://arxiv.org/abs/1008.2768
Asymptotic Safety, Singularities, and Gravitational Collapse
Roberto Casadio, Stephen D.H. Hsu, Behrouz Mirza
6 pages
(Submitted on 16 Aug 2010)
"Asymptotic safety (an ultraviolet fixed point with finite-dimensional critical surface) offers the possibility that a predictive theory of quantum gravity can be obtained from the quantization of classical general relativity. However, it is unclear what becomes of the singularities of classical general relativity, which, it is hoped, might be resolved by quantum effects. We study dust collapse with a running gravitational coupling and find that a future singularity can be avoided if the coupling becomes exactly zero at some finite energy scale. The singularity can also be avoided (pushed off to infinite proper time) if the coupling approaches zero sufficiently rapidly at high energies. However, the evolution deduced from perturbation theory still implies a singularity at finite proper time."

Noted in view of author's prominence:
http://arxiv.org/abs/1008.2764
Nonlocality as Evidence for a Multiverse Cosmology
Frank J. Tipler
(Submitted on 16 Aug 2010)
"I show that observations of quantum nonlocality can be interpreted as purely local phenomena, provided one assumes that the cosmos is a multiverse..."

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

http://arxiv.org/abs/1008.2962
Kinematics of a relativistic particle with de Sitter momentum space
Michele Arzano, Jerzy Kowalski-Glikman
12 pages
(Submitted on 17 Aug 2010)
"We discuss kinematical properties of a free relativistic particle with deformed phase space in which momentum space is given by (a submanifold of) de Sitter space. We provide a detailed derivation of the action, Hamiltonian structure and equations of motion for such free particle. We study the action of deformed relativistic symmetries on the phase space and derive explicit formulas for the action of the deformed Poincare' group. Finally we provide a discussion on parametrization of the particle worldlines stressing analogies and differences with ordinary relativistic kinematics."

 

[MTd2]

Asymptotic safety bibliography on Percacci's website.

http://www.percacci.it/roberto/physics/as/biblio.html

This is constantly updated. From the beginnings to the actual days.

 

[MTd2]

Another answer to the Smolin vs. Bee discussion about VSL:

http://arxiv.org/abs/1008.2962

Kinematics of a relativistic particle with de Sitter momentum space

Michele Arzano, Jerzy Kowalski-Glikman
(Submitted on 17 Aug 2010)
We discuss kinematical properties of a free relativistic particle with deformed phase space in which momentum space is given by (a submanifold of) de Sitter space. We provide a detailed derivation of the action, Hamiltonian structure and equations of motion for such free particle. We study the action of deformed relativistic symmetries on the phase space and derive explicit formulas for the action of the deformed Poincare' group. Finally we provide a discussion on parametrization of the particle worldlines stressing analogies and differences with ordinary relativistic kinematics.

 

[John86]

something different from Stu Kauffman

http://www.npr.org/blogs/13.7/2010/08/17/129250892/res-extensa-res-potentia-and-the-poised-realm

 

[MTd2]

http://arxiv.org/abs/1008.3345

Deformed Special Relativity from Asymptotically Safe Gravity

Xavier Calmet, Sabine Hossenfelder, Roberto Percacci
(Submitted on 19 Aug 2010)
By studying the notion of a fundamentally minimal length scale in asymptotically safe gravity we find that a specific version of deformed special relativity (DSR) naturally arises in this approach. We then consider two thought experiments to examine the interpretation of the scenario and discuss similarities and differences to other approaches to DSR.

 

[atyy]

http://arxiv.org/abs/1008.3439
Measuring Black Hole Formations by Entanglement Entropy via Coarse-Graining
Tadashi Takayanagi, Tomonori Ugajin
(Submitted on 20 Aug 2010)
We argue that the entanglement entropy offers us a useful coarse-grained entropy in time-dependent AdS/CFT. We show that the total von-Neumann entropy remains vanishing even when a black hole is created in a gravity dual, being consistent with the fact that its corresponding CFT is described by a time-dependent pure state. We analytically calculate the time evolution of entanglement entropy for a free Dirac fermion on a circle following a quantum quench. This is interpreted as a toy holographic dual of black hole creations and annihilations. It is manifestly free from the black hole information problem.

http://arxiv.org/abs/1008.3494
Measurement of Hawking Radiation with Ions in the Quantum Regime
Birger Horstmann, Ralf Schützhold, Benni Reznik, Serena Fagnocchi, J. Ignacio Cirac
(Submitted on 20 Aug 2010)
Abstract: This article discusses a recent proposal for the simulation of acoustic black holes with ions http://arxiv.org/abs/0904.4801. The ions are rotating on a ring with an inhomogeneous, but stationary velocity profile. Phonons cannot leave a region, in which the ion velocity exceeds the group velocity of the phonons, like light cannot escape from a black hole. The system is described by a discrete field theory with a nonlinear dispersion relation. Hawking radiation is emitted by the black hole, generating entanglement between its inside and its outside. We study schemes to detect the Hawking effect in this setup.

