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## Intuitive content of Loop Gravity--Rovelli's program

http://arxiv.org/abs/1302.3833
Loop Quantum Cosmology
Ivan Agullo, Alejandro Corichi
(Submitted on 15 Feb 2013)
This Chapter provides an up to date, pedagogical review of some of the most relevant advances in loop quantum cosmology. We review the quantization of homogeneous cosmological models, their singularity resolution and the formulation of effective equations that incorporate the main quantum corrections to the dynamics. We also summarize the theory of quantized metric perturbations propagating in those quantum backgrounds. Finally, we describe how this framework can be applied to obtain a self-consistent extension of the inflationary scenario to incorporate quantum aspects of gravity, and to explore possible phenomenological consequences.
52 pages, 5 figures. To appear as a Chapter of "The Springer Handbook of Spacetime," edited by A. Ashtekar and V. Petkov. (Springer-Verlag, at Press).
 http://arxiv.org/abs/1302.1496 Standard Model Higgs field and energy scale of gravity F.R. Klinkhamer (Submitted on 6 Feb 2013 (v1), last revised 14 Feb 2013 (this version, v3)) The effective potential of the Higgs scalar field in the Standard Model may have a second degenerate minimum at an ultrahigh vacuum expectation value. This second minimum then determines, by radiative corrections, the values of the top-quark and Higgs-boson masses at the standard minimum corresponding to the electroweak energy scale. An argument is presented that this ultrahigh vacuum expectation value is proportional to the energy scale of gravity, E_{Planck} \equiv \sqrt{\hbar c^5/G_N}, considered to be characteristic of a spacetime foam. In the context of a simple model, the existence of kink-type wormhole solutions places a lower bound on the ultrahigh vacuum expectation value and this lower bound is of the order of E_{Planck}. http://arxiv.org/abs/1302.3680 Quantum Gravity on a Quantum Computer? Achim Kempf (Submitted on 15 Feb 2013) EPR-type measurements on spatially separated entangled spin qubits allow one, in principle, to detect curvature. Also the entanglement of the vacuum state is affected by curvature. Here, we ask if the curvature of spacetime can be expressed entirely in terms of the spatial entanglement structure of the vacuum. This would open up the prospect that quantum gravity could be simulated on a quantum computer and that quantum information techniques could be fully employed in the study of quantum gravity. http://arxiv.org/abs/1302.3648 Causality and non-equilibrium second-order phase transitions in inhomogeneous systems A. del Campo, T. W. B. Kibble, W. H. Zurek (Submitted on 14 Feb 2013) When a second-order phase transition is crossed at fine rate, the evolution of the system stops being adiabatic as a result of the critical slowing down in the neighborhood of the critical point. In systems with a topologically nontrivial vacuum manifold, disparate local choices of the ground state lead to the formation of topological defects. The universality class of the transition imprints a signature on the resulting density of topological defects: It obeys a power law in the quench rate, with an exponent dictated by a combination of the critical exponents of the transition. In inhomogeneous systems the situation is more complicated, as the spontaneous symmetry breaking competes with bias caused by the influence of the nearby regions that already chose the new vacuum. As a result, the choice of the broken symmetry vacuum may be inherited from the neighboring regions that have already entered the new phase. This competition between the inherited and spontaneous symmetry breaking enhances the role of causality, as the defect formation is restricted to a fraction of the system where the front velocity surpasses the relevant sound velocity and phase transition remains effectively homogeneous. As a consequence, the overall number of topological defects can be substantially suppressed. When the fraction of the system is small, the resulting total number of defects is still given by a power law related to the universality class of the transition, but exhibits a more pronounced dependence on the quench rate. This enhanced dependence complicates the analysis but may also facilitate experimental test of defect formation theories.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1302.5265 The loop quantum gravity black hole Rodolfo Gambini, Jorge Pullin (Submitted on 21 Feb 2013) We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes. The new observables that arise suggest a possible resolution for the "firewall" problem of evaporating black holes. Comments: 4 pages,
