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Our picks for first quarter 2013 MIP (most important QG paper)

  1. Relative Locality in Curved Space-time

    6.3%
  2. Symmetry and Evolution in Quantum Gravity

    18.8%
  3. Loop quantum dynamics of the gravitational collapse

    12.5%
  4. A Gravitational Entropy Proposal

    37.5%
  5. Loop Quantum Gravity and the The Planck Regime of Cosmology

    18.8%
  6. Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity

    0 vote(s)
    0.0%
  7. Cosmology from Group Field Theory

    6.3%
  8. BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models

    18.8%
  9. Inclusion of matter in inhomogeneous loop quantum cosmology

    0 vote(s)
    0.0%
  10. Holonomy Operator and Quantization Ambiguities on Spinor Space

    12.5%
  11. The loop quantum gravity black hole

    18.8%
  12. Loop Quantum Cosmology

    6.3%
  13. Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity

    6.3%
  14. Death and resurrection of the zeroth principle of thermodynamics

    31.3%
  15. The pre-inflationary dynamics of loop quantum cosmology

    12.5%
  16. Embedding loop quantum cosmology without piecewise linearity

    12.5%
  17. Hamiltonian spinfoam gravity

    18.8%
  18. Gauge networks in noncommutative geometry

    12.5%
  19. Quantum-Reduced Loop Gravity: Cosmology

    0 vote(s)
    0.0%
  20. Inflation as a prediction of loop quantum cosmology

    25.0%
Multiple votes are allowed.
  1. Mar 28, 2013 #1

    marcus

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    Dearly Missed

    Of the twenty candidates, please choose the one(s) you think will prove most significant for future research in Loop-and-allied quantum gravity. Since the poll is multiple choice, it's possible to vote for several papers. Abstract summaries follow in the next post.


    http://arxiv.org/abs/1303.7216
    Relative Locality in Curved Space-time
    Jerzy Kowalski-Glikman, Giacomo Rosati

    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    http://arxiv.org/abs/1303.6157
    Loop quantum dynamics of the gravitational collapse
    Yaser Tavakoli, Joao Marto, Andrea Dapor

    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    http://arxiv.org/abs/1303.4752
    Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
    Norbert Bodendorfer, Yasha Neiman

    http://arxiv.org/abs/1303.3576
    Cosmology from Group Field Theory
    Steffen Gielen, Daniele Oriti, Lorenzo Sindoni

    http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    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

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    http://arxiv.org/abs/1302.3833
    Loop Quantum Cosmology
    Ivan Agullo, Alejandro Corichi

    http://arxiv.org/abs/1302.1781
    Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
    Aldo Riello

    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom

    http://arxiv.org/abs/1301.2245
    Quantum-Reduced Loop Gravity: Cosmology
    Emanuele Alesci, Francesco Cianfrani

    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau
     
  2. jcsd
  3. Mar 28, 2013 #2

    marcus

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    http://arxiv.org/abs/1303.7216
    Relative Locality in Curved Space-time
    Jerzy Kowalski-Glikman, Giacomo Rosati
    (Submitted on 28 Mar 2013)
    In this paper we construct the action describing dynamics of the particle moving in curved spacetime, with a non-trivial momentum space geometry. Curved momentum space is the core feature of theories where relative locality effects are presents. So far aspects of nonlinearities in momentum space have been studied only for flat or constantly expanding (De Sitter) spacetimes, relying on the their maximally symmetric nature. The extension of curved momentum space frameworks to arbitrary spacetime geometries could be relevant for the opportunities to test Planck-scale curvature/deformation of particles momentum space. As a first example of this construction we describe the particle with kappa-Poincaré momentum space on a circular orbit in Schwarzschild spacetime, where the contributes of momentum space curvature turn out to be negligible. The analysis of this problem relies crucially on the solution of the soccer ball problem.
    8 pages

    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault
    (Submitted on 28 Mar 2013)
    We propose an operator constraint equation for the wavefunction of the Universe that admits genuine evolution. While the corresponding classical theory is equivalent to the canonical decomposition of General Relativity, the quantum theory makes predictions that are distinct from Wheeler-DeWitt cosmology. Furthermore, the local symmetry principle - and corresponding observables - of the theory have a direct interpretation in terms of a conventional gauge theory, where the gauge symmetry group is that of spatial conformal diffeomorphisms (that preserve the spatial volume of the Universe). The global evolution is in terms of an arbitrary parameter that serves only as an unobservable label for successive states of the Universe. Our proposal follows unambiguously from a suggestion of York whereby the independently specifiable initial data in the action principle of General Relativity is given by a conformal geometry and the spatial average of the York time on the spacelike hypersurfaces that bound the variation. Remarkably, such a variational principle uniquely selects the form of the constraints of the theory so that we can establish a precise notion of both symmetry and evolution in quantum gravity.
    36 preprint pages. 1 table

    http://arxiv.org/abs/1303.6157
    Loop quantum dynamics of the gravitational collapse
    Yaser Tavakoli, Joao Marto, Andrea Dapor
    (Submitted on 25 Mar 2013)
    We consider a quantum description for a spherically symmetric gravitational collapse of a massless scalar field. The effective scenario from loop quantum gravity is applied to a homogeneous interior spacetime. The classical singularity that arises at the final stage of our collapsing system is resolved and replaced by a quantum bounce. Our main purpose is to investigate the evolution of trapped surfaces during the collapse in semiclassical regime. We show that, in this regime, there exists a threshold scale below which no horizon can form as collapse evolves towards the bounce. By employing the matching conditions at the boundary shell, quantum effects are carried out to the exterior region, leading to an improved Vaidya geometry. In addition, the effective mass loss emerging in this model predicts an outward energy flux from the interior quantum geometry regime.
    11 pages, 5 figures

