Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1103.5993
Vacuum Fluctuations and the Small Scale Structure of Spacetime
S. Carlip, R. A. Mosna, J. P. M. Pitelli
4 pages
(Submitted on 30 Mar 2011)
"We show that vacuum fluctuations of the stress-energy tensor in two-dimensional dilaton gravity lead to a sharp focusing of light cones near the Planck scale, effectively breaking space up into a large number of causally disconnected regions. This phenomenon, called 'asymptotic silence' when it occurs in cosmology, might help explain several puzzling features of quantum gravity, including evidence of spontaneous dimensional reduction at short distances. While our analysis focuses on a simplified two-dimensional model, we argue that the qualitative features should still be present in four dimensions."

 

[marcus]

http://arxiv.org/abs/1103.6264
Spin foam models with finite groups
Benjamin Bahr, Bianca Dittrich, James P. Ryan
47 pages, 6 figures
(Submitted on 31 Mar 2011)
"Spin foam models, loop quantum gravity and group field theory are discussed as quantum gravity candidate theories and usually involve a continuous Lie group. We advocate here to consider quantum gravity inspired models with finite groups, firstly as a test bed for the full theory and secondly as a class of new lattice theories possibly featuring an analogue diffeomorphism symmetry. To make these notes accessible to readers outside the quantum gravity community we provide an introduction to some essential concepts in the loop quantum gravity, spin foam and group field theory approach and point out the many connections to lattice field theory and condensed matter systems."

brief mention:
http://arxiv.org/abs/1103.6272
Directions in Causal Set Quantum Gravity
Sumati Surya
31 pages, 7 figures, To appear in Recent Research in Quantum Gravity, edited by A. Dasgupta (Nova Science Publishers NY)
(Submitted on 31 Mar 2011)
"... Recent highlights include a causal set expression for the Einstein-Hilbert action and the construction of a scalar field Feynman propagator on a fixed causal set. The aim of the present article is to give a broad overview of the results in causal set theory while pointing out directions for future investigations."

 

[atyy]

http://arxiv.org/abs/1104.0561
Modeling of Time with Metamaterials
Igor I. Smolyaninov, Yu-Ju Hung
(Submitted on 4 Apr 2011)
Metamaterials have been already used to model various exotic "optical spaces". Here we demonstrate that mapping of monochromatic extraordinary light distribution in a hyperbolic metamaterial along some spatial direction may model the "flow of time". This idea is demonstrated in experiments performed with plasmonic hyperbolic metamaterials. Appearance of the "statistical arrow of time" is examined in an experimental scenario which emulates a Big Bang-like event.

 

[marcus]

http://arxiv.org/abs/1104.0723
A phenomenology analysis of the tachyon warm inflation in loop quantum cosmology
Kui Xiao, Jian-Yang Zhu
7 pages,accepted for publication in Physics Letters B
(Submitted on 5 Apr 2011)
"We investigate the warm inflation condition in loop quantum cosmology. In our consideration, the system is described by a tachyon field interacted with radiation. The exponential potential function, V(\phi)=V_0 e^{-\alpha\phi}\label{exp-p}, with the same order parameters V_0 and \alpha, is taken as an example of this tachyon warm inflation model. We find that, for the strong dissipative regime, the total number of e-folds is less than the one in the classical scenario, and for the weak dissipative regime, the beginning time of the warm inflation will be later than the tachyon (cool) inflation."

 

[John86]

http://arxiv.org/abs/1104.0997
Toward a "fundamental theorem of quantal measure theory"
Authors: Rafael D. Sorkin (Perimeter Institute and Syracuse University)
(Submitted on 6 Apr 2011)

Abstract: We address the extension problem for quantal measures of path-integral type, concentrating on two cases: sequential growth of causal sets, and a particle moving on the finite lattice Z_n. In both cases the dynamics can be coded into a vector-valued measure mu on Omega, the space of all histories. Initially mu is defined only on special subsets of Omega called cylinder-events, and one would like to extend it to a larger family of subsets (events) in analogy to the way this is done in the classical theory of stochastic processes. Since quantally mu is generally not of bounded variation, a new method is required. We propose a method that defines the measure of an event by means of a sequence of simpler events which in a suitable sense converges to the event whose measure one is seeking to define. To this end, we introduce canonical sequences approximating certain events, and we propose a measure-based criterion for the convergence of such sequences. Applying the method, we encounter a simple event whose measure is zero classically but non-zero quantally.

 

[marcus]

http://arxiv.org/abs/1104.1384
Effective action and semiclassical limit of spin foam models
A. Mikovic, M. Vojinovic
15 pages
(Submitted on 7 Apr 2011)
"We define an effective action for spin foam models of quantum gravity by adapting the background field method from quantum field theory. We show that the Regge action is the leading term in the semi-classical expansion of the spin foam effective action if the vertex amplitude has the large-spin asymptotics which is proportional to an exponential function of the vertex Regge action. In the case of the known three-dimensional and four-dimensional spin foam models this amounts to modifying the vertex amplitude such that the exponential asymptotics is obtained. In particular, we show that the ELPR/FK model vertex amplitude can be modified such that the new model is finite and has the Einstein-Hilbert action as its classical limit. We also calculate the first-order and some of the second-order quantum corrections in the semi-classical expansion of the effective action."

 

[marcus]

http://arxiv.org/abs/1104.1800
Inflationary tensor fluctuations, as viewed by Ashtekar variables and their imaginary friends
Laura Bethke, Joao Magueijo
17pages
(Submitted on 10 Apr 2011)
"We investigate tensor modes in infllationary scenarios from the point of view of Ashtekar variables and their generalizations labelled by Immirzi parameter gamma, which we'll assume imaginary. By defining the classical perturbed Hamiltonian, we reproduce, on-shell, the usual expression found in cosmological perturbation theory. However the quantum Hamiltonian displays significant differences, namely in the vacuum energy and fluctuations of the various modes. Graviton states are represented by combinations of metric and connection variables. It turns out that half of these modes have negative energy but after defining the inner product we conclude that they are non-physical and should be selected out. We are left with the usual graviton modes but with a chiral asymmetry in the the vacuum energy and fluctuations. The latter depends on gamma and on the ordering prescription (namely in the Hamiltonian constraint). Such an effect would leave a distinctive imprint in the polarization of the cosmic microwave background, thus finally engaging quantum gravity in meaningful experimental test."