 

[marcus]

http://arxiv.org/abs/1008.3621
Asymptotic Safety, Emergence and Minimal Length
R. Percacci, G. P. Vacca
20 pages, 2 figures
(Submitted on 21 Aug 2010)
"There seems to be a common prejudice that asymptotic safety is either incompatible with, or at best unrelated to, the other topics in the title. This is not the case. In fact, we show that 1) the existence of a fixed point with suitable properties is a promising way of deriving emergent properties of gravity, and 2) there is a precise sense in which asymptotic safety implies a minimal length. In so doing we also discuss possible signatures of asymptotic safety in scattering experiments."

 

[MTd2]

http://arxiv.org/abs/1008.3886

The Photon Dispersion as an Indicator for New Physics ?

Wolfgang Bietenholz
(Submitted on 23 Aug 2010)
We first comment on the search for a deviation from the linear photon dispersion relation, in particular based on cosmic photons from Gamma Ray Bursts. Then we consider the non-commutative space as a theoretical concept that could lead to such a deviation, which would be a manifestation of Lorentz Invariance Violation. In particular we review a numerical study of pure U(1) gauge theory in a 4d non-commutative space. Starting from a finite lattice, we explore the phase diagram and the extrapolation to the continuum and infinite volume. These simultaneous limits - taken at fixed non-commutativity - lead to a phase of broken Poincare symmetry, where the photon appears to be IR stable, despite a negative IR divergence to one loop.

 

[marcus]

http://arxiv.org/abs/1008.4093
Lifting SU(2) Spin Networks to Projected Spin Networks
Maité Dupuis, Etera R. Livine
14 pages
(Submitted on 24 Aug 2010)
"Projected spin network states are the canonical basis of quantum states of geometry for the most recent EPR-FK spinfoam models for quantum gravity. They are functionals of both the Lorentz connection and the time normal field. We analyze in details the map from these projected spin networks to the standard SU(2) spin networks of loop quantum gravity. We show that this map is not one-to-one and that the corresponding ambiguity is parameterized by the Immirzi parameter. We conclude with a comparison of the scalar products between projected spin networks and SU(2) spin network states."

http://arxiv.org/abs/1008.3980
Noncommutative Geometric Spaces with Boundary: Spectral Action
Ali H. Chamseddine, Alain Connes
26 pages
(Submitted on 24 Aug 2010)
"We study spectral action for Riemannian manifolds with boundary, and then generalize this to noncommutative spaces which are products of a Riemannian manifold times a finite space. We determine the boundary conditions consistent with the hermiticity of the Dirac operator. We then define spectral triples of noncommutative spaces with boundary. In particular we evaluate the spectral action corresponding to the noncommutative space of the standard model and show that the Einstein-Hilbert action gets modified by the addition of the extrinsic curvature terms with the right sign and coefficient necessary for consistency of the Hamiltonian. We also include effects due to the addition of dilaton field."

 

[marcus]

http://arxiv.org/abs/1008.4351
Stability of the aether
William Donnelly, Ted Jacobson
4 pages
(Submitted on 25 Aug 2010)
"The requirements for stability of a Lorentz violating theory are analyzed. In particular we conclude that Einstein-aether theory can be stable when its modes have any phase velocity, rather than only the speed of light as was argued in a recent paper."

 

[MTd2]

http://arxiv.org/abs/1008.4147

Emergent Gauge Fields

Peter G.O. Freund
(Submitted on 24 Aug 2010)
Erik Verlinde's proposal of the emergence of the gravitational force as an entropic force is extended to abelian and non-abelian gauge fields and to matter fields. This suggests a picture with no fundamental forces or forms of matter whatsoever.