    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol
    (Submitted on 22 Mar 2013)
    We propose a thermodynamically motivated measure of gravitational entropy based on the Bel-Robinson tensor, which has a natural interpretation as the effective super-energy-momentum tensor of free gravitational fields. The specific form of this measure differs depending on whether the gravitational field is Coulomb-like or wave-like, and reduces to the Bekenstein-Hawking value when integrated over the interior of a Schwarzschild black hole. For scalar perturbations of a Robertson-Walker geometry we find that the entropy goes like the Hubble weighted anisotropy of the gravitational field, and therefore increases as structure formation occurs. This is in keeping with our expectations for the behaviour of gravitational entropy in cosmology, and provides a thermodynamically motivated arrow of time for cosmological solutions of Einstein's field equations. It is also in keeping with Penrose's Weyl curvature hypothesis.
    17 pages

    http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar
    (Submitted on 20 Mar 2013)
    The very early universe provides the best arena we currently have to test quantum gravity theories. The success of the inflationary paradigm in accounting for the observed inhomogeneities in the cosmic microwave background already illustrates this point to a certain extent because the paradigm is based on quantum field theory on the curved cosmological space-times. However, this analysis excludes the Planck era because the background space-time satisfies Einstein's equations all the way back to the big bang singularity. Using techniques from loop quantum gravity, the paradigm has now been extended to a self-consistent theory from the Planck regime to the onset of inflation, covering some 11 orders of magnitude in curvature. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects, such as a modification of the consistency relation involving the scalar and tensor power spectra and a new source for non-Gaussianities. Thus, the genesis of the large scale structure of the universe can be traced back to quantum gravity fluctuations in the Planck regime. This report provides a bird's eye view of these developments for the general relativity community.
    23 pages, 4 figures. Plenary talk at the Conference: Relativity and Gravitation: 100 Years after Einstein in Prague. To appear in the Proceedings to be published by Edition Open Access. Summarizes results that appeared in journal articles [2-13]

    http://arxiv.org/abs/1303.4752
    Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
    Norbert Bodendorfer, Yasha Neiman
    (Submitted on 19 Mar 2013)
    It was recently noted that the on-shell Einstein-Hilbert action with York-Gibbons-Hawking boundary term has an imaginary part, proportional to the area of the codimension-2 surfaces on which the boundary normal becomes null. We extend this result to first-order formulations of gravity, by generalizing a previously proposed boundary term to closed boundaries. As a side effect, we settle the issue of the Holst modification vs. the Nieh-Yan density by demanding a well-defined variational principle. We then set out to find the imaginary action in the large-spin 4-simplex limit of the Lorentzian EPRL/FK spinfoam. It turns out that the spinfoam's effective action indeed has the correct imaginary part, but only if the Barbero-Immirzi parameter γ is set to ± i after the quantum calculation. An interpretation and a connection to other recent results is discussed. In particular, we propose that the large-spin limit of loop quantum gravity can be viewed as a high-energy 'transplanckian' regime.
    22 pages, 5 figures

    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.
    5 pages

    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

    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 T3 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 à 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

    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/1302.5265
    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.
    4 pages

    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/1302.1781
    Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
    Aldo Riello
    (Submitted on 7 Feb 2013)
    We calculate the most divergent contribution to the self-energy (or "melonic") graph in the context of the Lorentzian EPRL-FK Spin Foam model of Quantum Gravity. We find that such a contribution is logarithmically divergent in the cut-off over the SU(2)-representation spins when one chooses the face amplitude guaranteeing the face-splitting invariance of the foam. We also find that the dependence on the boundary data is different from that of the bare propagator. This fact has its origin in the non-commutativity of the EPRL-FK Y-map with the projector onto SL(2,C)-invariant states. In the course of the paper, we discuss in detail the approximations used during the calculations, its geometrical interpretation as well as the physical consequences of our result.
    55 pages, 8 figures

    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli
    (Submitted on 4 Feb 2013)
    The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
    5 pages, 2 figures

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson
    (Submitted on 1 Feb 2013)
    Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. ... departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
    64 pages, 15 figures

    http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle
    (Submitted on 26 Jan 2013)
    An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, ...The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. ... The construction is made possible by a recent result proven by Fleischhack.
    18 pages

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland
    (Submitted on 24 Jan 2013)
    This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. ... Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
    28 pages, 2 figures

    http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom
    (Submitted on 15 Jan 2013)
    We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. ... gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
    ... we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. ...
    30 pages

    http://arxiv.org/abs/1301.2245
    Quantum-Reduced Loop Gravity: Cosmology
    Emanuele Alesci, Francesco Cianfrani
    (Submitted on 10 Jan 2013)
    We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory...The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.
    26 pages.

    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau
    (Submitted on 7 Jan 2013)
    Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems...
    6 pages, 4 figures
     
    Last edited: Mar 28, 2013
  4. Mar 30, 2013 #3

    marcus

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    Tom, thanks for responding! I noticed that your responses here address the problem of theoretical consistency, and I recall they have in the past as well. That's a priority which you emphasize, overcoming weaknesses in the theory---exploring ways to make the formulation more solid. Particularly linking the Hamiltonian and the Foam approaches, which is exactly what Wolfgang Wieland is doing in one of the papers you picked.

    This gives me the idea to sort the 20 candidates into types according to what sort of advance they aim at. It's one way to understand/explain why we voted as we did. I'm very interested in seeing the theory applied to cosmology---I think that is where the most important developments are occurring and will occur for the foreseeable future. (Partly because of all the new data on the early universe, and partly because LQG leads to a specific very concrete model of the start of expansion and the pre-inflationary period. It gives an interesting view of very early universe that many researchers want to elaborate in more detail.)
    Here's how I would categorize the 20 papers on the poll.

