Intuitive content of Loop Gravity--Rovelli's program

MTd2

http://arxiv.org/abs/1209.0881

The Physics of Events: A Potential Foundation for Emergent Space-Time

Kevin H. Knuth, Newshaw Bahreyni
Comments: 42 pages, 16 figures
Subjects: Mathematical Physics (math-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Everything that is detected or measured is the direct result of something influencing something else. This is the essence of the concept of force, which has become central to physics. By considering both the act of influencing and the response to such influence as a pair of events, we can describe a universe of interactions as a partially-ordered set of events. In this paper, we take the partially-ordered set of events as a fundamental picture of influence and aim to determine what interesting physics can be recovered. This is accomplished by identifying a means by which events in a partially-ordered set can be aptly and consistently quantified. Since, in general, a partially-ordered set lacks symmetries to constraint any quantification, we propose to distinguish a chain of events, which represents an observer, and quantify some subset of events with respect to the observer chain. We demonstrate that consistent quantification with respect to pairs of observer chains exhibiting a constant relationship with one another results in a metric analogous to the Minkowski metric and that transformation of the quantification with respect to one pair of chains to quantification with respect to another pair of chains results in the Bondi k-calculus, which represents a Lorentz transformation under a simple change of variables. We further demonstrate that chain projection induces geometric structure in the partially-ordered set, which itself is inherently both non-geometric and non-dimensional. Collectively, these results suggest that the concept of space-time geometry may emerge as a unique way for an embedded observer to aptly and consistently quantify a partially-ordered set of events. In addition to having potential implications for space-time physics, this also may serve as a foundation for understanding analogous space-time in condensed matter systems.

marcus

http://arxiv.org/abs/1209.1344
Point particles in 2+1 dimensions: toward a semiclassical loop gravity formulation
Jonathan Ziprick
(Submitted on 6 Sep 2012)
We study point particles in 2+1 dimensional first order gravity using a triangulation to fix the connection and frame-field. The Hamiltonian is reduced to a boundary term which yields the total mass. The triangulation is dynamical with non-trivial transitions occurring when a particle meets an edge. This framework facilitates a description in terms of the loop gravity phase space.
3 pages, for Theory Canada 7 conference proceedings in Canadian Journal of Physics

http://arxiv.org/abs/1209.1110
Introduction to multifractional spacetimes
Gianluca Calcagni
(Submitted on 5 Sep 2012)
We informally review the construction of spacetime geometries with multifractal and, more generally, multiscale properties. Based on fractional calculus, these continuous spacetimes have their dimension changing with the scale; they display discrete symmetries in the ultraviolet and ordinary Poincar\'e symmetries in the infrared. Under certain reasonable assumptions, field theories (including gravity) on multifractional geometries are generally argued to be perturbatively renormalizable. We also sketch the relation with other field theories of quantum gravity based on the renormalization group.
27 pages, 6 figures. Lectures given at Sixth International School on Field Theory and Gravitation 2012 (Petropolis, Brazil). To appear in PoS

MTd2

http://arxiv.org/abs/1209.1266

Machian Time Is To Be Abstracted From What Change?

Edward Anderson
(Submitted on 6 Sep 2012)
"It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction at which we arrive through the changes of things." Ernst Mach [1].
What change? Three answers to this are `any change' (Rovelli), 'all change' (Barbour) and my argument here for the middle ground of a `sufficient totality of locally relevant change' (STLRC) giving a generalization of the astronomers' ephemeris time. I then use STLRC as a selection principle on existing and new approaches to the Problem of Time in Quantum Gravity. Emergent Jacobi-Barbour-Bertotti time can be interpreted as arising from a STLRC, resolves the classical Problem of Time and has an emergent semiclassical counterpart as regards facing the QM Problem of Time.

marcus

Frank Hellmann's 4 September online seminar talk
http://relativity.phys.lsu.edu/ilqgs/hellmann090412.pdf
http://relativity.phys.lsu.edu/ilqgs/hellmann090412.wav
To follow, download the slides PDF first and have them ready when you turn on the audio.