 http://arxiv.org/abs/1302.5273 There exist no 4-dimensional geodesically equivalent metrics with the same stress-energy tensor Volodymir Kiosak, Vladimir S. Matveev (Submitted on 21 Feb 2013) We show that if two 4-dimensional metrics of arbitrary signature on one manifold are geodesically equivalent (i.e., have the same geodesics considered as unparameterized curves) and are solutions of the Einstein field equation with the same stress-energy tensor, then they are affinely equivalent or flat. Under the additional assumption that the metrics are complete or the manifold is closed, the result survives in all dimensions >2. http://arxiv.org/abs/1302.5162 On CCC-predicted concentric low-variance circles in the CMB sky V. G. Gurzadyan, R. Penrose (Submitted on 21 Feb 2013) A new analysis of the CMB, using WMAP data, supports earlier indications of non-Gaussian features of concentric circles of low temperature variance. Conformal cyclic cosmology (CCC) predicts such features from supermassive black-hole encounters in an aeon preceding our Big Bang. The significance of individual low-variance circles in the true data has been disputed; yet a recent independent analysis has confirmed CCC's expectation that CMB circles have a non-Gaussian temperature distribution. Here we examine concentric sets of low-variance circular rings in the WMAP data, finding a highly non-isotropic distribution. A new "sky-twist" procedure, directly analysing WMAP data, without appeal to simulations, shows that the prevalence of these concentric sets depends on the rings being circular, rather than even slightly elliptical, numbers dropping off dramatically with increasing ellipticity. This is consistent with CCC's expectations; so also is the crucial fact that whereas some of the rings' radii are found to reach around $15^\circ$, none exceed $20^\circ$. The non-isotropic distribution of the concentric sets may be linked to previously known anomalous and non-Gaussian CMB features.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1302.5695 Quantum matter in quantum space-time Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, Casey Tomlin (Submitted on 22 Feb 2013) Quantum matter in quantum space-time is discussed using general properties of energy-conservation laws. As a rather radical conclusion, it is found that standard methods of differential geometry and quantum field theory on curved space-time are inapplicable in canonical quantum gravity, even at the level of effective equations. 16 pages http://arxiv.org/abs/1302.5564 Spin-cube Models of Quantum Gravity A. Mikovic (Submitted on 22 Feb 2013) We study the state-sum models of quantum gravity based on a representation 2-category of the Poincare 2-group. We call them spin-cube models, since they are categorical generalizations of spin-foam models. A spin-cube state sum can be considered as a path integral for a constrained 2-BF theory, and depending on how the constraints are imposed, a spin-cube state sum can be reduced to a path integral for the area-Regge model with the edge-length constraints, or to a path integral for the Regge model. We also show that the effective actions for these spin-cube models have the correct classical limit. 16 pages brief mention (Shapo is always interesting): http://arxiv.org/abs/1302.5619 Spontaneously Broken Conformal Symmetry: Dealing with the Trace Anomaly Roberta Armillis, Alexander Monin, Mikhail Shaposhnikov (Submitted on 22 Feb 2013) The majority of renormalizable field theories possessing the scale invariance at the classical level exhibits the trace anomaly once quantum corrections are taken into account. This leads to the breaking of scale and conformal invariance. At the same time any realistic theory must contain gravity and is thus non-renormalizable. We show that discarding the renormalizability it is possible to construct viable models allowing to preserve the scale invariance at the quantum level. We present explicit one-loop computations for two toy models to demonstrate the main idea of the approach. Constructing the renormalized energy momentum tensor we show that it is traceless, meaning that the conformal invariance is also preserved. 20 pages, 5 figures
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1302.6264 The Solution to the Problem of Time in Shape Dynamics Julian Barbour, Tim Koslowski, Flavio Mercati (Submitted on 25 Feb 2013) The absence of unique time evolution in Einstein's spacetime description of gravity leads to the hitherto unresolved 'problem of time' in quantum gravity. Shape Dynamics is an objectively equivalent representation of gravity that trades spacetime refoliation invariance for three-dimensional conformal invariance. Its logical completion presented here gives a dimensionless description of gravitational dynamics. We show that in this framework the classical problem of time is completely solved. Since a comparable definitive solution is impossible within the spacetime description, we believe Shape Dynamics provides a key ingredient for the creation of quantum gravity. 14 pages http://arxiv.org/abs/1302.6566 Singularity resolution from polymer quantum matter Andreas Kreienbuehl, Tomasz Pawlowski (Submitted on 26 Feb 2013) We study the polymeric nature of quantum matter fields using the example of a Friedmann-Lemaitre-Robertson-Walker universe sourced by a minimally coupled massless scalar field. The model is treated in the symmetry reduced regime via deparametrization techniques, with the scale factor playing the role of time. Subsequently the remaining dynamic degrees of freedom are polymer quantized. The analysis of the resulting dynamic shows that the big bang singularity is resolved, although with the form of the resolution differing significantly from that of the models with matter clocks: dynamically, the singularity is made passable rather than avoided. Furthermore, the results of the genuine quantum analysis expose crucial limitations to the so-called effective dynamics in loop quantum cosmology when applied outside of the simplest isotropic settings. 12 pages and 4 figures