http://arxiv.org/abs/1104.2019
Relative locality and the soccer ball problem
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
4 pages
(Submitted on 11 Apr 2011)
"We consider the behavior of macroscopic bodies within the framework of relative locality, which is a recent proposal for Planck scale modifications of the relativistic dynamics of particles which are described as arising from deformations in the geometry of momentum space. These lead to the addition of non-linear terms to the energy-momentum relations and conservation laws, which are suppressed by powers of ratio between the energy E of the particles involved and the Planck mass M_P. We consider and resolve a common objection against such proposals, which is that, even if the corrections are small for elementary particles in current experiments, they are huge when applied to composite systems such as soccer balls, planets and stars, with energies E_macro much larger than M_P. We show that this "soccer-ball problem" does not arise within the framework of relative locality, because the non-linear effects for the dynamics of a composite system with N elementary particles appear at most of order E_macro/ N M_P."

brief mention:
http://arxiv.org/abs/1104.1733
Arrows of Time in the Bouncing Universes of the No-boundary Quantum State
James Hartle, Thomas Hertog
13 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1104.2066
Reformulating and Reconstructing Quantum Theory
Lucien Hardy
159 pages. Many pictures
(Submitted on 11 Apr 2011)
"We provide a reformulation of finite dimensional quantum theory in the circuit framework in terms of mathematical axioms, and a reconstruction of quantum theory from operational postulates. The mathematical axioms for quantum theory are the following:
[Axiom 1] Operations correspond to operators.
[Axiom 2] Every complete set of positive operators corresponds to a complete set of operations.

The following operational postulates are shown to be equivalent to these mathematical axioms:
[P1] Definiteness. Associated with any given pure state is a unique maximal effect giving probability equal to one. This maximal effect does not give probability equal to one for any other pure state.
[P2] Information locality. A maximal measurement on a composite system is effected if we perform maximal measurements on each of the components.
[P3] Tomographic locality. The state of a composite system can be determined from the statistics collected by making measurements on the components.
[P4] Compound permutatability. There exists a compound reversible transformation on any system effecting any given permutation of any given maximal set of distinguishable states for that system.
[P5] Preparability. Filters are non-mixing and non-flattening.

Hence, from these postulates we can reconstruct all the usual features of quantum theory: States are represented by positive operators, transformations by completely positive trace non-increasing maps, and effects by positive operators. The Born rule (i.e. the trace rule) for calculating probabilitieso follows. A more detailed abstract is provided in the paper."

 

[marcus]

http://arxiv.org/abs/1104.2765
Spin Foam Models for Quantum Gravity and semi-classical limit
Maité Dupuis
PhD Thesis; Ecole Normale Supérieure de Lyon. 192pages, many figures
(Submitted on 14 Apr 2011)
"The spinfoam framework is a proposal for a regularized path integral for quantum gravity. Spinfoams define quantum space-time structures describing the evolution in time of the spin network states for quantum geometry derived from Loop Quantum Gravity (LQG). The construction of this covariant approach is based on the formulation of General Relativity as a topological theory plus the so-called simplicity constraints which introduce local degrees of freedom. The simplicity constraints are essential in turning the non-physical topological theory into 4d gravity.

In this PhD manuscript, an original way to impose the simplicity constraints in 4d Euclidean gravity using harmonic oscillators is proposed and new coherent states, solutions of the constraints, are given. Moreover, a consistent spinfoam model for quantum gravity has to be connected to LQG and must have the right semi-classical limit. An explicit map between the spin network states of LQG and the boundary states of spinfoam models is given connecting the canonical and the covariant approaches. Finally, new techniques to compute semiclassical asymptotic expressions for the transition amplitudes of 3d quantum gravity and to extract semi-classical information from a spinfoam model are introduced. Explicit computations based on approximation methods and on the use of recurrence relations on spinfoam amplitudes have been performed. The results are relevant to derive quantum corrections to the dynamics of the gravitational field."

Maité is the PhD student of Etera Livine.

http://arxiv.org/abs/1104.2822
A real ensemble interpretation of quantum mechanics
Lee Smolin
14 pages
(Submitted on 14 Apr 2011)
"A new ensemble interpretation of quantum mechanics is proposed according to which the ensemble associated to a quantum state really exists: it is the ensemble of all the systems in the same quantum state in the universe. Individual systems within the ensemble have microscopic states, described by beables. The probabilities of quantum theory turn out to be just ordinary relative frequencies probabilities in these ensembles. Laws for the evolution of the beables of individual systems are given such that their ensemble relative frequencies evolve in a way that reproduces the predictions of quantum mechanics. These laws are highly non-local and involve a new kind of interaction between the members of an ensemble that define a quantum state. These include a stochastic process by which individual systems copy the beables of other systems in the ensembles of which they are a member. The probabilities for these copy processes do not depend on where the systems are in space, but do depend on the distribution of beables in the ensemble. Macroscopic systems then are distinguished by being large and complex enough that they have no copies in the universe. They then cannot evolve by the copy law, and hence do not evolve stochastically according to quantum dynamics. This implies novel departures from quantum mechanics for systems in quantum states that can be expected to have few copies in the universe. At the same time, we are able to argue that the centre of masses of large macroscopic systems do satisfy Newton's laws."

 

[MTd2]

http://arxiv.org/abs/1104.2889

Black holes in Einstein-aether and Horava-Lifgarbagez gravity

Enrico Barausse, Ted Jacobson, Thomas P. Sotiriou
(Submitted on 14 Apr 2011)
We study spherical black-hole solutions in Einstein-aether theory, a Lorentz-violating gravitational theory consisting of General Relativity with a dynamical unit timelike vector (the "aether") that defines a preferred timelike direction. These are also solutions to the infrared limit of Horava-Lifgarbagez gravity. We explore parameter values of the two theories where all presently know experimental constraints are satisfied, and find that spherical black-hole solutions of the type expected to form by gravitational collapse exist for all those parameters. Outside the metric horizon, the deviations away from the Schwarzschild metric are typically no more than a few percent for most of the explored parameter regions, which makes them difficult to observe with electromagnetic probes, but in principle within reach of future gravitational-wave detectors. Remarkably, we find that the solutions possesses a universal horizon, not far inside the metric horizon, that traps waves of any speed relative to the aether. A notion of black hole thus persists in these theories, even in the presence of arbitrarily high propagation speeds.