It is about 3 papers, one of which has already appeared on Arxiv, the other two soon to appear.
Papers:
* B. Bahr, B. Dittrich, FH, W. Kaminski:
Holonomy Spin Foam Models: Definition and coarse graining.
(arxiv:1208:3388),
Holonomy Spin Foam Models: Boundary Hilbert spaces and canonical dynamics. (arxiv:soon)
* FH, W. Kaminski:
Holonomy Spin Foam Models: Asymptotic Dynamics of EPRL type
models. (arxiv: soon+ε)

The authors are at Perimeter, MPI-Potsdam (Albert Einstein Institute), and Cambridge DAMPT. The abstract for the first of the three papers is:
==quote==
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two–complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods , which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
==endquote==
For more information about the ILQGS series of talks:
http://relativity.phys.lsu.edu/ilqgs/
http://relativity.phys.lsu.edu/ilqgs/schedulefa12.html

marcus

http://arxiv.org/abs/1209.1609
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 7 Sep 2012)
Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the paradigm is extended to a self-consistent theory from the Planck scale to the onset of slow roll inflation, covering some 11 orders of magnitude in energy density and curvature. This pre-inflationary dynamics also opens a small window for novel effects, e.g. a source for non-Gaussianities, which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
4 pages, 2 figures

marcus

http://arxiv.org/abs/1209.2752
On the choice of time in the continuum limit of polymeric effective theories
Alejandro Corichi, Tatjana Vukasinac
(Submitted on 12 Sep 2012)
In polymeric quantum theories, a natural question pertains to the so called continuum limit, corresponding to the limit where the 'discreteness parameter' λ approaches zero. In particular one might ask whether the limit exists and, in that case, what the limiting theory is. Here we review recent results on the classical formulation of the problem for a soluble model in loop quantum cosmology. We show that it is only through the introduction of a particular λ-dependent internal time function that the limit λ→0 can be well defined. We then compare this result with the existing analysis in the quantum theory, where the dynamics was cast in terms of an internal (λ-independent) parameter for which the limit does not exist. We briefly comment on the steps needed to define the corresponding time parameter in the quantum theory for which the limit was shown to exist classically.
12 pages

http://arxiv.org/abs/1209.2766
Anomaly freedom of the vector modes with holonomy corrections in perturbative Euclidean loop quantum gravity
Jian-Pin Wu, Yongge Ma
(Submitted on 13 Sep 2012)
We study the perturbation of the effective Hamiltonian constraint with holonomy correction from Euclidean loop quantum gravity. The Poisson bracket between the corrected Hamiltonian constraint and the diffeomorphism constraint is derived for vector modes. Some specific form of the holonomy correction function fⁱ_cd is found, which satisfies that the constraint algebra is anomaly-free. This result confirms the possibility of non-trivial holonomy corrections from full theory while preserving anomaly-free constraint algebra in the perturbation framework. It also gives valuable hints on the possible form of holonomy corrections in effective loop quantum gravity.
16 pages, no figures

marcus

http://arxiv.org/abs/1209.3087
Entropy and entanglement in polymer quantization
Tommaso F. Demarie, Daniel R. Terno
(Submitted on 14 Sep 2012)
Polymer quantization is as a useful toy model for the mathematical aspects of loop quantum gravity and is interesting in its own right. Analyzing entropies in the standard Hilbert space and the polymer Hilbert space we show that they converge in the limit of vanishing polymer scale. We derive a general bound that relates entropies of physically equivalent states in unitarily inequivalent representations.
5 pages

http://arxiv.org/abs/1209.3252
A review of the 1/N expansion in random tensor models
Razvan Gurau
(Submitted on 14 Sep 2012)
Matrix models are a highly successful framework for the analytic study of random two dimensional surfaces with applications to quantum gravity in two dimensions, string theory, conformal field theory, statistical physics in random geometry, etc. Their success relies crucially on the so called 1/N expansion introduced by 't Hooft.
In higher dimensions matrix models generalize to tensor models. In the absence of a viable 1/N expansion tensor models have for a long time been less successful in providing an analytically controlled theory of random higher dimensional topological spaces. This situation has drastically changed recently. Models for a generic complex tensor have been shown to admit a 1/N expansion dominated by graphs of spherical topology in arbitrary dimensions and to undergo a phase transition to a continuum theory.
11 pages. Proceedings of the International Congress on Mathematical Physics 2012 - Topical Section: Quantum Field Theory