    Cosmology[8] numbers 5, 7, 9, 12, 15, 16, 19, 20

    Internal Consistency[4] numbers 6, 10, 13, 17

    Black hole (and collapse) modeling[3] numbers 3, 8, 11

    Thermodynamics of geometry[2] numbers 4, 14

    "Shape dynamics"[1] number 2

    Noncommutative geometry[1] number 18

    Relative locality[1] number 1

    To make it easy to find the papers in each category I will number them:


    1. http://arxiv.org/abs/1303.7216
    Relative Locality in Curved Space-time
    Jerzy Kowalski-Glikman, Giacomo Rosati

    2. http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    3. http://arxiv.org/abs/1303.6157
    Loop quantum dynamics of the gravitational collapse
    Yaser Tavakoli, Joao Marto, Andrea Dapor

    4. http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    5. http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    6. http://arxiv.org/abs/1303.4752
    Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
    Norbert Bodendorfer, Yasha Neiman

    7. http://arxiv.org/abs/1303.3576
    Cosmology from Group Field Theory
    Steffen Gielen, Daniele Oriti, Lorenzo Sindoni

    8. http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    9. 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

    10. http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    11. http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    12. http://arxiv.org/abs/1302.3833
    Loop Quantum Cosmology
    Ivan Agullo, Alejandro Corichi

    13. http://arxiv.org/abs/1302.1781
    Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
    Aldo Riello

    14. http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    15. http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    16. http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle

    17. http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    18. http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom

    19. http://arxiv.org/abs/1301.2245
    Quantum-Reduced Loop Gravity: Cosmology
    Emanuele Alesci, Francesco Cianfrani

    20. http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    Perhaps I should sort them out and group them by type: cosmology, consistency, black hole, thermodynamics, and miscellaneous. And say why I think each category is important (unless it's obvious.)
     
    Last edited: Mar 30, 2013
  5. Mar 30, 2013 #4

    marcus

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    See how this grouping into categories looks to you. Would you suggest any changes, or that different types be considered?


    Cosmology
    5. http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    7. http://arxiv.org/abs/1303.3576
    Cosmology from Group Field Theory
    Steffen Gielen, Daniele Oriti, Lorenzo Sindoni

    9. 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

    12. http://arxiv.org/abs/1302.3833
    Loop Quantum Cosmology
    Ivan Agullo, Alejandro Corichi

    15. http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    16. http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle

    19. http://arxiv.org/abs/1301.2245
    Quantum-Reduced Loop Gravity: Cosmology
    Emanuele Alesci, Francesco Cianfrani

    20. http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    Consistency
    6. http://arxiv.org/abs/1303.4752
    Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
    Norbert Bodendorfer, Yasha Neiman

    10. http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    13. http://arxiv.org/abs/1302.1781
    Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
    Aldo Riello

    17. http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    Black hole/collapse
    3. http://arxiv.org/abs/1303.6157
    Loop quantum dynamics of the gravitational collapse
    Yaser Tavakoli, Joao Marto, Andrea Dapor

    8. http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    11. http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    Temperature/entropy of geometry
    4. http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    14. http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    Miscellaneous (connections to other QG research)
    "Shape dynamics"
    2. http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    Noncommutative geometry
    18. http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom

    Relative locality
    1. http://arxiv.org/abs/1303.7216
    Relative Locality in Curved Space-time
    Jerzy Kowalski-Glikman, Giacomo Rosati
     
  6. Apr 4, 2013 #5

    marcus

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    Now that we've had a few days to think about the last quarter's QG research, which paper would you name as the MOST significant of the lot? It might not be one you picked if you responded earlier to the poll, afterthoughts and reassessment are natural and not entirely unexpected.

    BTW thanks to those of us who registered their picks already! It's an interesting bunch of choices.

    I find that for me it is very easy to say which paper looms largest: Clifton Ellis Tavakol.

    George Ellis should be well-known to everybody--worldwide reputation as relativist/cosmologist. But some may wish to know a bit more about the two authors at QMUL (Queen Mary University London):

    http://astro.qmul.ac.uk/directory/t.clifton

    http://astro.qmul.ac.uk/directory/r.tavakol

    What makes, or one of the things that makes, this CET paper so important is that if we are to understand the thermodynamics of the cosmic bounce, or whatever did occur around the start of expansion, we absolutely need a definition of the geometric entropy. The entropy of the gravitational field, in other words, especially under extreme conditions. There is, so far, no satisfactory definition, so it is impossible to talk meaningfully about thermodynamics around the start of expansion.

    And these people have a proposal which, for various reasons, is remarkably persuasive as I see it.
     
  7. Apr 8, 2013 #6

    marcus

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    Maybe it's interesting and I should mention this. The poll leader is Wolfgang Wieland's
    "Hamiltonian Spinfoam Gravity" paper, and that is exactly what he is giving an online ILQGS talk about tomorrow:

    Tuesday, Apr. 9th
    Wolfgang Wieland, CPT Marseille
    Title: Hamiltonian spinfoam gravity

    Thanks to everyone who responded in the poll so far!

    The leading papers currently are:

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    followed by the ELLIS ENTROPY one I mentioned in preceding post and two others:

    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli
     
  8. Apr 17, 2013 #7

    marcus

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    I'm really happy about the results of this quarter's poll so far! I'll try to comment in a few minutes.

    We have a interesting bunch of papers to evaluate this time. Non-trivial investigation of the thermodynamics of geometry, also serious efforts to reform LQG at the foundations level and to develop a alternate but related "spinorial" QG. Advances in quantum cosmology towards making testable predictions, modeling pre-inflation era, introducing matter and non-uniformity into our understanding of the start of expansion. Evaluating this bunch of papers took a fair amount of thought.

    Eight respondents so far--thanks all!

    Four votes
    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    three votes
    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    Two votes
    http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson
     
    Last edited: Apr 17, 2013
  9. Apr 28, 2013 #8

    marcus

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    It's a non-trivial job to sort out and weigh the importance of this batch of papers. It takes some knowledge, thought, and time. Ten of us have responded: thanks Atyy, Chronos, Devils, Nonlinearity, Nullius, Sam, Skydive, Tom and Vasu! I always find I learn by seeing how other people rate the recent research papers and coming to understand other's perspective---this quarter, because of the range of interesting topics, that was especially so.

    We had papers on the thermodynamics of geometry, reforming LQG at foundations level, "spinorial"/holonomy QG, spinfoam Hamiltonian using twistors, shape dynamics, quantum cosmology including testable predictions, the pre-inflation era and a prediction of inflation, bounce with matter and non-uniformity. Also an important strengthening of the link between Loop cosmology and the main theory, so that observational tests at the level of cosmology can now serve to test full LQG.

    Five votes
    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    four votes
    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    three votes
    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    Two votes
    http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle
     
    Last edited: Apr 28, 2013
  10. Apr 30, 2013 #9

    marcus

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    Eleven of us have responded to the poll so far in the first month! Thanks again to all who registered their picks for first quarter MIP. The tally of votes is the same as I listed in the preceding post except that the Clifton Ellis Tavakol paper on geometric entropy now has 6 votes. Seems to be a strong favorite with more than half of us who entered the poll choosing it. And the Gambini Pullin went from two votes to three.