 

[marcus]

http://arxiv.org/abs/1104.3126
Mass Inflation in the Loop Black Hole
Eric G. Brown, Robert B. Mann, Leonardo Modesto
20 pages, 7 figures
(Submitted on 15 Apr 2011)
"In classical general relativity the Cauchy horizon within a two-horizon black hole is unstable via a phenomenon known as mass inflation, in which the mass parameter (and the spacetime curvature) of the black hole diverges at the Cauchy horizon. Here we study this effect for loop black holes -- quantum gravitationally corrected black holes from loop quantum gravity -- whose construction alleviates the r=0 singularity present in their classical counterparts. We use a simplified model of mass inflation, which makes use of the generalized DTR[*] relation, to conclude that the Cauchy horizon of loop black holes indeed results in a curvature singularity similar to that found in classical black holes. The DTR relation is of particular utility in the loop black hole because it does not directly rely upon Einstein's field equations. We elucidate some of the interesting and counterintuitive properties of the loop black hole, and corroborate our results using an alternate model of mass inflation due to Ori."

[*By DTR they refer to work by Dray 't Hooft, and Redmount.]

brief mention:
http://arxiv.org/abs/1104.2910
The Relation Between Geometry and Matter in classical and Quantum Gravity and Cosmology
Daniele Regoli
PhD thesis
(Submitted on 14 Apr 2011)
"The present thesis is divided into two main research areas: Classical Cosmology and (Loop) Quantum Gravity. The first part concerns cosmological models... The second part concerns the spinfoam approach to (Loop) Quantum Gravity,..."

 

[marcus]

http://arxiv.org/abs/1104.3683
Holomorphic Simplicity Constraints for 4d Spinfoam Models
Maité Dupuis, Etera R. Livine
27 pages
(Submitted on 19 Apr 2011)
"Within the framework of spinfoam models, we revisit the simplicity constraints reducing topological BF theory to 4d Riemannian gravity. We use the reformulation of SU(2) intertwiners and spin networks in term of spinors, which has come out from both the recently developed U(N) framework for SU(2) intertwiners and the twisted geometry approach to spin networks and spinfoam boundary states. Using these tools, we are able to perform a holomorphic/anti-holomorphic splitting of the simplicity constraints and define a new set of holomorphic simplicity constraints, which are equivalent to the standard ones at the classical level and which can be imposed strongly on intertwiners at the quantum level. We then show how to solve these new holomorphic simplicity constraints using coherent intertwiner states. We further define the corresponding coherent spin network functionals and introduce a new spinfoam model for 4d Riemannian gravity based on these holomorphic simplicity constraints and whose amplitudes are defined from the evaluation of the new coherent spin networks."

http://arxiv.org/abs/1104.3688
Pre-Big-Bang Cosmology and Circles in the Cosmic Microwave Background
William Nelson, Edward Wilson-Ewing
21 pages, 3 figures
(Submitted on 19 Apr 2011)
"We examine the possibility that circles in the cosmic microwave background could be formed by the interaction of a gravitational wave pulse emitted in some pre-big-bang phase of the universe with the last scattering surface. We derive the expected size distribution of such circles, as well as their typical width and (for concentric circles) angular separation. We apply these results in particular to conformal cyclic cosmology, ekpyrotic cosmology as well as loop quantum cosmology with and without inflation in order to determine how the predicted geometric properties of these circles would vary from one model to the other, and thus, if detected, could allow us to differentiate between various pre-big-bang cosmological models. We also show that the angular width and the sine of the angular radius of such circles are inversely proportional. This relation can be used in order to determine whether or not circles observed in the cosmic microwave background are due to energetic pre-big-bang events."

 

[John86]

http://arxiv.org/abs/1104.3443
Constructive Renormalization for $\Phi^{4}_2$ Theory with Loop Vertex Expansion
Vincent Rivasseau, Zhituo Wang
(Submitted on 18 Apr 2011)
In this paper we construct the 2 dimensional Euclidean $\phi^4$ quantum field theory using the method of loop vertex expansion. We reproduce the results of standard constructive theory, for example the Borel summability of the Schwinger functions in the coupling constant. Our method should be also suitable for the future construction of Grosse-Wulkenhaar models on non-commutative space-time.

 

[marcus]

http://arxiv.org/abs/1104.4028
Perturbative quantum gravity with the Immirzi parameter
Dario Benedetti, Simone Speziale
(Submitted on 20 Apr 2011)
"We study perturbative quantum gravity in the first-order tetrad formalism. The lowest order action corresponds to Einstein-Cartan plus a parity-odd term, and is known in the literature as the Holst action. The coupling constant of the parity-odd term can be identified with the Immirzi parameter of loop quantum gravity. We compute the quantum effective action in the one-loop expansion. As in the metric second-order formulation, we find that in the case of pure gravity the theory is on-shell finite, and the running of Newton's constant and the Immirzi parameter is inessential. In the presence of fermions, the situation changes in two fundamental aspects. First, non-renormalizable logarithmic divergences appear, as usual. Second, the Immirzi parameter becomes a priori observable, and we find that it is renormalized by a four-fermion interaction generated by radiative corrections. We compute its beta function and discuss possible implications. The sign of the beta function depends on whether the Immirzi parameter is larger or smaller than one in absolute value, and the values plus or minus one are UV fixed-points (we work in Euclidean signature). Finally, we find that the Holst action is stable with respect to radiative corrections in the case of minimal coupling, up to higher order non-renormalizable interactions."