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1302.7142 Holonomy Operator and Quantization Ambiguities on Spinor Space Etera R. Livine (Submitted on 28 Feb 2013) We construct the holonomy-flux operator algebra in the recently developed spinor formulation of loop gravity. We show that, when restricting to SU(2)-gauge invariant operators, the familiar grasping and Wilson loop operators are written as composite operators built from the gauge-invariant 'generalized ladder operators' recently introduced in the U(N) approach to intertwiners and spin networks. We comment on quantization ambiguities that appear in the definition of the holonomy operator and use these ambiguities as a toy model to test a class of quantization ambiguities which is present in the standard regularization and definition of the Hamiltonian constraint operator in loop quantum gravity. 14 pages http://arxiv.org/abs/1302.7135 Fields and Laplacians on Quantum Geometries Johannes Thürigen (Submitted on 28 Feb 2013) In fundamentally discrete approaches to quantum gravity such as loop quantum gravity, spin-foam models, group field theories or Regge calculus observables are functions on discrete geometries. We present a bra-ket formalism of function spaces and discrete calculus on abstract simplicial complexes equipped with geometry and apply it to the mentioned theories of quantum gravity. In particular we focus on the quantum geometric Laplacian and discuss as an example the expectation value of the heat kernel trace from which the spectral dimension follows. 3 pages, submitted to the Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, July 1-7, 2012 http://arxiv.org/abs/1302.7037 Loop Quantization of Shape Dynamics Tim Koslowski (Submitted on 28 Feb 2013) Loop Quantum Gravity (LQG) is a promising approach to quantum gravity, in particular because it is based on a rigorous quantization of the kinematics of gravity. A difficult and still open problem in the LQG program is the construction of the physical Hilbert space for pure quantum gravity. This is due to the complicated nature of the Hamilton constraints. The Shape Dynamics description of General Relativity (GR) replaces the Hamilton constraints with spatial Weyl constraints, so the problem of finding the physical Hilbert space reduces to the problem of quantizing the Weyl constraints. Unfortunately, it turns out that a loop quantization of Weyl constraints is far from trivial despite their intuitive physical interpretation. A tentative quantization proposal and interpretation proposal is given in this contribution. 3 pages, talk given at the 13th Marcel Grossmann Meeting, Stockholm 1-7 July 2012 I don't normally list online seminar talks but these might be of particular interest to people following QG research: http://pirsa.org/13020132/ Quantum Gravity as Random Geometry Vincent Rivasseau Abstract: Matrix models, random maps and Liouville field theory are prime tools which connect random geometry and quantum gravity in two dimensions. The tensor track is a new program to extend this connection to higher dimensions through the corresponding notions of tensor models, colored triangulations and tensor group field theories. 27/02/2013 http://pirsa.org/13020146/ The universe as a process of unique events Lee Smolin 26/02/2013 (Covers material from a new book "Time Reborn" listed on Amazon to appear April 2013) also of interest, possibly related: http://arxiv.org/abs/1302.7291 General Relativity and Quantum Cosmology The arrow of time and the nature of spacetime George F R Ellis (Submitted on 28 Feb 2013) This paper extends the work of a previous paper [arXiv:1108.5261] on top-down causation and quantum physics, to consider the origin of the arrow of time. It proposes that a past condition' cascades down from cosmological to micro scales, being realized in many microstructures and setting the arrow of time at the quantum level by top-down causation. This physics arrow of time then propagates up, through underlying emergence of higher level structures, to geology, astronomy, engineering, and biology. The appropriate space-time picture to view all this is an emergent block universe (EBU'), that recognizes the way the present is different from both the past and the future. This essential difference is the ultimate reason the arrow of time has to be the way it is. 56 pages, 6 figures