    It's time to make a tentative list of especially interesting/significant papers that appeared in April, becoming possible candidates on the 2013 2nd quarter MIP poll. Although the maximum number of papers that can be on any quarterly poll is 20, on this preliminary list there are already 13 from the month of April alone! It's going to take some narrowing down, which will not be a trivial job with this selection.

    http://arxiv.org/abs/1304.8050
    Beyond the Spectral Standard Model: Emergence of Pati-Salam Unification
    Ali H. Chamseddine, Alain Connes, Walter D. van Suijlekom
    (Submitted on 30 Apr 2013)
    The assumption that space-time is a noncommutative space formed as a product of a continuous four dimensional manifold times a finite space predicts, almost uniquely, the Standard Model with all its fermions, gauge fields, Higgs field and their representations. A strong restriction on the noncommutative space results from the first order condition which came from the requirement that the Dirac operator is a differential operator of order one. Without this restriction, invariance under inner automorphisms requires the inner fluctuations of the Dirac operator to contain a quadratic piece expressed in terms of the linear part. We apply the classification of product noncommutative spaces without the first order condition and show that this leads immediately to a Pati-Salam SU(2)R x SU(2)L x SU(4) type model which unifies leptons and quarks in four colors. Besides the gauge fields, there are 16 fermions in the (2,2,4) representation, fundamental Higgs fields in the (2,2,1), (2,1,4) and (1,1,1+15) representations. Interestingly we encounter a new phenomena where the Higgs fields in the high energy sector are composite and depend quadratically on the fundamental Higgs fields. The Pati-Salam symmetries are broken spontaneously at high energies to those of the Standard Model.
    37 pages, 6 figures

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh
    (Submitted on 29 Apr 2013)
    We investigate the φ2 inflationary model in the Bianchi-I spacetime using effective spacetime description of loop quantum cosmology to understand the issues of the resolution of initial singularity, isotropization, effect of anisotropies on amount of inflation, and the phase space attractors in the presence of non-perturbative quantum gravitational modifications. A comparative analysis with the classical theory by including more general initial conditions than the ones previously considered in the latter is also performed. We show that, in general, the classical singularity is replaced by a bounce of the mean scale factor in loop quantum cosmology. Due to the underlying quantum geometric effects, the energy density of the inflaton and the anisotropic shear remain bounded throughout the non-singular evolution. Starting from arbitrary anisotropic initial conditions, a loop quantum universe isotropizes either before or soon after the onset of slow-roll inflation. We find a double attractor behavior in the phase space dynamics of loop quantum cosmology, similar to the one in classical theory, but with some additional subtle features. Quantum modifications to the dynamical equations are such that, unlike the classical theory, the amount of inflation does not monotonically depend on the initial anisotropy in loop quantum cosmology. Our results suggest that a viable non-singular inflationary model can be constructed from highly anisotropic initial conditions in the Planck regime.
    34 pages, 19 figures

    http://arxiv.org/abs/1304.7583
    Inner Fluctuations in Noncommutative Geometry without the first order condition
    Ali H. Chamseddine, Alain Connes, Walter D. van Suijlekom
    (Submitted on 29 Apr 2013)
    We extend inner fluctuations to spectral triples that do not fulfill the first-order condition. This involves the addition of a quadratic term to the usual linear terms. We find a semi-group of inner fluctuations, which only depends on the involutive algebra A and which extends the unitary group of A. This has a key application in noncommutative spectral models beyond the Standard Model, of which we consider here a toy model.
    22 pages, 2 figures

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig
    (Submitted on 26 Apr 2013)
    Many approaches to quantum gravity have resorted to diffusion processes to characterize the spectral properties of the resulting quantum spacetimes. We critically discuss these quantum-improved diffusion equations and point out that a crucial property, namely positivity of their solutions, is not preserved automatically. We then construct a novel set of diffusion equations with positive semi-definite probability densities, applicable to Asymptotically Safe gravity, Horava-Lifgarbagez gravity and Loop Quantum Gravity. These recover all previous results on the spectral dimension and shed further light on the structure of the quantum spacetimes by assessing the underlying stochastic processes. Pointing out that manifestly different diffusion processes lead to the same spectral dimension, we propose the probability distribution of the diffusion process as a refined probe of quantum spacetime.
    14 pages, 5 figures

    http://arxiv.org/abs/1304.6946
    Pure connection gravity at one loop: Instanton background
    Kai Groh, Kirill Krasnov, Christian F. Steinwachs
    (Submitted on 25 Apr 2013)
    In the "pure connection" formulation General Relativity becomes a particular diffeomorphism invariant SL(2) gauge theory. Using this formalism, we compute the divergent contributions to the gravitational one-loop effective action. Calculations of the on-shell effective action simplify greatly if one specialises to an instanton background where only the anti-self-dual part of the Weyl curvature is non-vanishing. One of the most striking features of the connection formulation is that the (linearised) Euclidean action is non-negative, unlike in the metric case. As in the metric GR, we find the logarithmically divergent contribution to consist of the volume and Euler character terms, but the arising numerical constants are different. However, surprisingly, the difference between the two results turns out to be always an integer. This suggests that there exists a relation between the connection and metric based quantum theories.
    21 pages

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan
    (Submitted on 24 Apr 2013)
    We examine spinfoam cosmology by use of a simple graph adapted to homogeneous cosmological models. We calculate dynamics in the isotropic limit, and provide the framework for the aniostropic case. The dynamical behaviour is calculating transition amplitudes between holomorphic coherent states on a single node graph. The resultant dynamics is peaked on solutions which have no support on the zero volume state, indicating that big bang type singularities are avoided within such models.
    19 pages, 4 figures

    http://arxiv.org/abs/1304.6632
    Bubble divergences and gauge symmetries in spin foams
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 24 Apr 2013)
    Spin foams are candidate state-sum models for transition amplitudes in quantum gravity. An active research subject is to identify the possible divergences of spin foam models, or alternatively to show that models are finite. We will discuss in detail the (non--occurrence of) divergences in the Barrett-Crane model, formulated as an integral of delta function weights only. We will furthermore present a simple method to estimate the divergence degree of the so-called bubbles for general spin foam models. Divergences in spin foams are expected to be related to the existence of gauge symmetries (diffeomorphisms). Thus we have to conclude that such gauge symmetries are not (fully) present in the model we consider. But we will identify a class of gauge symmetries which occur at special solutions of the equations imposed by the delta function weights. This situation is surprisingly similar to the case of broken diffeomorphism symmetries in discrete gravity, which are present around flat solutions. We introduce a method to derive (Ward-identity-like) equations for the vertex amplitude of the model in the case of broken gauge symmetries.
    28 pages