 

[jason11]

http://arxiv.org/abs/1104.1499"

Semiclassical Analysis of the Wigner $9J$-Symbol with Small and Large Angular Momenta
Robert G. Littlejohn, Liang Yu
(Submitted on 8 Apr 2011 (v1), last revised 13 Apr 2011 (this version, v2))
We derive a new asymptotic formula for the Wigner $9j$-symbol, in the limit of one small and eight large angular momenta, using a novel gauge-invariant factorization for the asymptotic solution of a set of coupled wave equations. Our factorization eliminates the geometric phases completely, using gauge-invariant non-canonical coordinates, parallel transports of spinors, and quantum rotation matrices. Our derivation generalizes to higher $3nj$-symbols. We display without proof some asymptotic formulas for the $12j$-symbol and the $15j$-symbol in the appendices. This work contributes a new asymptotic formula of the Wigner $9j$-symbol to the quantum theory of angular momentum, and serves as an example of a new general method for deriving asymptotic formulas for $3nj$-symbols.



http://arxiv.org/abs/1104.3275"

Semiclassical Analysis of the Wigner $12J$-Symbol with One Small Angular Momentum: Part I
Liang Yu
(Submitted on 17 Apr 2011)
We derive a new asymptotic formula for the Wigner $12j$-symbol, in the limit of one small and eleven large angular momenta. There are two kinds of asymptotic formulas for the $12j$-symbol with one small angular momentum. We present the first kind in this paper. Our derivation relies on the techniques developed in the semiclassical analysis of the Wigner $9j$-symbol, where we used a gauge-invariant form of the multicomponent WKB wavefunctions to derive new asymptotic formulas for the $9j$-symbol with small and large angular momenta. When applying the same technique to the $12j$-symbol in this paper, we find that the spinor is diagonalized in the direction of an intermediate angular momentum. In addition, we find that the geometry of the new asymptotic formula for the $12j$-symbol is expressed in terms of the vector diagram for a $9j$-symbol. This illustrates a general geometric connection between asymptotic limits of the various $3nj$-symbols. This work contributes the first known asymptotic formula for the $12j$-symbol to the quantum theory of angular momentum, and serves as a basis for finding asymptotic formulas for the Wigner $15j$-symbol with two small angular momenta.



http://arxiv.org/abs/1104.3641"

Asymptotic Limits of the Wigner $15J$-Symbol with Small Quantum Numbers
Liang Yu
(Submitted on 19 Apr 2011)
"We present new asymptotic formulas for the Wigner $15j$-symbol with two, three, or four small quantum numbers, and provide numerical evidence of their validity. These formulas are of the WKB form and are of a similar nature as the Ponzano-Regge formula for the Wigner $6j$-symbol. They are expressed in terms of edge lengths and angles of geometrical figures associated with angular momentum vectors. In particular, the formulas for the $15j$-symbol with two, three, and four small quantum numbers are based on the geometric figures of the $9j$-, $6j$-, and $3j$-symbols, respectively, The geometric nature of these new asymptotic formulas pave the way for further analysis of the semiclassical limits of vertex amplitudes in loop quantum gravity models."

 

[marcus]

http://arxiv.org/abs/1104.4691
Black hole horizons from within loop quantum gravity
Hanno Sahlmann
13 pages, 10 figures
(Submitted on 25 Apr 2011)
"In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. Similarly, in quantum gravity, the quantized horizon degrees of freedom should result from restricting, or pulling-back, the quantized bulk degrees of freedom. This is not yet fully realized in the - otherwise very successful - quantization of isolated horizons in loop quantum gravity. In this work we outline a setting in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. There is no need to quantize them separately. We present evidence that for a horizon of sphere topology, the resulting horizon theory is remarkably similar to what has been found before."

http://arxiv.org/abs/1104.4546
Implications of the gauge-fixing in Loop Quantum Cosmology
Francesco Cianfrani, Giovanni Montani
12 pages
(Submitted on 23 Apr 2011)
"The restriction to invariant connections in a Friedmann-Robertson-Walker space-time is discussed via the analysis of the Dirac brackets associated with the corresponding gauge fixing. This analysis allows us to infer the proper relation between fluxes and reduced phase-space variables. In this respect, it is outlined how the holonomy-flux algebra coincides with the one of Loop Quantum Gravity if edges are parallel to simplicial vectors and the quantization of the model is performed via standard techniques by restricting admissible paths. Within this scheme, the discretization of the area spectrum is emphasized, while the fundamental path underlying the continuous picture is described by a cubical lattice. Finally, the perspectives for a consistent dynamical treatment are discussed."

brief mention (not Loop-and-allied QG, but possibly of general interest)
http://arxiv.org/abs/1104.4543
A class of elementary particle models without any adjustable real parameters
Gerard 't Hooft
28 pages
(Submitted on 23 Apr 2011)
"Conventional particle theories such as the Standard Model have a number of freely adjustable coupling constants and mass parameters, depending on the symmetry algebra of the local gauge group and the representations chosen for the spinor and scalar fields. There seems to be no physical principle to determine these parameters as long as they stay within certain domains dictated by the renormalization group. Here however, reasons are given to demand that, when gravity is coupled to the system, local conformal invariance should be a spontaneously broken exact symmetry. The argument has to do with the requirement that black holes obey a complementarity principle relating ingoing observers to outside observers, or equivalently, initial states to final states. This condition fixes all parameters, including masses and the cosmological constant. We suspect that only examples can be found where these are all of order one in Planck units, but the values depend on the algebra chosen. This paper combines findings reported in two previous preprints, and puts these in a clearer perspective by shifting the emphasis towards the implications for particle models."

 

[marcus]

http://relativity.phys.lsu.edu/ilqgs/gurau042611.pdf
The 1/N expansion in colored tensor models
Razvan Gurau, Perimeter Institute
ILQGS talk Tuesday, Apr 26th
I expect the audio to be posted within a day or so.
Gurau is a frequent collaborator with Vincent Rivasseau, central in the development of a certain group-field-theory approach applicable to QG including spinfoam LQG.
Assuming the audio for this seminar lecture is posted soon, it will be found here:
http://relativity.phys.lsu.edu/ilqgs/
In the meantime I will look for related technical papers to give more of an idea what he is talking about.

http://arxiv.org/abs/1011.2726
The 1/N expansion of colored tensor models
http://arxiv.org/abs/1101.4182
The 1/N expansion of colored tensor models in arbitrary dimension
http://arxiv.org/abs/1102.5759
The complete 1/N expansion of colored tensor models in arbitrary dimension

brief mention (not directly involved with Loop-and-allied QG, but possibly of wider interest)
http://arxiv.org/abs/1104.4849
Noncommutative Geometry Inspired Entropic Inflation
Kourosh Nozari, Siamak Akhshabi
14 pages, 5 figuers, Accepted for Publication in Phys. Lett. B
(Submitted on 26 Apr 2011)
"Recently Verlinde proposed that gravity can be described as an emergent phenomena arising from changes in the information associated with the positions of material bodies. By using noncommutative geometry as a way to describe the microscopic, microstructure of quantum spacetime, we derive modified Friedmann equation in this setup and study the entropic force modifications to the inflationary dynamics of early universe."