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1303.0195 Living in Curved Momentum Space J. Kowalski-Glikman (Submitted on 1 Mar 2013) In this paper we review some aspects of relativistic particles' mechanics in the case of a non-trivial geometry of momentum space. We start with showing how the curved momentum space arises in the theory of gravity in 2+1 dimensions coupled to particles, when (topological) degrees of freedom of gravity are solved for. We argue that there might exist a similar topological phase of quantum gravity in 3+1 dimensions. Then we characterize the main properties of the theory of interacting particles with curved momentum space and the symmetries of the action. We discuss the spacetime picture and the emergence of the principle of relative locality, according to which locality of events is not absolute but becomes observer dependent, in the controllable, relativistic way. We conclude with the detailed review of the most studied kappa-Poincare framework, which corresponds to the de Sitter momentum space. 23 pages http://arxiv.org/abs/1303.0196 Inhomogeneous Universe in Loop Quantum Gravity Francesco Cianfrani (Submitted on 1 Mar 2013) It is discussed a truncation of the kinematical Hilbert space of Loop Quantum Gravity, which describes the dynamical system associated with an inhomogeneous cosmological model. 3 pages, contribution to the Proceedings of the 13th Marcel Grossman Meeting (Stockholm, Sweden, July 1-7 2012) http://arxiv.org/abs/1303.0060 Differences and similarities between Shape Dynamics and General Relativity Henrique Gomes, Tim Koslowski (Submitted on 1 Mar 2013) The purpose of this contribution is to elucidate some of the properties of Shape Dynamics (SD) and is largely based on a recent longer article. We shall point out some of the key differences between SD and related theoretical constructions, illustrate the central mechanism of symmetry trading in electromagnetism and finally point out some new quantization strategies inspired by SD. We refrain from describing mathematical detail and from citing literature. For both we refer to the longer article. 3 pages, talk given at the 13th Marcel Grossmann Meeting in Stockholm, July 2012 http://arxiv.org/abs/1303.0174 MG13 Proceedings: A lattice Universe as a toy-model for inhomogeneous cosmology Jean-Philippe Bruneton (Submitted on 1 Mar 2013) We briefly report on a previously found new, approximate, solution to Einstein field equations, describing a cubic lattice of spherical masses. This model mimics in a satisfactory way a Universe which can be strongly inhomogeneous at small scales, but quite homogeneous at large ones. As a consequence of field equations, the lattice Universe is found to expand or contract in the same way as the solution of a Friedmann Universe filled with dust having the same average density. The study of observables indicates however the possible existence of a fitting problem, i.e. the fact that the Friedmann model obtained from past-lightcone observables does not match with the one obtained by smoothing the matter content of the Universe. 3 pages. Prepared for MG13 conference
 http://arxiv.org/abs/1303.0762 A new perspective on early cosmology Emanuele Alesci (Submitted on 4 Mar 2013) We present a new perspective on early cosmology based on Loop Quantum Gravity. We use projected spinnetworks, coherent states and spinfoam techniques, to implement a quantum reduction of the full Kinematical Hilbert space of LQG, suitable to describe inhomogeneous cosmological models. Some preliminary results on the solutions of the Scalar constraint of the reduced theory are also presented. http://arxiv.org/abs/1303.0433 A Dynamics for Discrete Quantum Gravity Stan Gudder (Submitted on 2 Mar 2013) This paper is based on the causal set approach to discrete quantum gravity. We first describe a classical sequential growth process (CSGP) in which the universe grows one element at a time in discrete steps. At each step the process has the form of a causal set (causet) and the "completed" universe is given by a path through a discretely growing chain of causets. We then quantize the CSGP by forming a Hilbert space $H$ on the set of paths. The quantum dynamics is governed by a sequence of positive operators $\rho_n$ on $H$ that satisfy normalization and consistency conditions. The pair $(H,\brac{\rho_n})$ is called a quantum sequential growth process (QSGP). We next discuss a concrete realization of a QSGP in terms of a natural quantum action. This gives an amplitude process related to the sum over histories" approach to quantum mechanics. Finally, we briefly discuss a discrete form of Einstein's field equation and speculate how this may be employed to compare the present framework with classical general relativity theory.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1303.0752 Inclusion of matter in inhomogeneous loop quantum cosmology Daniel Martín-de Blas, Mercedes Martín-Benito, Guillermo A. Mena Marugán (Submitted on 4 Mar 2013) We study the hybrid quantization of the linearly polarized Gowdy $T^3$ model with a massless scalar field with the same symmetries as the metric. For simplicity, we quantize its restriction to the model with local rotational symmetry. Using this hybrid approach, the homogeneous degrees of freedom of the geometry are quantized \`a la loop, leading to the resolution of the cosmological singularity. A Fock quantization is employed both for the matter and the gravitational inhomogeneities. Owing to the inclusion of the massless scalar field this system allows us to modelize flat Friedmann-Robertson-Walker cosmologies filled with inhomogeneities propagating in one direction, providing a perfect scenario to study the quantum back-reaction of the inhomogeneities on the polymeric homogeneous and isotropic background. 4 pages

 Quote by marcus I told you I would make several false starts. Eventually there should be a non-technical description of loop gravity in only one to ten pages. Let's keep this thread going until we have one, or find one in the literature.