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 22 Apr 2013)
    The diffeomorphism symmetry of general relativity leads in the canonical formulation to constraints, which encode the dynamics of the theory. These constraints satisfy a complicated algebra, known as Dirac's hypersurface deformation algebra. This algebra has been a long standing challenge for quantization. One reason is that discretizations, on which many quantum gravity approaches rely, generically break diffeomorphism symmetry. In this work we find a representation for the Dirac constraint algebra of hypersurface deformations in a formulation of discrete 3D gravity and for the flat as well as homogeneously curved sector of discrete 4D gravity. In these cases diffeomorphism symmetry can be preserved. Furthermore we present different versions of the hypersurface deformation algebra for the boundary of a simplex in arbitrary dimensions.
    30 pages

    http://arxiv.org/abs/1304.5913
    How to Resum Feynman Graphs
    Vincent Rivasseau, Zhituo Wang
    (Submitted on 22 Apr 2013)
    In this paper we reformulate in a simpler way the combinatoric core of constructive quantum field theory We define universal rational combinatoric weights for pairs made of a graph and one of its spanning trees. These weights are nothing but the percentage of Hepp's sectors in which the tree is leading the ultraviolet analysis. We explain how they allow to reshuffle the divergent series formulated in terms of Feynman graphs into convergent series indexed by the trees that these graphs contain. The Feynman graphs to be used are not the ordinary ones but those of the intermediate field representation, and the result of the reshuffling is called the Loop Vertex Expansion.
    18 pages, 6 figures

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski
    (Submitted on 20 Apr 2013)
    A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
    36 pages

    http://arxiv.org/abs/1304.3025
    The Wald entropy formula and loop quantum gravity
    Norbert Bodendorfer, Yasha Neiman
    (Submitted on 10 Apr 2013)
    We outline how the Wald entropy formula naturally arises in loop quantum gravity based on recently introduced dimension-independent connection variables. The key observation is that in a loop quantization of a generalized gravity theory, the analog of the area operator turns out to measure, morally speaking, the Wald entropy rather than the area. We discuss the explicit example of (higher-dimensional) Lovelock gravity and comment on recent work on finding the correct numerical prefactor of the entropy by comparing it to a semiclassical effective action.
    16 pages

    http://arxiv.org/abs/1304.2679
    New Variables for Classical and Quantum Gravity in all Dimensions V. Isolated Horizon Boundary Degrees of Freedom
    Norbert Bodendorfer, Thomas Thiemann, Andreas Thurn
    (Submitted on 9 Apr 2013)
    In this paper, we generalise the treatment of isolated horizons in loop quantum gravity, resulting in a Chern-Simons theory on the boundary in the four-dimensional case, to non-distorted isolated horizons in 2(n+1)-dimensional spacetimes. The key idea is to generalise the four-dimensional isolated horizon boundary condition by using the Euler topological density of a spatial slice of the black hole horizon as a measure of distortion. The resulting symplectic structure on the horizon coincides with the one of higher-dimensional SO(2(n+1))-Chern-Simons theory in terms of a Peldan-type hybrid connection and resembles closely the usual treatment in 3+1 dimensions. We comment briefly on a possible quantisation of the horizon theory. Here, some subtleties arise since higher-dimensional non-Abelian Chern-Simons theory has local degrees of freedom. However, when replacing the natural generalisation to higher dimensions of the usual boundary condition by an equally natural stronger one, it is conceivable that the problems originating from the local degrees of freedom are avoided, thus possibly resulting in a finite entropy.
    49 pages

    http://arxiv.org/abs/1304.2666
    The 1/N Expansion of Tensor Models Beyond Perturbation Theory
    Razvan Gurau
    (Submitted on 9 Apr 2013)
    We analyze in full mathematical rigor the most general quartically perturbed invariant probability measure for a random tensor. Using a version of the Loop Vertex Expansion (which we call the mixed expansion) we show that the cumulants write as explicit series in 1/N plus bounded rest terms. The mixed expansion recasts the problem of determining the subleading corrections in 1/N into a simple combinatorial problem of counting trees decorated by a finite number of loop edges.
    As an aside, we use the mixed expansion to show that the (divergent) perturbative expansion of the tensor models is Borel summable and to prove that the cumulants respect an uniform scaling bound. In particular the quartically perturbed measures fall, in the N to infinity limit, in the universality class of Gaussian tensor models.
    45 pages
     
    Last edited: Apr 30, 2013
  11. May 1, 2013 #10

    marcus

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    12 respondents so far. Thanks Atyy, Chronos, Devils, John, Martin, Nonlinearity, Nullius, Sam, Skydive, Tom and Vasu! Seeing how you rate the recent research and getting a better understanding of different people's perspectives teaches me something. Maybe we all learn from each other. Interesting topics this past quarter!
    In tallying votes, to keep it simple I only list papers that got two or more:

    Six votes
    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    four votes
    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    three votes
    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    Two votes
    http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle

    http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom
     
    Last edited: May 1, 2013
  12. May 1, 2013 #11

    martinbn

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    I have to admit that I haven't read or even looked at all of them. Also I cannot judge them about significance, my votes are based on what I liked. And I was planning to add more votes after I browsed them, but I don't think I can do that now.
     
  13. May 24, 2013 #12

    marcus

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    Martin, thanks for participating! No one can study everything which is why it helps so much to have a group of people looking over the QG research that has appeared. Our areas of knowledge and perceptions both overlap and complement each other---so we get better coverage. I'm really pleased with the results of the 2013 first quarter's poll, in which you and a dozen other people took part!