 

[marcus]

http://arxiv.org/abs/1104.5158
Bounding bubbles: the vertex representation of 3d Group Field Theory and the suppression of pseudo-manifolds
Sylvain Carrozza, Daniele Oriti
28 pages, 17 figures
(Submitted on 27 Apr 2011)
"Based on recent work on simplicial diffeomorphisms in colored group field theories, we develop a representation of the colored Boulatov model, in which the GFT fields depend on variables associated to vertices of the associated simplicial complex, as opposed to edges. On top of simplifying the action of diffeomorphisms, the main advantage of this representation is that the GFT Feynman graphs have a different stranded structure, which allows a direct identification of subgraphs associated to bubbles, and their evaluation is simplified drastically. As a first important application of this formulation, we derive new scaling bounds for the regularized amplitudes, organized in terms of the genera of the bubbles, and show how the pseudo-manifolds configurations appearing in the perturbative expansion are suppressed as compared to manifolds. Moreover, these bounds are proved to be optimal."

 

[marcus]

http://arxiv.org/abs/1104.5471
Tensor models and embedded Riemann surfaces
James P. Ryan
9 pages, 7 figures
(Submitted on 28 Apr 2011)
"Tensor models and, more generally, group field theories are candidates for higher-dimensional quantum gravity, just as matrix models are in the 2d setting. With the recent advent of a 1/N-expansion for coloured tensor models, more focus has been given to the study of the topological aspects of their Feynman graphs. Crucial to the aforementioned analysis were certain subgraphs known as bubbles and jackets. We demonstrate in the 3d case that these graphs are generated by matrix models embedded inside the tensor theory. Moreover, we show that the jacket graphs represent (Heegaard) splitting surfaces for the triangulation dual to the Feynman graph. With this in hand, we are able to re-express the Boulatov model as a quantum field theory on these Riemann surfaces."
Ryan is at AEI Potsdam, in Dittrich's group. he just gave a talk on 12 April at ILQGS
http://relativity.phys.lsu.edu/ilqgs/

The audio for Razvan Gurau's ILQGS seminar talk is now available online. When I posted about the talk two days ago only the slides PDF was available.http://arxiv.org/abs/1104.5486
Examination of the nature of the Bianchi type cosmological singularities
Piotr Dzierzak
PhD Thesis, about 80 pages
(Submitted on 28 Apr 2011)
"We present quantum (and classical) Bianchi I model, with free massless scalar field, of the Universe. Our model may be treated as the simplest prototype of the quantum BKL (Belinskii-Khalatnikov-Lifgarbagez) scenario. The quantization is done by making use of the nonstandard Loop Quantum Cosmology (LQC). Since the method is quite new, we present in details its motivation and the formalism. To make the nonstandard method easily understandable, we include its application to the FRW model. In the nonstandard LQC, we first solve the Hamiltonian constraint of the theory at the classical level and find elementary observables. Physical compound observables are defined in terms of elementary ones. We find that classical Big Bang singularity is replaced by quantum Big Bounce transition due to modification of classical theory by holonomy around a loop with finite size. The energy density of matter fields at the Big Bounce depends on a free parameter lambda, which value is expected to be determined from future cosmological observations. The phase space is divided it into two distinct regions: Kasner-like and Kasner-unlike. We use the elementary observables to quantize volume and directional volume operators in both cases. Spectra of these operators are bounded from below and discrete, and depend on lambda. The discreteness may imply a foamy structure of spacetime at semiclassical level. At the quantum level an evolution of the model is generated by the so-called true Hamiltonian. This enables introducing a time parameter valued in the set of all real numbers."
Dzierzak is at Warsaw. Jerzy Lewandowski heads the LQG group there.

 

[marcus]

http://arxiv.org/abs/1104.5509
Effective Hamiltonian Constraint from Group Field Theory
Etera R. Livine, Daniele Oriti, James P. Ryan
14 pages
(Submitted on 28 Apr 2011)
"Spinfoam models provide a covariant formulation of the dynamics of loop quantum gravity. They are non-perturbatively defined in the group field theory (GFT) framework: the GFT partition function defines the sum of spinfoam transition amplitudes over all possible (discretized) geometries and topologies. The issue remains, however, of explicitly relating the specific form of the group field theory action and the canonical Hamiltonian constraint. Here, we suggest an avenue for addressing this issue. Our strategy is to expand group field theories around non-trivial classical solutions and to interpret the induced quadratic kinematical term as defining a Hamiltonian constraint on the group field and thus on spin network wave functions. We apply our procedure to Boulatov group field theory for 3d Riemannian gravity. Finally, we discuss the relevance of understanding the spectrum of this Hamiltonian operator for the renormalization of group field theories."

http://arxiv.org/abs/1104.5527
Affine holomorphic quantization
Robert Oeckl (UNAM)
38 pages
(Submitted on 28 Apr 2011)
"We present a rigorous and functorial quantization scheme for affine field theories, i.e., field theories where local spaces of solutions are affine spaces. The target framework for the quantization is the general boundary formulation, allowing to implement manifest locality without the necessity for metric or causal background structures. The quantization combines the holomorphic version of geometric quantization for state spaces with the Feynman path integral quantization for amplitudes. We also develop an adapted notion of coherent states, discuss vacuum states, and consider observables and their Berezin-Toeplitz quantization. Moreover, we derive a factorization identity for the amplitude in the special case of a linear field theory modified by a source-like term and comment on its use as a generating functional for a generalized S-matrix."