marcus

http://arxiv.org/abs/1209.3403
Quantum Cosmology: Effective Theory
Martin Bojowald
(Submitted on 15 Sep 2012)
Quantum cosmology has traditionally been studied at the level of symmetry-reduced minisuperspace models, analyzing the behavior of wave functions. However, in the absence of a complete full setting of quantum gravity and detailed knowledge of specific properties of quantum states, it remained difficult to make testable predictions. For quantum cosmology to be part of empirical science, it must allow for a systematic framework in which corrections to well-tested classical equations can be derived, with any ambiguities and ignorance sufficiently parameterized. As in particle and condensed-matter physics, a successful viewpoint is one of effective theories, adapted to specific issues one encounters in quantum cosmology. This review presents such an effective framework of quantum cosmology, taking into account, among other things, space-time structures, covariance, the problem of time and the anomaly issue.
75 pages, 3 figures, Invited Topical Review for Class. Quantum Grav. 29 (2012) 213001

http://arxiv.org/abs/1209.3623
An Approach to Loop Quantum Cosmology Through Integrable Discrete Heisenberg Spin Chains
Christine C. Dantas
(Submitted on 17 Sep 2012)
The quantum evolution equation of Loop Quantum Cosmology (LQC) -- the quantum Hamiltonian constraint -- is a difference equation. We relate the LQC constraint equation in vacuum Bianchi I separable (locally rotationally symmetric) models with an integrable differential-difference nonlinear Schrödinger type equation, which in turn is known to be associated with integrable, discrete Heisenberg spin chain models in condensed matter physics. We illustrate the similarity between both systems with a simple constraint in the linear regime.
6 pages, submitted to Foundations of Physics

http://arxiv.org/abs/1209.3649
Functional renormalization with fermions and tetrads
Pietro Donà, Roberto Percacci
(Submitted on 17 Sep 2012)
We investigate some aspects of the renormalization group flow of gravity in the presence of fermions, which have remained somewhat puzzling so far. The first is the sign of the fermionic contribution to the running of Newton's constant, which depends on details of the cutoff. We argue that only one of the previously used schemes correctly implements the cutoff on eigenvalues of the Dirac operator, and it acts in the sense of screening Newton's constant. We also show that Kähler fermions give the same contribution to the running of the cosmological and Newton constant as four Dirac spinors. We then calculate the graviton contributions to the beta functions by imposing the cutoffs on the irreducible spin components of the tetrad. In this way we can probe the gauge dependence of the off-shell flow. The results resemble closely those of the metric formalism, except for an increased scheme-- and (off shell) gauge--dependence.
28 pages, 4 figures

brief mention:
http://arxiv.org/abs/1209.3511
The effective field theory treatment of quantum gravity
John F. Donoghue
(Submitted on 16 Sep 2012)
This is a pedagogical introduction to the treatment of quantum general relativity as an effective field theory...
22 pages, 3 figures. Presented at the Sixth International School on Field Theory and Gravitation, Petropolis, Brazil, April 2012, to be published in the proceedings.

http://arxiv.org/abs/1209.3339
Annual Modulation of Dark Matter: A Review
Katherine Freese, Mariangela Lisanti, Christopher Savage
(Submitted on 14 Sep 2012)
Direct detection experiments, which are designed to detect the scattering of dark matter off nuclei in detectors, are a critical component in the search for the Universe's missing matter. The count rate in these experiments should experience an annual modulation due to the relative motion of the Earth around the Sun...
37 pages, 7 figures. To appear in Review of Modern Physics

marcus

http://arxiv.org/abs/1209.4038
Fixed points and infrared completion of quantum gravity
Nicolai Christiansen, Daniel F. Litim, Jan M. Pawlowski, Andreas Rodigast
(Submitted on 18 Sep 2012)
The phase diagram of four-dimensional Einstein-Hilbert gravity is studied using Wilson's renormalization group. Smooth trajectories connecting the ultraviolet fixed point at short distances with attractive infrared fixed points at long distances are derived from the non-perturbative graviton propagator. Implications for the asymptotic safety conjecture and further results are discussed.
4 pages, 4 figures

marcus

http://arxiv.org/abs/1209.4295
A review of the large N limit of tensor models
Razvan Gurau
(Submitted on 19 Sep 2012)
Random matrix models encode a theory of random two dimensional surfaces with applications to string theory, conformal field theory, statistical physics in random geometry and quantum gravity in two dimensions. The key to their success lies in the 1/N expansion introduced by 't Hooft. Random tensor models generalize random matrices to theories of random higher dimensional spaces. For a long time, no viable 1/N expansion for tensors was known and their success was limited.
A series of recent results has changed this situation and the extension of the 1/N expansion to tensors has been achieved. We review these results in this paper.
12 pages. Proceedings of The XXIX International Colloquium on Group-Theoretical Methods in Physics, August 20-26 2012, Chern Institute of Mathematics, Nankai University, Tianjin, China