Hi, sorry for the interruption, I quoted the above from post #17 of this thread.

I'm just wondering if a non-technical discussion of loop gravity in only one to ten pages has ever been achieved?

I would love to have a nice concise and refined synopsis of the main ideas of LQG along with perhaps a well-organized outline of what types of topics are most required.

My specific interests in LQG is to try to understand how and why it is 'background independent", and also any information that might be known on how it might relate to the entropy associated with the horizon area of a black hole. Or actually any information on how a spin network itself relates to the concept of entropy.

Thanks.
 Recognitions: Gold Member Science Advisor Hi Leucippus, this thread started out with discussion (in 2003) but evolved into a Loop and allied QG bibliography. We can start separate discussion threads. You have some good questions so I gathered excerpts (including from this latest) and started a new thread. I hope the new one will prove satisfactory. http://www.physicsforums.com/showthr...=1#post4298277
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1303.1687 Quantum states of the bouncing universe Jean Pierre Gazeau, Jakub Mielczarek, Wlodzimierz Piechocki (Submitted on 7 Mar 2013) In this paper we study quantum dynamics of the bouncing cosmological model. We focus on the model of the flat Friedman-Robertson-Walker universe with a free scalar field. The bouncing behavior, which replaces classical singularity, appears due to the modification of general relativity along the methods of loop quantum cosmology. We show that there exist a unitary transformation that enables to describe the system as a free particle with Hamiltonian equal to canonical momentum. We examine properties of the various quantum states of the Universe: boxcar state, standard coherent state, and soliton-like state, as well as Schrödinger's cat states constructed from these states. Characteristics of the states such as quantum moments and Wigner functions are investigated. We show that each of these states have, for some range of parameters, a proper semiclassical limit fulfilling the correspondence principle. Decoherence of the superposition of two universes is described and possible interpretations in terms of triad orientation and Belinsky-Khalatnikov-Lifshitz conjecture are given. Some interesting features regarding the area of the negative part of the Wigner function have emerged. 18 pages, 19 figures brief mention: http://arxiv.org/abs/1303.1535 The Structure of the Gravitational Action and its relation with Horizon Thermodynamics and Emergent Gravity Paradigm Krishnamohan Parattu, Bibhas Ranjan Majhi, T. Padmanabhan (Submitted on 6 Mar 2013) http://arxiv.org/abs/1303.1782 Non-Associative Geometry and the Spectral Action Principle Shane Farnsworth, Latham Boyle (Submitted on 7 Mar 2013) Chamseddine and Connes have shown how the action for Einstein gravity, coupled to the SU(3) × SU(2) × U(1) standard model of particle physics, may be elegantly recast as the "spectral action" on a certain "non-commutative geometry." In this paper, we show how this formalism may be extended to "non-associative geometries," and explain the motivations for doing so. As a guiding illustration, we present the simplest non-associative geometry (based on the octonions) and evaluate its spectral action: it describes Einstein gravity coupled to a G2 gauge theory, with 8 Dirac fermions (which transform as a singlet and a septuplet under G2). We use this example to illustrate how non-associative geometries may be naturally linked to ordinary (associative) geometries by a certain twisting procedure. This is just the simplest example: in a forthcoming paper we show how to construct realistic models that include Higgs fields, spontaneous symmetry breaking and fermion masses.