We should get familiar with the acronym UNAM. It is where Corichi is based (although he also seems to be at Penn State with Ashtekar group sometimes as well.) The Loops 2007 conference was held at UNAM. I guess it means University National Autonomous Mexico. Or else the M stands for Morelia, the beautiful old city where the university is located and where they had the conference.

 

[marcus]

http://arxiv.org/abs/1105.0216
Regge gravity from spinfoams
Elena Magliaro, Claudio Perini
8 pages
(Submitted on 1 May 2011)
We consider spinfoam quantum gravity in the double scaling limit \gamma\rightarrow 0, j\rightarrow\infty with \gamma j constant, where \gamma is the Immirzi parameter, j is the spin and \gamma j gives the physical area in Planck units. We show how in this regime the partition function for a 2-complex takes the form of a path integral over continuous Regge metrics and enforces Einstein equations in the semiclassical regime. The Immirzi parameter must be considered as dynamical in the sense that it runs towards zero when the small wavelengths are integrated out. In addition to quantum corrections which vanish for \hbar\rightarrow 0, we find new corrections due to the discreteness of geometric spectra which is controlled by \gamma.


http://arxiv.org/abs/1105.0194
From Quantum Gravity to Quantum Field Theory via Noncommutative Geometry
Johannes Aastrup, Jesper M. Grimstrup
34 pages, 3 figures
(Submitted on 1 May 2011)
"A link between canonical quantum gravity and fermionic quantum field theory is established in this paper. From a spectral triple construction which encodes the kinematics of quantum gravity semi-classical states are constructed which, in a semi-classical limit, give a system of interacting fermions in an ambient gravitational field. The interaction involves flux tubes of the gravitational field. In the additional limit where all gravitational degrees of freedom are turned off, a free fermionic quantum field theory emerges."

http://arxiv.org/abs/1105.0183
Shape Dynamics. An Introduction
Julian Barbour
48 pages, 8 figures,46 references. To be published in the refereed proceedings of the conference Quantum Field Theory and Gravity (Regensburg 2010)
(Submitted on 1 May 2011)
"Shape dynamics is a completely background-independent universal framework of dynamical theories from which all absolute elements have been eliminated. For particles, only the variables that describe the shapes of the instantaneous particle configurations are dynamical. In the case of Riemannian three-geometries, the only dynamical variables are the parts of the metric that determine angles. The local scale factor plays no role. This leads to a shape-dynamic theory of gravity in which the four-dimensional diffeomorphism invariance of general relativity is replaced by three-dimensional diffeomorphism invariance and three-dimensional conformal invariance. Despite this difference of symmetry groups, it is remarkable that the predictions of the two theories -- shape dynamics and general relativity -- agree on spacetime foliations by hypersurfaces of constant mean extrinsic curvature. However, the two theories are distinct, with shape dynamics having a much more restrictive set of solutions. There are indications that the symmetry group of shape dynamics makes it more amenable to quantization and thus to the creation of quantum gravity. This introduction presents in simple terms the arguments for shape dynamics, its implementation techniques, and a survey of existing results."

 

[marcus]

http://arxiv.org/abs/1105.0566
Euclidean three-point function in loop and perturbative gravity
Carlo Rovelli, Mingyi Zhang
16 pages
(Submitted on 3 May 2011)
"We compute the leading order of the three-point function in loop quantum gravity, using the vertex expansion of the Euclidean version of the new spin foam dynamics, in the region of gamma<1. We find results consistent with Regge calculus in the limit gamma->0 and j->infinity. We also compute the tree-level three-point function of perturbative quantum general relativity in position space, and discuss the possibility of directly comparing the two results."

http://arxiv.org/abs/1105.0439
Canonical quantization of non-commutative holonomies in 2+1 loop quantum gravity
Karim Noui, Alejandro Perez, Daniele Pranzetti
(Submitted on 2 May 2011)
In this work we investigate the canonical quantization of 2+1 gravity with cosmological constant Lambda > 0 in the canonical framework of loop quantum gravity. The unconstrained phase space of gravity in 2+1 dimensions is coordinatized by an SU(2) connection A and the canonically conjugate triad field e. A natural regularization of the constraints of 2+1 gravity can be defined in terms of the holonomies of A+ = A + sqrt(Lambda) e. As a first step towards the quantization of these constraints we study the canonical quantization of the holonomy of the connection A_lambda = A+ lambda e on the kinematical Hilbert space of loop quantum gravity. The holonomy operator associated to a given path acts non trivially on spin network links that are transversal to the path (a crossing). We provide an explicit construction of the quantum holonomy operator. In particular, we exhibit a close relationship between the action of the quantum holonomy at a crossing and Kauffman's q-deformed crossing identity. The crucial difference is that (being an operator acting on the kinematical Hilbert space of LQG) the result is completely described in terms of standard SU(2) spin network states (in contrast to q-deformed spin networks in Kauffman's identity). We discuss the possible implications of our result.

http://arxiv.org/abs/1105.0636
The Diffeomorphism Constraint Operator in Loop Quantum Gravity
Alok Laddha, Madhavan Varadarajan
37 pages, 6 figures
(Submitted on 3 May 2011)
"We construct the smeared diffeomorphism constraint operator at finite triangulation from the basic holonomy-flux operators of Loop Quantum Gravity, evaluate its continuum limit on the Lewandowski-Marolf habitat and show that the action of the continuum operator provides an anomaly free representation of the Lie algebra of diffeomorphisms of the 3-manifold. Key features of our analysis include: (i) finite triangulation approximants to the curvature, F_abⁱ of the Ashtekar-Barbero connection which involve not only small loop holonomies but also small surface fluxes as well as an explicit dependence on the edge labels of the spin network being acted on (ii) the dependence of the small loop underlying the holonomy on both the direction and magnitude of the shift vector field (iii) continuum constraint operators which do not have finite action on the kinematic Hilbert space, thus implementing a key lesson from recent studies of parameterised field theory by the authors.
Features (i) and (ii) provide the first hints in LQG of a conceptual similarity with the so called "mu-bar" scheme of Loop Quantum Cosmology. We expect our work to be of use in the construction of an anomaly free quantum dynamics for LQG."