marcus

http://arxiv.org/abs/1209.4374
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia
(Submitted on 19 Sep 2012)
The holonomy-flux algebra A of loop quantum gravity is known to admit a natural representation that is uniquely singled out by the requirement of covariance under spatial diffeomorphisms. In the cosmological context, the requirement of spatial homogeneity naturally reduces A to a much smaller algebra, A_Red, used in loop quantum cosmology. In Bianchi I models, it is shown that the requirement of covariance under residual diffeomorphism symmetries again uniquely selects the representation of A_Red that has been commonly used. We discuss the close parallel between the two uniqueness results and also point out a difference.
9 pages

http://arxiv.org/abs/1209.4539
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 20 Sep 2012)
In this and the companion paper a novel holonomy formulation of so called Spin Foam models of lattice gauge gravity are explored. After giving a natural basis for the space of simplicity constraints we define a universal boundary Hilbert space, on which the imposition of different forms of the simplicity constraints can be studied. We detail under which conditions this Hilbert space can be mapped to a Hilbert space of projected spin networks or an ordinary spin network space.
These considerations allow to derive the general form of the transfer operators which generates discrete time evolution. We will describe the transfer operators for some current models on the different boundary Hilbert spaces and highlight the role of the simplicity constraints determining the concrete form of the time evolution operators.
51 pages, 18 figures

http://arxiv.org/abs/1209.4376
Multi-fractional spacetimes, asymptotic safety and Hořava-Lifshitz gravity
Gianluca Calcagni
(Submitted on 19 Sep 2012)
We compare the recently formulated multi-fractional spacetimes with field theories of quantum gravity based on the renormalization group (RG), such as asymptotic safety and Horava-Lifshitz gravity. The change of spacetime dimensionality with the probed scale is realized in both cases by an adaptation of the measurement tools (`rods') to the scale, but in different ways. In the multi-fractional case, by an adaptation of the position-space measure, roughly corresponding to a scale dependence of the coordinates. In the case of RG-based theories, by an adaptation of the momenta. The two pictures are mapped into each other, thus presenting the fractal structure of spacetime in RG-based theories under an alternative perspective.
17 pages, 1 table

http://arxiv.org/abs/1209.4606
Addendum to "A Renormalizable 4-Dimensional Tensor Field Theory"
Joseph Ben Geloun, Vincent Rivasseau
(Submitted on 20 Sep 2012)
This note fills a gap in the article with title above [1]. We provide the proof of Equation (82) of Lemma 5 in [1] and thereby complete its power counting analysis with a more precise next-to-leading-order estimate.
10 pages, 4 figures

marcus

http://arxiv.org/abs/1209.4892
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
Bianca Dittrich, James P. Ryan
(Submitted on 21 Sep 2012)
The 1-parameter family of transformations identified by Barbero and Immirzi plays a significant role in non-perturbative approaches to quantum gravity, among them Loop Quantum Gravity and Spin Foams. It facilitates the loop quantization programme and subsequently the Barbero-Immirzi parameter (gamma) arises in both the spectra of geometrical operators and in the dynamics provided by Spin Foams. However, the debate continues as to whether quantum physics should be Barbero-Immirzi parameter dependent. Starting from a discrete SO(4)-BF theory phase space, we find two possible reductions with respect to a discrete form of the simplicity constraints. The first reduces to a phase space with gamma-dependent symplectic structure and more generally in agreement with the phase space underlying Loop Quantum Gravity restricted to a single graph - a.k.a. Twisted Geometries. The second, fuller reduction leads to a gamma-independent symplectic structure on the phase space of piecewise-flat-linear geometries - a.k.a. Regge geometries. Thus, the gamma-dependence of physical predictions is related to the choice of phase space underlying the quantization.
16 + 12 pages

brief mention:
http://arxiv.org/abs/1209.4786
Spectral dimension flow on continuum random multigraph
Georgios Giasemidis, John F. Wheater, Stefan Zohren
(Submitted on 21 Sep 2012)
We review a recently introduced effective graph approximation of causal dynamical triangulations (CDT), the multigraph ensemble. We argue that it is well suited for analytical computations and that it captures the physical degrees of freedom which are important for the reduction of the spectral dimension as observed in numerical simulations of CDT. ...
6 pages, 1 figure, to appear in the Proceedings of Sixth International School on Field Theory and Gravitation 2012 (Petropolis, Brazil)