 

[marcus]

http://arxiv.org/abs/1008.4787
Comment on Asymptotically Safe Inflation
S.-H. Henry Tye, Jiajun Xu
4 pages
(Submitted on 27 Aug 2010)
"We comment on Weinberg's interesting analysis of asymptotically safe inflation (arXiv:0911.3165). We find that even if the gravity theory exhibits an ultraviolet fixed point, the energy scale during inflation is way too low to drive the theory close to the fixed point value. We choose the specific renormalization groupflow away from the fixed point towards the infrared region that reproduces the Newton's constant and today's cosmological constant. We follow this RG flow path to scales below the Planck scale to study the stability of the inflationary scenario. Again, we find that some fine tuning is necessary to get enough efolds of infflation in the asymptotically safe inflationary scenario."
[edit: http://arxiv.org/abs/0912.4093 ?]

 

[MTd2]

http://arxiv.org/abs/1008.4805

Space-time and special relativity from causal networksGiacomo Mauro D'Ariano, Alessandro Tosini
(Submitted on 27 Aug 2010)
We show how the Minkowskian space-time emerges from a topologically homogeneous causal network, presenting a simple analytical derivation of the Lorentz transformations, with metric as pure event-counting. The derivation holds generally for d=1 space dimension, however, it can be extended to d>1 for special causal networks.

 

[MTd2]

http://arxiv.org/abs/1008.5132

Massive Gravity: Resolving the Puzzles

Lasma Alberte, Ali H. Chamseddine, Viatcheslav Mukhanov
(Submitted on 30 Aug 2010)
We consider the massless limit of Higgs gravity, where the graviton becomes massive when the scalar fields acquire expectation values. We determine the Vainshtein scale and prove that massive gravity smoothly goes to General Relativity below this scale. We find that the Vainshtein scale depends on the particular action of scalar fields used to give mass to the graviton.

 

[John86]

http://arxiv.org/abs/1008.4639
Any spacetime has a Bianchi type I spacetime as a limit
Authors: Bethan Cropp (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 27 Aug 2010)
Abstract: Pick an arbitrary timelike geodesic in an arbitrary spacetime. We demonstrate that there is a particular limiting process, an "ultra-local limit", in which the immediate neighborhood of the timelike geodesic can be "blown up" to yield a general (typically non-diagonal) Bianchi type I spacetime. This process shares some (but definitely not all) of the features of the Penrose limit, whereby the immediate neighborhood of an arbitrary null geodesic is "blown up" to yield a pp-wave as a limit.

http://arxiv.org/abs/1008.5006
New Horizons in Gravity: The Trace Anomaly, Dark Energy and Condensate Stars
Authors: Emil Mottola
(Submitted on 30 Aug 2010)
Abstract: General Relativity receives quantum corrections relevant at macroscopic distance scales and near event horizons. These arise from the conformal scalar degrees of freedom in the extended effective field theory of gravity generated by the trace anomaly of massless quantum fields in curved space. The origin of these conformal scalar degrees of freedom as massless poles in two-particle intermediate states of anomalous amplitudes in flat space is exposed. At event horizons the conformal anomaly scalar degrees of freedom can have macroscopically large effects on the geometry, potentially removing the classical event horizon of black hole and cosmological spacetimes, replacing them with a quantum boundary layer where the effective value of the gravitational vacuum energy density can change. In the effective theory, the cosmological term becomes a dynamical condensate, whose value depends upon boundary conditions near the horizon. In the conformal phase where the anomaly induced fluctutations dominate, and the condensate dissolves, the effective cosmological "constant" is a running coupling which has an infrared stable fixed point at zero. By taking a positive value in the interior of a fully collapsed star, the effective cosmological term removes any singularity, replacing it with a smooth dark energy interior. The resulting gravitational condensate star configuration resolves all black hole paradoxes, and provides a testable alternative to black holes as the final state of complete gravitational collapse. The observed dark energy of our universe likewise may be a macroscopic finite size effect whose value depends not on microphysics but on the cosmological horizon scale.

 

[marcus]

http://arxiv.org/abs/1008.5348
Noncommutative Geometry Spectral Action as a framework for unification: Introduction and phenomenological/cosmological consequences
Mairi Sakellariadou
11 pages; Invited contribution for Mario Castagnino's Festschrift, to be published in IJMPD
(Submitted on 31 Aug 2010)
"I will summarize Noncommutative Geometry Spectral Action, an elegant geometrical model valid at unification scale, which offers a purely gravitational explanation of the Standard Model, the most successful phenomenological model of particle physics. Noncommutative geometry states that close to the Planck energy scale, space-time has a fine structure and proposes that it is given as the product of a four-dimensional continuum compact Riemaniann manifold by a tiny discrete finite noncommutative space. The spectral action principle, a universal action functional on spectral triples which depends only on the spectrum of the Dirac operator, applied to this almost commutative product geometry, leads to the full Standard Model, including neutrino mixing which has Majorana mass terms and a see-saw mechanism, minimally coupled to gravity. It also makes various predictions at unification scale. I will review some of the phenomenological and cosmological consequences of this beautiful and purely geometrical approach to unification."