 Don't miss these http://arxiv.org/abs/1303.1537 On the theory of composition in physics Lucien Hardy (Submitted on 6 Mar 2013) We develop a theory for describing composite objects in physics. These can be static objects, such as tables, or things that happen in spacetime (such as a region of spacetime with fields on it regarded as being composed of smaller such regions joined together). We propose certain fundamental axioms which, it seems, should be satisfied in any theory of composition. A key axiom is the order independence axiom which says we can describe the composition of a composite object in any order. Then we provide a notation for describing composite objects that naturally leads to these axioms being satisfied. In any given physical context we are interested in the value of certain properties for the objects (such as whether the object is possible, what probability it has, how wide it is, and so on). We associate a generalized state with an object. This can be used to calculate the value of those properties we are interested in for for this object. We then propose a certain principle, the composition principle, which says that we can determine the generalized state of a composite object from the generalized states for the components by means of a calculation having the same structure as the description of the generalized state. The composition principle provides a link between description and prediction. http://arxiv.org/abs/1303.1538 Reconstructing quantum theory Lucien Hardy (Submitted on 6 Mar 2013) We discuss how to reconstruct quantum theory from operational postulates. In particular, the following postulates are consistent only with for classical probability theory and quantum theory. Logical Sharpness: There is a one-to-one map between pure states and maximal effects such that we get unit probability. This maximal effect does not give probability equal to one for any other pure state. Information Locality: A maximal measurement is effected on a composite system if we perform maximal measurements on each of the components. Tomographic Locality: The state of a composite system can be determined from the statistics collected by making measurements on the components. Permutability: There exists a reversible transformation on any system effecting any given permutation of any given maximal set of distinguishable states for that system. Sturdiness: Filters are non-flattening. To single out quantum theory we need only add any requirement that is inconsistent with classical probability theory and consistent with quantum theory. http://arxiv.org/abs/1303.1632 Higgs potential and confinement in Yang-Mills theory on exotic R^4 Torsten Asselmeyer-Maluga, Jerzy Król (Submitted on 7 Mar 2013) We show that pure SU(2) Yang-Mills theory formulated on certain exotic R^4 from the radial family shows confinement. The condensation of magnetic monopoles and the qualitative form of the Higgs potential are derived from the exotic R^4, e. A relation between the Higgs potential and inflation is discussed. Then we obtain a formula for the Higgs mass and discuss a particular smoothness structure so that the Higgs mass agrees with the experimental value. The singularity in the effective dual U(1) potential has its cause by the exotic 4-geometry and agrees with the singularity in the maximal abelian gauge scenario. We will describe the Yang-Mills theory on e in some limit as the abelian-projected effective gauge theory on the standard R^4. Similar results can be derived for SU(3) Yang-Mills theory on an exotic R^4 provided dual diagonal effective gauge bosons propagate in the exotic 4-geometry. http://arxiv.org/abs/1303.1803 Classifying gauge anomalies through SPT orders and classifying anomalies through topological orders Xiao-Gang Wen (Submitted on 7 Mar 2013) In this paper, we systematically study gauge anomalies in bosonic and fermionic weak-coupling gauge theories with gauge group G (which can be continuous or discrete). We argue that, in d space-time dimensions, the gauge anomalies are described by the elements in Free[H^{d+1}(G,R/Z)]\oplus H_\pi^{d+1}(BG,R/Z). The well known Adler-Bell-Jackiw anomalies are classified by the free part of the group cohomology class H^{d+1}(G,R/Z) of the gauge group G (denoted as Free[H^{d+1}(G,\R/\Z)]). We refer other kinds of gauge anomalies beyond Adler-Bell-Jackiw anomalies as nonABJ gauge anomalies, which include Witten SU(2) global gauge anomaly. We introduce a notion of \pi-cohomology group, H_\pi^{d+1}(BG,R/Z), for the classifying space BG, which is an Abelian group and include Tor[H^{d+1}(G,R/Z)] and topological cohomology group H^{d+1}(BG,\R/\Z) as subgroups. We argue that