    Looking ahead to the second quarter poll (April-June), it's time to start assembling possible candidates from the May papers. The first one that comes to mind is a Freidel Hnybida paper that MTd2 spotted and added to our bibliography:

    http://arxiv.org/abs/1305.3326
    A Discrete and Coherent Basis of Intertwiners
    Laurent Freidel, Jeff Hnybida
    (Submitted on 15 May 2013)
    We construct a new discrete basis of 4-valent SU(2) intertwiners. This basis possesses both the advantage of being discrete, while at the same time representing accurately the classical degrees of freedom; hence it is coherent. The closed spin network amplitude obtained from these intertwiners depends on twenty spins and can be evaluated by a generalization of the Racah formula for an arbitrary graph. The asymptotic limit of these amplitudes is found. We give, for the first time, the asymptotics of 15j symbols in the real basis. Remarkably it gives a generalization of the Regge action to twisted geometries.
    31 pages.

    Another potentially important May paper is one that Atyy added to the biblio thread:

    http://arxiv.org/abs/1305.2207
    The imaginary part of the gravitational action at asymptotic boundaries and horizons
    Yasha Neiman
    (Submitted on 9 May 2013)
    We study the imaginary part of the Lorentzian gravitational action for bounded regions, as described in arXiv:1301.7041. By comparing to a Euclidean calculation, we explain the agreement between the formula for this imaginary part and the formula for black hole entropy. We also clarify the topological structure of the imaginary part in Lovelock gravity. We then evaluate the action's imaginary part for some special regions. These include cylindrical slabs spanning the exterior of a stationary black hole spacetime, 'maximal diamonds' in various symmetric spacetimes, as well as local near-horizon regions. In the first setup, the black hole's entropy and conserved charges contribute to the action's imaginary and real parts, respectively. In the other two setups, the imaginary part coincides with the relevant entropy.
    34 pages, 10 figures.

    A third May paper that appears to stand out is again one that MTd2 spotted for us:

    http://arxiv.org/abs/1305.0822
    On the Origin of Gravitational Lorentz Covariance
    Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley
    (Submitted on 3 May 2013)
    We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.
    40 pages.

    Here are two other May papers that might be good to consider for the poll:

    http://arxiv.org/abs/1305.1487
    Shape Dynamics and Effective Field Theory
    Tim Koslowski
    (Submitted on 7 May 2013)
    Shape Dynamics is a gauge theory based on spatial diffeomorphism- and Weyl-invariance which is locally indistinguishable form classical General Relativity. If taken seriously, it suggests that the spacetime--geometry picture that underlies General Relativity can be replaced by a picture based on spatial conformal geometry. This classically well understood trading of gauge symmetries opens new conceptual avenues in many approaches to quantum gravity. I focus on the general implications for quantum gravity and effective field theory and consider the application of the Shape Dynamics picture in the exact renormalization group approaches to gravity, loop- and polymer- quantization approaches to gravity and low energy effective field theories. I also discuss the interpretation of known results through in the Shape Dynamics picture, in particular holographic renormalization and the problem of time in canonical quantum gravity.
    56 pages, 1 figure

    http://arxiv.org/abs/1305.0310
    A Birkhoff theorem for Shape Dynamics
    Henrique Gomes
    (Submitted on 1 May 2013)
    Here we use the equations of motion of Shape Dynamics in its asymptotically flat version to derive a Birkhoff theorem. There are three significant differences with respect to the usual Birkhoff theorem in GR. The first regards the posing of the problem: in Shape Dynamics we must establish from the start the boundary conditions of our phase space variables. Thus unlike the GR Birkhoff theorem, which yields a static 4-metric from vacuum and spherical symmetry irrespectively of the boundary conditions, we have to postulate asymptotically flat boundary conditions from the start. The second difference regards the construction of the solution: we heavily use the Shape Dynamics spatial Weyl gauge freedom to simplify the problem. The remaining difference is that the solution obtained is uniquely the isotropic wormhole solution, in which no singularity is present, as opposed to maximally extended Schwarzschild.
    6 pages
    =====================
    Adding to the above five, here's the short list (5 also) of April papers:

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh
    (Submitted on 29 Apr 2013)
    We investigate the φ2 inflationary model in the Bianchi-I spacetime using effective spacetime description of loop quantum cosmology to understand the issues of the resolution of initial singularity, isotropization, effect of anisotropies on amount of inflation, and the phase space attractors in the presence of non-perturbative quantum gravitational modifications. A comparative analysis with the classical theory by including more general initial conditions than the ones previously considered in the latter is also performed. We show that, in general, the classical singularity is replaced by a bounce of the mean scale factor in loop quantum cosmology. Due to the underlying quantum geometric effects, the energy density of the inflaton and the anisotropic shear remain bounded throughout the non-singular evolution. Starting from arbitrary anisotropic initial conditions, a loop quantum universe isotropizes either before or soon after the onset of slow-roll inflation. We find a double attractor behavior in the phase space dynamics of loop quantum cosmology, similar to the one in classical theory, but with some additional subtle features. Quantum modifications to the dynamical equations are such that, unlike the classical theory, the amount of inflation does not monotonically depend on the initial anisotropy in loop quantum cosmology. Our results suggest that a viable non-singular inflationary model can be constructed from highly anisotropic initial conditions in the Planck regime.34 pages, 19 figures

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig
    (Submitted on 26 Apr 2013)
    Many approaches to quantum gravity have resorted to diffusion processes to characterize the spectral properties of the resulting quantum spacetimes. We critically discuss these quantum-improved diffusion equations and point out that a crucial property, namely positivity of their solutions, is not preserved automatically. We then construct a novel set of diffusion equations with positive semi-definite probability densities, applicable to Asymptotically Safe gravity, Horava-Lifgarbagez gravity and Loop Quantum Gravity. These recover all previous results on the spectral dimension and shed further light on the structure of the quantum spacetimes by assessing the underlying stochastic processes. Pointing out that manifestly different diffusion processes lead to the same spectral dimension, we propose the probability distribution of the diffusion process as a refined probe of quantum spacetime.
    14 pages, 5 figures