http://arxiv.org/abs/1209.4798
Aspects of dynamical dimensional reduction in multigraph ensembles of CDT
Georgios Giasemidis, John F. Wheater, Stefan Zohren
(Submitted on 21 Sep 2012)
We study the continuum limit of a "radially reduced" approximation of Causal Dynamical Triangulations (CDT), so-called multigraph ensembles, and explain why they serve as realistic toy models to study the dimensional reduction observed in numerical simulations of four-dimensional CDT...
4 pages, 1 figure, Presented at "Gravity, Quantum, and Black Holes" session of IC-MSQUARE 2012, Budapest

John86

http://arxiv.org/abs/1209.5284
The Tensor Track: an Update
Vincent Rivasseau
(Submitted on 24 Sep 2012)
The tensor track approach to quantum gravity is based on a new class of quantum field theories, called tensor group field theories (TGFTs). We provide a brief review of recent progress and list some desirable properties of TGFTs. In order to narrow the search for interesting models, we also propose a set of guidelines for TGFT's loosely inspired by the Osterwalder-Schrader axioms of ordinary Euclidean QFT.

http://arxiv.org/abs/1209.5060
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 23 Sep 2012)
A new approach to a unified theory of quantum gravity based on noncommutative geometry and canonical quantum gravity is presented. The approach is built around a *-algebra generated by local holonomy-diffeomorphisms on a 3-manifold and a quantized Dirac type operator; the two capturing the kinematics of quantum gravity formulated in terms of Ashtekar variables. We prove that the separable part of the spectrum of the algebra is contained in the space of measurable connections modulo gauge transformations and we give limitations to the non-separable part. The construction of the Dirac type operator -- and thus the application of noncommutative geometry -- is motivated by the requirement of diffeomorphism invariance. We conjecture that a semi-finite spectral triple, which is invariant under volume-preserving diffeomorphisms, arise from a GNS construction of a semi-classical state. Key elements of quantum field theory emerge from the construction in a semi-classical limit, as does an almost commutative algebra. Finally, we note that the spectrum of loop quantum gravity emerges from a discretization of our construction. Certain convergence issues are left unresolved. This paper is the first of two where the second paper is concerned with mathematical details and proofs concerning the spectrum of the holonomy-diffeomorphism algebra.

http://arxiv.org/abs/1209.5057
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity II
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 23 Sep 2012)
We introduce the holonomy-diffeomorphism algebra, a C*-algebra generated by flows of vectorfields and the compactly supported smooth functions on a manifold. We show that the separable representations of the holonomy-diffeomorphism algebra are given by measurable connections, and that the unitary equivalence of the representations corresponds to measured gauge equivalence of the measurable connections. We compare the setup to Loop Quantum Gravity and show that the generalized connections found there are not contained in the spectrum of the holonomy-diffeomorphism algebra in dimensions higher than one. This is the second paper of two, where the prequel gives an exposition of a framework of quantum gravity based on the holonomy-diffeomorphism algebra.

http://arxiv.org/abs/1209.4948
Processing quantum information with relativistic motion of atoms
Eduardo Martin-Martinez, David Aasen, Achim Kempf
(Submitted on 22 Sep 2012)
We show that particle detectors, such as 2-level atoms, in non-inertial motion (or in gravitational fields) could be used to build quantum gates for the processing of quantum information. Concretely, we show that through suitably chosen non-inertial trajectories of the detectors the interaction Hamiltonian's time dependence can be modulated to yield arbitrary rotations in the Bloch sphere due to relativistic quantum effects.

http://arxiv.org/abs/1209.5196
Evidence for Bohmian velocities from conditional Schrodinger equation
H. Nikolic
(Submitted on 24 Sep 2012)
It is often argued that measurable predictions of Bohmian mechanics cannot be distinguished from those of a theory with arbitrarily modified particle velocities satisfying the same equivariance equation. By considering the wave function of a closed system in a state with definite total energy, we argue that a distinction in measurable predictions is possible. Even though such a wave function is time-independent, the conditional wave function for a subsystem depends on time through the time-dependent particle trajectories not belonging to the subsystem. If these trajectories can be approximated by classical trajectories, then the conditional wave function can be approximated by a wave function which satisfies Schrodinger equation in a classical time-dependent potential, which is in good agreement with observations. However, such an approximation cannot be justified for particle velocities significantly deviating from the Bohmian ones, implying that Bohmian velocities are observationally preferred.