http://arxiv.org/abs/1105.0667
Quantum scalar field in quantum gravity: the propagator and Lorentz invariance in the spherically symmetric case
Rodolfo Gambini, Jorge Pullin, Saeed Rastgoo
Dedicated to Josh Goldberg, to appear in special issue of Gen. Rel. Grav., 14 pages
(Submitted on 3 May 2011)
"We recently studied gravity coupled to a scalar field in spherical symmetry using loop quantum gravity techniques. Since there are local degrees of freedom one faces the 'problem of dynamics'. We attack it using the 'uniform discretization technique'. We find the quantum state that minimizes the value of the master constraint for the case of weak fields and curvatures. The state has the form of a direct product of Gaussians for the gravitational variables times a modified Fock state for the scalar field. In this paper we do three things. First, we verify that the previous state also yields a small value of the master constraint when one polymerizes the scalar field in addition to the gravitational variables. We then study the propagators for the polymerized scalar field in flat space-time using the previously considered ground state in the low energy limit. We discuss the issue of the Lorentz invariance of the whole approach. We note that if one uses real clocks to describe the system, Lorentz invariance violations are small. We discuss the implications of these results in the light of Horava's Gravity at the Lifgarbagez point and of the argument about potential large Lorentz violations in interacting field theories of Collins et. al.

 

[marcus]

http://arxiv.org/abs/1105.0938
The gravity/CFT correspondence
Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati
5 pages, 1 figure
(Submitted on 4 May 2011)
"We prove a general correspondence between classical gravity in 3+1 dimensions and a pair of classical conformal field theories in 3 dimensions (the generalization to higher dimensions is straightforward). The proof relies on a novel formulation of general relativity called shape dynamics that, despite having different local symmetries, leads to classical trajectories identical to those of general relativity in a particular gauge. The key difference is that general relativity's refoliation invariance is traded for volume-preserving three-dimensional conformal invariance, i.e., local spatial Weyl invariance. It is precisely this symmetry that allows us to establish the general correspondence while resolving exactly the local degrees of freedom, a feat that is not possible in general relativity, without a derivative expansion, due to non-linearity."

 

[marcus]

http://arxiv.org/abs/1105.1334
On the Expansions in Spin Foam Cosmology
Frank Hellmann
(Submitted on 6 May 2011)
We discuss the expansions used in spin foam cosmology. We point out that already at the one vertex level arbitrarily complicated amplitudes contribute, and discuss the geometric asymptotics of the five simplest ones. We discuss what type of consistency conditions would be required to control the expan- sion. We show that the factorisation of the amplitude originally considered is best interpreted in topological terms. We then consider the next higher term in the graph expansion. We demonstrate the tension between the truncation to small graphs and going to the homogeneous sector, and conclude that it is necessary to truncate the dynamics as well.

http://arxiv.org/abs/1105.1340
Black-hole horizons in modified space-time structures arising from canonical quantum gravity
Martin Bojowald, George M. Paily, Juan D. Reyes, Rakesh Tibrewala
(Submitted on 6 May 2011)
Several properties of canonical quantum gravity modify space-time structures, sometimes to the degree that no effective line elements exist to describe the geometry. An analysis of solutions, for instance in the context of black holes, then requires new insights. In this article, standard definitions of horizons in spherical symmetry are first reformulated canonically, and then evaluated for solutions of equations and constraints modified by inverse-triad corrections of loop quantum gravity. When possible, a space-time analysis is performed which reveals a mass threshold for black holes and small changes to Hawking radiation. For more general conclusions, canonical perturbation theory is developed to second order to include back-reaction from matter. The results shed light on the questions of whether renormalization of Newton's constant or other modifications of horizon conditions should be taken into account in computations of black-hole entropy in loop quantum gravity.
Comments: 43 pages, 3 figures

 

[MTd2]

http://arxiv.org/abs/1105.1557

Topological quantum D-branes and wild embeddings from exotic smooth R^4

T. Asselmeyer-Maluga, J.Krol
(Submitted on 8 May 2011)
This is the next step of uncovering the relation between string theory and exotic smooth R^4. Exotic smoothness of R^4 is correlated with D6 brane charges in IIA string theory. We construct wild embeddings of spheres and relate them to a class of topological quantum Dp-branes as well to KK theory. These branes emerge when there are non-trivial NS-NS H-fluxes where the topological classes are determined by wild embeddings S^2 -> S^3. Then wild embeddings of higher dimensional $p$-complexes into S^n correspond to Dp-branes. These wild embeddings as constructed by using gropes are basic objects to understand exotic smoothness as well Casson handles. Next we build C*-algebras corresponding to the embeddings. Finally we consider topological quantum D-branes as those which emerge from wild embeddings in question. We construct an action for these quantum D-branes and show that the classical limit agrees with the Born-Infeld action such that flat branes = usual embeddings.

 

[marcus]

http://arxiv.org/abs/1105.1898
A note on the geometrical interpretation of quantum groups and non-commutative spaces in gravity
Eugenio Bianchi, Carlo Rovelli
(Submitted on 10 May 2011)
Quantum groups and non-commutative spaces have been repeatedly utilized in approaches to quantum gravity. They provide a mathematically elegant cut-off, often interpreted as related to the Planck-scale quantum uncertainty in position. We consider here a different geometrical interpretation of this cut-off, where the relevant non-commutative space is the space of directions around any spacetime point. The limitations in angular resolution expresses the finiteness of the angular size of a Planck-scale minimal surface at a maximum distance 1/\sqrt{\Lambda} related the cosmological constant Lambda.
This yields a simple geometrical interpretation for the relation between the quantum deformation parameter
q=e^{i \Lambda l_{Planck}^2}
and the cosmological constant, and resolves a difficulty of more conventional interpretations of the physical geometry described by quantum groups or fuzzy spaces.
Comments: 2 pages, 1 figure

http://arxiv.org/abs/1105.1968
Fermions and Goldstone bosons in an asymptotically safe model
F. Bazzocchi, M. Fabbrichesi, R. Percacci, A. Tonero, L. Vecchi
(Submitted on 10 May 2011)
We consider a model in which Goldstone bosons, described by a SU(N) chiral nonlinear sigma model, are coupled to an N-plet of colored fermions by means of a Yukawa interaction. We study the one-loop renormalization group flow and show that the non-Gaussian UV fixed point, which is present in the purely bosonic model, is lost because of fermion loop effects unless N is sufficiently large. We then add four-fermion contact interactions to the lagrangian and show that in this case there exist several non-Gaussian fixed points. The strength of the contact interactions, predicted by the requirement that the theory flows towards a fixed point in the UV, is compared to the current experimental bounds. This toy model could provide an important building block of an asymptotically safe model of the weak interactions.
Comments: 7 pages, 3 figures, 1 table