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan
    (Submitted on 24 Apr 2013)
    We examine spinfoam cosmology by use of a simple graph adapted to homogeneous cosmological models. We calculate dynamics in the isotropic limit, and provide the framework for the aniostropic case. The dynamical behaviour is calculating transition amplitudes between holomorphic coherent states on a single node graph. The resultant dynamics is peaked on solutions which have no support on the zero volume state, indicating that big bang type singularities are avoided within such models.
    19 pages, 4 figures

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 22 Apr 2013)
    The diffeomorphism symmetry of general relativity leads in the canonical formulation to constraints, which encode the dynamics of the theory. These constraints satisfy a complicated algebra, known as Dirac's hypersurface deformation algebra. This algebra has been a long standing challenge for quantization. One reason is that discretizations, on which many quantum gravity approaches rely, generically break diffeomorphism symmetry. In this work we find a representation for the Dirac constraint algebra of hypersurface deformations in a formulation of discrete 3D gravity and for the flat as well as homogeneously curved sector of discrete 4D gravity. In these cases diffeomorphism symmetry can be preserved. Furthermore we present different versions of the hypersurface deformation algebra for the boundary of a simplex in arbitrary dimensions.
    30 pages

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski
    (Submitted on 20 Apr 2013)
    A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
    36 pages
     
    Last edited: May 24, 2013
  14. May 28, 2013 #13

    marcus

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    This has to be on the 2nd quarter MIP poll!
    http://arxiv.org/abs/1305.6315
    Why gravity codes the renormalization of conformal field theories
    Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin
    (Submitted on 27 May 2013)
    We give a new demonstration that General Relativity in d+1 dimensions with negative or positive cosmological constant codes the renormalization group behaviour of conformal field theories (CFT) in d dimensions. This utilizes Shape Dynamics, which is a conformally invariant theory known to be equivalent to General Relativity. A key result of Shape Dynamics is that the evolution of observables under local conformal transformations and spatial diffeomorphisms is shown to be equivalent to many fingered time, i.e., d+1-dimensional spacetime diffeomorphisms. This relationship explains why the renormalization group flow of a CFT is governed by a geometry with d+1-dimensional spacetime diffeomorphism invariance.
    25 pages

    This, and the Freidel Hnybida paper listed in the preceding post have got to be two of the most important QG papers appearing this quarter. Of the year perhaps, but it's too early to say.
     
  15. May 29, 2013 #14

    marcus

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    Thanks to the two most recent respondents: Chemist@ and Jason_0! So far fourteen of us have registered our choices for the first quarter's most interesting/important QG papers. Here's how the votes add up at present. To keep it simple, in tallying votes I've only listed papers that got two or more:

    Six votes
    http://arxiv.org/abs/1303.5612
    A Gravitational Entropy Proposal
    Timothy Clifton, George F R Ellis, Reza Tavakol

    four votes
    http://arxiv.org/abs/1302.0724
    Death and resurrection of the zeroth principle of thermodynamics
    Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1301.1264
    Inflation as a prediction of loop quantum cosmology
    Linda Linsefors, Aurelien Barrau

    three votes
    http://arxiv.org/abs/1303.7139
    Symmetry and Evolution in Quantum Gravity
    Sean Gryb, Karim Thebault

    http://arxiv.org/abs/1303.2773
    BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
    J.Manuel Garcia-Islas

    http://arxiv.org/abs/1302.5265
    The loop quantum gravity black hole
    Rodolfo Gambini, Jorge Pullin

    http://arxiv.org/abs/1301.5859
    Hamiltonian spinfoam gravity
    Wolfgang M. Wieland

    Two votes
    http://arxiv.org/abs/1303.6157
    Loop quantum dynamics of the gravitational collapse
    Yaser Tavakoli, Joao Marto, Andrea Dapor

    http://arxiv.org/abs/1303.4989
    Loop Quantum Gravity and the The Planck Regime of Cosmology
    Abhay Ashtekar

    http://arxiv.org/abs/1302.7142
    Holonomy Operator and Quantization Ambiguities on Spinor Space
    Etera R. Livine

    http://arxiv.org/abs/1302.0254
    The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
    Ivan Agullo, Abhay Ashtekar, William Nelson

    http://arxiv.org/abs/1301.6210
    Embedding loop quantum cosmology without piecewise linearity
    Jonathan Engle

    http://arxiv.org/abs/1301.3480
    Gauge networks in noncommutative geometry
    Matilde Marcolli, Walter D. van Suijlekom
     
    Last edited: May 29, 2013
  16. May 31, 2013 #15

    marcus

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    We now have 13 possible candidates for the second quarter poll (April-June), from papers that appeared in the past two months.

    http://arxiv.org/abs/1305.6714
    Black hole entropy from KMS-states of quantum isolated horizons
    Daniele Pranzetti
    (Submitted on 29 May 2013)
    By reintroducing Lorentz invariance via a complex connection formulation in canonical loop quantum gravity, we define a geometrical notion of temperature for quantum isolated horizons. Upon imposition of the reality conditions in the form of the linear simplicity constraints for an imaginary Barbero-Immirzi parameter, the exact formula for the temperature can be derived by demanding that the horizon state satisfying the boundary conditions be a KMS-state. In this way, our analysis reveals the connection between the passage to the Ashtekar self-dual variables and the thermality of the horizon. The horizon equilibrium state can then be used to compute both the von Neumann and the Boltzmann entropies. By means of a natural cut-off introduced by the topological theory on the boundary, we show that the two provide the same finite answer which allows us to recover the Bekenstein-Hawking formula in the semi-classical limit. The connection with Connes-Rovelli thermal time proposal for a general relativistic statistical mechanics is worked out.
    10 pages, 1 figure

    http://arxiv.org/abs/1305.6680
    CDT and the Search for a Theory of Quantum Gravity
    J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
    (Submitted on 29 May 2013)
    Causal Dynamical Triangulations provide a non-perturbative regularization of a theory of quantum gravity. We describe how this approach connects with the asymptotic safety program and Ho\vrava-Lifgarbagez gravity theory, and present the most recent results from computer simulations.
    23 pages, 5 figures. Based on plenary talk at MG13. To appear in the proceeding