 

[MTd2]

http://arxiv.org/abs/1008.5154

Structure of Lanczos-Lovelock Lagrangians in Critical Dimensions

Alexandre Yale, ***T. Padmanabhan***
(Submitted on 30 Aug 2010)
The Lanczos-Lovelock models of gravity constitute the most general theories of gravity in D dimensions which satisfy (a) the principle of of equivalence, (b) the principle of general co-variance, and (c) have field equations involving derivatives of the metric tensor only up to second order. The mth order Lanczos-Lovelock Lagrangian is a polynomial of degree m in the curvature tensor. The field equations resulting from it become trivial in the critical dimension $D = 2m$ and the action itself can be written as the integral of an exterior derivative of an expression involving the vierbeins, in the differential form language. While these results are well known, there is some controversy in the literature as to whether the Lanczos-Lovelock Lagrangian itself can be expressed as a total divergence of quantities built only from the metric and its derivatives (without using the vierbeins) in $D = 2m$. We settle this issue by showing that this is indeed possible and provide an algorithm for its construction. In particular, we demonstrate that, in two dimensions, $R \sqrt{-g} = \partial_j R^j$ for a doublet of functions $R^j = (R^0,R^1)$ which depends only on the metric and its first derivatives. We explicitly construct families of such R^j -s in two dimensions. We also address related questions regarding the Gauss-Bonnet Lagrangian in $D = 4$. Finally, we demonstrate the relation between the Chern-Simons form and the mth order Lanczos-Lovelock Lagrangian.

¨¬Somewhere in the beginning of the text he says that this generalizations have a natural generalization of the thermo-holograhic principle to any dimension.
http://arxiv.org/abs/1008.5288

Relative entropy as a measure of inhomogeneity in general relativity

Nikolas Akerblom, Gunther Cornelissen
(Submitted on 31 Aug 2010)
We introduce the notion of relative volume entropy for two spacetimes with preferred compact spacelike foliations. This is accomplished by applying the notion of Kullback-Leibler divergence to the volume elements induced on spacelike slices. The resulting quantity gives a lower bound on the number of bits which are necessary to describe one metric given the other. For illustration, we study some examples, in particular gravitational waves, and conclude that the relative volume entropy is a suitable device for quantitative comparison of the inhomogeneity of two spacetimes.

http://arxiv.org/abs/1008.5192

Does Quantum Mechanics Clash with the Equivalence Principle - and Does it Matter?

Elias Okon, Craig Callender
(Submitted on 31 Aug 2010)
With an eye on developing a quantum theory of gravity, many physicists have recently searched for quantum challenges to the equivalence principle of general relativity. However, as historians and philosophers of science are well aware, the principle of equivalence is not so clear. When clarified, we think quantum tests of the equivalence principle won't yield much. The problem is that the clash/not-clash is either already evident or guaranteed not to exist. Nonetheless, this work does help teach us what it means for a theory to be geometric.

 

[qsa]

http://arxiv.org/PS_cache/arxiv/pdf/1008/1008.5169v1.pdf

Discrete mechanics: a kinematics for a particular case of causal sets
Authors: Alexey L. Krugly

Subjects: General Relativity and Quantum Cosmology (gr-qc)

The model is a particular case of causal set. This is a discrete model of spacetime in a microscopic level. In paper the most general properties of the model are investigated without any reference to a dynamics. The dynamics of the model is introduced in [arXiv: 1004.5077]. These two papers introduce a consistent description of the model.

 

[qsa]

http://arxiv.org/abs/1004.5077


Discrete mechanics: a sequential growth dynamics for causal sets, and a self-organization of particles


Authors: Alexey L. Krugly

(Submitted on 28 Apr 2010)
Abstract: A model of a discrete pregeometry on a microscopic scale is introduced. This model is a finite network of finite elementary processes. The mathematical description is a d-graph that is a generalization of a graph. This is the particular case of a causal set. The aim of this study is to construct the particles as emergent structures. The particles in this model must be cyclic processes. The general dynamics and several examples are given. A simple dynamics generates a hierarchy of cyclic processes. An algebraic representation of this dynamics is given. It is based on the algebra of creation and destruction operators. Loops are described by bosonic operators and causal connections are described by fermionic operators.