H_\pi^{d+1}(BG,R/Z) classifies the bosonic nonABJ gauge anomalies, and partially classifies fermionic nonABJ anomalies. We also show a very close relation between gauge anomalies and symmetry-protected trivial (SPT) orders [also known as symmetry-protected topological (SPT) orders] in one-higher dimensions. Such a connection will allow us to use many well known results and well developed methods for gauge anomalies to study SPT states. In particular, the \pi-cohomology theory may give a more general description of SPT states than the group cohomology theory. http://arxiv.org/abs/1212.4863 Boundary Degeneracy of Topological Order Juven Wang, Xiao-Gang Wen (Submitted on 19 Dec 2012 (v1), last revised 23 Jan 2013 (this version, v2)) We introduce the notion of boundary degeneracy of topologically ordered states on a compact orientable spatial manifold with boundaries, and emphasize that it provides richer information than the bulk degeneracy. Beyond the bulk-edge correspondence, we find the ground state degeneracy of fully gapped edge states depends on boundary gapping conditions. We develop a quantitative description of different types of boundary gapping conditions by viewing them as different ways of non-fractionalized particle condensation on the boundary. Via Chern-Simons theory, this allows us to derive the ground state degeneracy formula in terms of boundary gapping conditions, which reveals more than the fusion algebra of fractionalized quasiparticles. We apply our results to Toric code and Levin-Wen string-net models. By measuring the boundary degeneracy on a cylinder, we predict Z_k gauge theory and U(1)_k x U(1)_k non-chiral fractional quantum hall state at even integer k can be experimentally distinguished. Our work refines definitions of symmetry protected topological order and intrinsic topological order.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1303.2773 BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models J.Manuel Garcia-Islas (Submitted on 12 Mar 2013) We present a comparison of the calculation of BTZ black hole entropy in loop quantum gravity and in spin foam models. We see that both give the same answer. 6 pages, 3 figures brief mention: http://arxiv.org/abs/1303.2719 Another Survey of Foundational Attitudes Towards Quantum Mechanics Christoph Sommer (Submitted on 11 Mar 2013) Although it has been almost 100 years since the beginnings of quantum mechanics, the discussions about its interpretation still do not cease. Therefore, a survey of opinions regarding this matter is of particular interest. This poll was conducted following an idea and using the methodology of Schlosshauer et al. (arXiv:1301.1069 [quant-ph]), but among a slightly different group. It is supposed to give another snapshot of attitudes towards the interpretation of quantum mechanics and keep discourse about this topic alive. 10 pages, 18 figures, 1 table
 Recognitions: Science Advisor There is one problem with these results, namely that some people may agree that they believe in interpretation x, but that a careful analysis may also show, that they do NOT agree what this interpretation x MEANS. So without such an analysis one always misses the fact that people agreeing on Copenhagen do unfortunately not agree on the same Copenhagen :-)
 http://arxiv.org/abs/1303.3576 Cosmology from Group Field Theory Steffen Gielen, Daniele Oriti, Lorenzo Sindoni (Submitted on 14 Mar 2013) We identify a class of condensate states in the group field theory (GFT) approach to quantum gravity that can be interpreted as macroscopic homogeneous spatial geometries. We then extract the dynamics of such condensate states directly from the fundamental quantum GFT dynamics, following the procedure used in ordinary quantum fluids. The effective dynamics is a non-linear and non-local extension of quantum cosmology. We also show that any GFT model with a kinetic term of Laplacian type gives rise, in a semi-classical (WKB) approximation and in the isotropic case, to a modified Friedmann equation. This is the first concrete, general procedure for extracting an effective cosmological dynamics directly from a fundamental theory of quantum geometry. http://arxiv.org/abs/1303.3497 The DeWitt Equation in Quantum Field Theory Parikshit Dutta, Krzysztof A. Meissner, Hermann Nicolai (Submitted on 14 Mar 2013) We take a new look at the DeWitt equation, a defining equation for the effective action functional in quantum field theory. We present a formal solution to this equation, and discuss the equation in various contexts, and in particular for models where it can be made completely well defined, such as the Wess-Zumino model in two dimensions.

 Tags loop quantum gravity