http://arxiv.org/abs/1209.5271
Using Newton's Law for Dark Energy
Paul Frampton
(Submitted on 24 Sep 2012)
A model is introduced in which Newton's law is modified between matter and dark energy corpuscles (DECs). The model predicts that the DEC component is presently decelerating in its expansion at 14% of the magnitude of the matter expansion acceleration. In the future, expansion of the DEC universe will continue to decelerate.

marcus

This paper is very beautiful. Clear writing makes it broadly understandable (partly John Baez influence on Derek I imagine):
http://arxiv.org/abs/1210.0019
Lifting General Relativity to Observer Space
Steffen Gielen, Derek K. Wise
(Submitted on 28 Sep 2012)
The 'observer space' of a Lorentzian spacetime is the space of future-timelike unit tangent vectors. Using Cartan geometry, we first study the structure a given spacetime induces on its observer space, then use this to define abstract observer space geometries for which no underlying spacetime is assumed. We propose taking observer space as fundamental in general relativity, and prove integrability conditions under which spacetime can be reconstructed as a quotient of observer space. Additional field equations on observer space then descend to Einstein's equations on the reconstructed spacetime. We also consider the case where no such reconstruction is possible, and spacetime becomes an observer-dependent, relative concept. Finally, we discuss applications of observer space, including a geometric link between covariant and canonical approaches to gravity.
34 pages

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell
(Submitted on 1 Oct 2012)
Loop quantum cosmology allows for arbitrary superpositions of the triad variable. We show here how these superpositions can become indistinguishable from a classical mixture by the interaction with fermions. We calculate the reduced density matrix for a locally rotationally symmetric Bianchi I model and show that the purity factor for the triads decreases by decoherence. In this way, the Universe assumes a definite orientation.
12 pages, 1 figure

Late addition, somehow overlooked in July when it came out:
http://arxiv.org/abs/1207.6734
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau
(Submitted on 28 Jul 2012)
We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.
33 pages, 8 figures
EDIT: I now see that this was added to the bibliography when it came out, in post #1781,
http://physicsforums.com/showthread....91#post4016291
but I overlooked it when sifting thru the quarter's papers to assemble a list for the poll.

EDIT: Tom, interesting comment about Kiefer! I had a vague notion of him as an independent, able to do research in LQG and make a significant contribution, but apt equally well three months later to post an article in some other branch of QG. I will look more carefully now.

tom.stoer

Claus Kiefer is joining the LQC community ...

marcus

http://arxiv.org/abs/1210.0849
The polymer quantization in LQG: massless scalar field
Marcin Domagala, Michal Dziendzikowski, Jerzy Lewandowski
(Submitted on 2 Oct 2012)
The polymer quantization of matter fields is a diffeomorphism invariant framework compatible with Loop Quantum Gravity. Whereas studied by itself, it is not explicitly used in the known completely quantizable models of matter coupled to LQG. In the current paper we apply the polymer quantization to the model of massless scalar field coupled to LQG. We show that the polymer Hilbert space of the field degrees of freedom times the LQG Hilbert space of the geometry degrees of freedom admit the quantum constraints of GR and accommodate their explicit solutions. In this way the quantization can be completed. That explicit way of solving the quantum constraints suggests interesting new ideas.
19 pages, no figures, Contribution to the Proceedings of the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

briefly noted, possibly of general interest:
http://arxiv.org/abs/1210.0544
Dark matter and cosmic structure
Carlos S. Frenk, Simon D. M. White
(Submitted on 1 Oct 2012)
We review the current standard model for the evolution of cosmic structure, tracing its development over the last forty years and focusing specifically on the role played by numerical simulations and on aspects related to the nature of dark matter.
27 pages. Invited review in Annalen der Physik - www.ann-phys.org

marcus

briefly noted:
http://arxiv.org/abs/1210.0944
Reverse Engineering Quantum Field Theory
Robert Oeckl (CCM-UNAM)
(Submitted on 2 Oct 2012)
An approach to the foundations of quantum theory is advertised that proceeds by "reverse engineering" quantum field theory. As a concrete instance of this approach, the general boundary formulation of quantum theory is outlined.
5 pages,