 

[atyy]

http://arxiv.org/abs/1009.0268
Stability of spin-0 graviton and strong coupling in Horava-Lifgarbagez theory of gravity
Anzhong Wang, Qiang Wu
(Submitted on 1 Sep 2010)
In this paper, we consider two different issues, stability and strong coupling, raised lately in the newly-proposed Horava-Lifgarbagez (HL) theory of quantum gravity with projectability condition. We find that all the scalar modes are stable in the de Sitter background, due to two different kinds of effects, one from high-order derivatives of the spacetime curvature, and the other from the exponential expansion of the de Sitter space. Combining these effects properly, one can make the instability found in the Minkowski background never raise even for small-scale modes, provided that the IR limit is sufficiently closed to the relativistic fixed point. At the fixed point, all the modes become stabilized, which is expected, as it is well-known that the de Sitter spacetime is stable in general relativity. We also show that the instability of Minkowski spacetime can be cured by introducing mass to the spin-0 graviton. The strong coupling problem is investigated following the effective field theory approach, and found that it cannot be cured by the Blas-Pujolas-Sibiryakov mechanism, initially designed for the case without projectability condition, but might be solved by the Vainshtein mechanism. In fact, we construct a class of non-perturbative solutions, and show explicitly that it reduces smoothly to the de Sitter spacetime in the relativistic limit.

 

[marcus]

http://arxiv.org/abs/1009.0669
Probing the small distance structure of canonical quantum gravity using the conformal group
Gerard 't Hooft
20 pages, 1 figure
(Submitted on 3 Sep 2010)
"In canonical quantum gravity, the formal functional integral includes an integration over the local conformal factor, and we propose to perform the functional integral over this factor before doing any of the other functional integrals. By construction, the resulting effective theory would be expected to be conformally invariant and therefore finite. However, also the conformal integral itself diverges, and therefore the actual situation is more delicate. The effects of a renormalization counter term are considered, including the associated problem of unitarity violation, such as a Landau-like ghost. Adding (massive or massless) matter fields does not change the picture; to confirm this, detailed calculations were necessary, and they are presented. Some alternative ideas are offered, including a more daring speculation, which is that no counter term should be allowed for at all. This has far-reaching and important consequences, which we discuss. A surprising picture emerges of quantized elementary particles interacting with a gravitational field that is "partly classical". This approach was inspired by a search towards the reconciliation of Hawking radiation with unitarity and locality, and it offers basic new insights there."

 

[atyy]

http://arxiv.org/abs/1008.4774
Tensor network states and algorithms in the presence of a global U(1) symmetrySukhwinder Singh, Robert N. C. Pfeifer, Guifre Vidal
(Submitted on 27 Aug 2010)
Tensor network decompositions offer an efficient description of certain many-body states of a lattice system and are the basis of a wealth of numerical simulation algorithms. In a recent paper [arXiv:0907.2994v1] we discussed how to incorporate a global internal symmetry, given by a compact, completely reducible group G, into tensor network decompositions and algorithms. Here we specialize to the case of Abelian groups and, for concreteness, to a U(1) symmetry, often associated with particle number conservation. We consider tensor networks made of tensors that are invariant (or covariant) under the symmetry, and explain how to decompose and manipulate such tensors in order to exploit their symmetry. In numerical calculations, the use of U(1) symmetric tensors allows selection of a specific number of particles, ensures the exact preservation of particle number, and significantly reduces computational costs. We illustrate all these points in the context of the multi-scale entanglement renormalization ansatz.

 

[marcus]

Steve Carlip and George Ellis are both major figures in GR and cosmology. So this is news. A book edited by Ellis with a QG chapter by Carlip.
http://arxiv.org/abs/1009.1136
The Small Scale Structure of Spacetime
Steven Carlip
14 pages, 2 figures; to appear in Foundations of Space and Time, edited by George Ellis, Jeff Murugan, Amanda Weltman (Cambridge University Press)
(Submitted on 6 Sep 2010)
"Several lines of evidence hint that quantum gravity at very small distances may be effectively two-dimensional. I summarize the evidence for such 'spontaneous dimensional reduction', and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests a fascinating relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity."

It sounds like the same talk he gave at the XXV Max Born conference on Planck Scale, in 2009.

Brief mention:
http://arxiv.org/abs/1009.0850
Natural Neutrino Dark Energy
Ilya Gurwich
14 pages, 6 figures
"A new class of neutrino dark energy models is presented. ... These predictions, can be verified (or disproved) with future experiments. At this point, the strongest constraints on these models are obtained from big bang nucleosynthesis, and lead to new bounds on the mass of the lightest neutrino."