http://pirsa.org/11050010/
The emergent gravity challenge: a few lessons from analogue gravity
Speaker(s): Stefano Liberati
Abstract: Emergent gravity scenarios have become increasingly popular in recent times. In this talk I will review some evidence in this sense and discuss some lessons from toy models based on condensed matter analogues of gravity. These lessons suggest some (possibly) general features of the emergent gravity framework which not only can be tested with current astrophysical observations but can also improve our understanding of cosmological puzzles such as the dark energy one. I shall review these tests and expectations and discuss the perspectives of this line of research and emergent gravity scenarios at large.
Date: 04/05/2011 - 4:00 pm
Tech Note: May be some slides missing - Auto was turned off

(My comment: the slide advance does seem to have gotten stuck around minutes 49-54. but this is not much of a problem, later on he flips back to those slides and one eventually sees them on the big screen.)

http://pirsa.org/11050022/
A real ensemble interpretation of quantum mechanics
Speaker(s): Lee Smolin
Abstract: A new ensemble interpretation of quantum mechanics is proposed according to which the ensemble associated to a quantum state really exists: it is the ensemble of all the systems in the same quantum state in the universe. Individual systems within the ensemble have microscopic states, described by beables. The probabilities of quantum theory turn out to be just ordinary relative frequencies probabilities in these ensembles. Laws for the evolution of the beables of individual systems are given such that their ensemble relative frequencies evolve in a way that reproduces the predictions of quantum mechanics. These laws are highly non-local and involve a new kind of interaction between the members of an ensemble that define a quantum state. These include a stochastic process by which individual systems copy the beables of other systems in the ensembles of which they are a member. The probabilities for these copy processes do not depend on where the systems are in space, but do depend on the distribution of beables in the ensemble. Macroscopic systems then are distinguished by being large and complex enough that they have no copies in the universe. They then cannot evolve by the copy law, and hence do not evolve stochastically according to quantum dynamics. This implies novel departures from quantum mechanics for systems in quantum states that can be expected to have few copies in the universe. At the same time, we are able to argue that the centre of masses of large macroscopic systems do satisfy Newton's laws.
Date: 03/05/2011 - 4:00 pm

This is the video talk relating to what was written up in a recent paper posted on arxiv, logged here in post #1458 on the preceding page.
http://arxiv.org/abs/1104.2822
A real ensemble interpretation of quantum mechanics
Lee Smolin
14 pages
(Submitted on 14 Apr 2011)

 

[MTd2]

http://arxiv.org/abs/1105.2212

Cosmological Constant in LQG Vertex Amplitude

Muxin Han
(Submitted on 11 May 2011)
A new q-deformation of the Euclidean EPRL/FK vertex amplitude is proposed by using the evaluation of the Vassiliev invariant associated with a 4-simplex graph (related to two copies of quantum SU(2) group at different roots of unity). We show that the large-j asymptotics of the q-deformed vertex amplitude gives the Regge action with cosmological constant (in the corresponding 4-simplex). In the end we also discuss its relation with a Chern-Simons theory on the boundary of 4-simplex.

 

[marcus]

http://arxiv.org/abs/1105.2330
Complex Ashtekar variables, the Kodama state and spinfoam gravity
Wolfgang Wieland
29 pages, 3 figures
(Submitted on 11 May 2011)
"Starting from a Hamiltonian description of four dimensional general relativity in presence of a cosmological constant we perform the program of canonical quantisation. This is done using complex Ashtekar variables while keeping the Barbero--Immirzi parameter real. Introducing the SL(2,C) Kodama state formally solving all first class constraints we propose a spinfoam vertex amplitude. We construct SL(2,C) boundary spinnetwork functions coloured by finite dimensional representations of the group, and derive the skein relations needed to calculate the amplitude. The space of boundary states is shown to carry a representation of the holonomy flux algebra and can naturally be equipped with an inner product. It fails to be positive definite, but cylindrical consistency is perfectly satisfied."

 

[marcus]

http://arxiv.org/abs/1105.2804
On the Semiclassical Limit of Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya
14 pages, 5 figures
(Submitted on 13 May 2011)
"In this note we consider a k=0 Friedman-Robertson-Walker (FRW) model within loop quantum cosmology and consider the issue of its semiclassical limit. For this system, we benefit from the existence of a solvable model that allows us to construct analytical coherent-state solutions. We explore the physical criteria that select from these coherent states, those that display semiclassical behavior, and study their properties in the deep Planck regime. Furthermore, we consider generalized squeezed states and compare them to the Gaussian states. The issue of semiclassicality preservation across the bounce is studied and shown to be generic for all the states considered. Finally, we comment on the different implications these results have, depending on the topology of the spatial slice. In particular the issue of the recovery, within our class of states, of a scaling symmetry present in the classical description of the system when the spatial topology is non-compact."

 

[marcus]

http://arxiv.org/abs/1105.3385
Spinor Representation for Loop Quantum Gravity
Etera R. Livine, Johannes Tambornino
36+1 pages
(Submitted on 17 May 2011)
"We perform a quantization of the loop gravity phase space purely in terms of spinorial variables, which have recently been shown to provide a direct link between spin network states and simplicial geometries. The natural Hilbert space to represent these spinors is the Bargmann space of holomorphic square-integrable functions over complex numbers. We show the unitary equivalence between the resulting generalized Bargmann space and the standard loop quantum gravity Hilbert space by explicitly constructing the unitary map. The latter maps SU(2)-holonomies, when written as a function of spinors, to their holomorphic part. We analyze the properties of this map in detail. We show that the subspace of gauge invariant states can be characterized particularly easy in this representation of loop gravity. Furthermore, this map provides a tool to efficiently calculate physical quantities since integrals over the group are exchanged for straightforward integrals over the complex plane."