    http://arxiv.org/abs/1305.6315
    Why gravity codes the renormalization of conformal field theories
    Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin
    (Submitted on 27 May 2013)
    We give a new demonstration that General Relativity in d+1 dimensions with negative or positive cosmological constant codes the renormalization group behaviour of conformal field theories (CFT) in d dimensions. This utilizes Shape Dynamics, which is a conformally invariant theory known to be equivalent to General Relativity. A key result of Shape Dynamics is that the evolution of observables under local conformal transformations and spatial diffeomorphisms is shown to be equivalent to many fingered time, i.e., d+1-dimensional spacetime diffeomorphisms. This relationship explains why the renormalization group flow of a CFT is governed by a geometry with d+1-dimensional spacetime diffeomorphism invariance.
    25 pages

    http://arxiv.org/abs/1305.3326
    A Discrete and Coherent Basis of Intertwiners
    Laurent Freidel, Jeff Hnybida
    (Submitted on 15 May 2013)
    We construct a new discrete basis of 4-valent SU(2) intertwiners. This basis possesses both the advantage of being discrete, while at the same time representing accurately the classical degrees of freedom; hence it is coherent. The closed spin network amplitude obtained from these intertwiners depends on twenty spins and can be evaluated by a generalization of the Racah formula for an arbitrary graph. The asymptotic limit of these amplitudes is found. We give, for the first time, the asymptotics of 15j symbols in the real basis. Remarkably it gives a generalization of the Regge action to twisted geometries.
    31 pages.

    http://arxiv.org/abs/1305.2207
    The imaginary part of the gravitational action at asymptotic boundaries and horizons
    Yasha Neiman
    (Submitted on 9 May 2013)
    We study the imaginary part of the Lorentzian gravitational action for bounded regions, as described in arXiv:1301.7041. By comparing to a Euclidean calculation, we explain the agreement between the formula for this imaginary part and the formula for black hole entropy. We also clarify the topological structure of the imaginary part in Lovelock gravity. We then evaluate the action's imaginary part for some special regions. These include cylindrical slabs spanning the exterior of a stationary black hole spacetime, 'maximal diamonds' in various symmetric spacetimes, as well as local near-horizon regions. In the first setup, the black hole's entropy and conserved charges contribute to the action's imaginary and real parts, respectively. In the other two setups, the imaginary part coincides with the relevant entropy.
    34 pages, 10 figures.

    http://arxiv.org/abs/1305.0822
    On the Origin of Gravitational Lorentz Covariance
    Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley
    (Submitted on 3 May 2013)
    We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.
    40 pages.

    http://arxiv.org/abs/1305.1487
    Shape Dynamics and Effective Field Theory
    Tim Koslowski
    (Submitted on 7 May 2013)
    Shape Dynamics is a gauge theory based on spatial diffeomorphism- and Weyl-invariance which is locally indistinguishable form classical General Relativity. If taken seriously, it suggests that the spacetime--geometry picture that underlies General Relativity can be replaced by a picture based on spatial conformal geometry. This classically well understood trading of gauge symmetries opens new conceptual avenues in many approaches to quantum gravity. I focus on the general implications for quantum gravity and effective field theory and consider the application of the Shape Dynamics picture in the exact renormalization group approaches to gravity, loop- and polymer- quantization approaches to gravity and low energy effective field theories. I also discuss the interpretation of known results through in the Shape Dynamics picture, in particular holographic renormalization and the problem of time in canonical quantum gravity.
    56 pages, 1 figure

    http://arxiv.org/abs/1305.0310
    A Birkhoff theorem for Shape Dynamics
    Henrique Gomes
    (Submitted on 1 May 2013)
    Here we use the equations of motion of Shape Dynamics in its asymptotically flat version to derive a Birkhoff theorem. There are three significant differences with respect to the usual Birkhoff theorem in GR. The first regards the posing of the problem: in Shape Dynamics we must establish from the start the boundary conditions of our phase space variables. Thus unlike the GR Birkhoff theorem, which yields a static 4-metric from vacuum and spherical symmetry irrespectively of the boundary conditions, we have to postulate asymptotically flat boundary conditions from the start. The second difference regards the construction of the solution: we heavily use the Shape Dynamics spatial Weyl gauge freedom to simplify the problem. The remaining difference is that the solution obtained is uniquely the isotropic wormhole solution, in which no singularity is present, as opposed to maximally extended Schwarzschild.
    6 pages
    =====================
    Adding to the above eight, here's a short list of five April papers:

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh
    (Submitted on 29 Apr 2013)
    We investigate the φ2 inflationary model in the Bianchi-I spacetime using effective spacetime description of loop quantum cosmology to understand the issues of the resolution of initial singularity, isotropization, effect of anisotropies on amount of inflation, and the phase space attractors in the presence of non-perturbative quantum gravitational modifications. A comparative analysis with the classical theory by including more general initial conditions than the ones previously considered in the latter is also performed. We show that, in general, the classical singularity is replaced by a bounce of the mean scale factor in loop quantum cosmology. Due to the underlying quantum geometric effects, the energy density of the inflaton and the anisotropic shear remain bounded throughout the non-singular evolution. Starting from arbitrary anisotropic initial conditions, a loop quantum universe isotropizes either before or soon after the onset of slow-roll inflation. We find a double attractor behavior in the phase space dynamics of loop quantum cosmology, similar to the one in classical theory, but with some additional subtle features. Quantum modifications to the dynamical equations are such that, unlike the classical theory, the amount of inflation does not monotonically depend on the initial anisotropy in loop quantum cosmology. Our results suggest that a viable non-singular inflationary model can be constructed from highly anisotropic initial conditions in the Planck regime.34 pages, 19 figures

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig
    (Submitted on 26 Apr 2013)
    Many approaches to quantum gravity have resorted to diffusion processes to characterize the spectral properties of the resulting quantum spacetimes. We critically discuss these quantum-improved diffusion equations and point out that a crucial property, namely positivity of their solutions, is not preserved automatically. We then construct a novel set of diffusion equations with positive semi-definite probability densities, applicable to Asymptotically Safe gravity, Horava-Lifgarbagez gravity and Loop Quantum Gravity. These recover all previous results on the spectral dimension and shed further light on the structure of the quantum spacetimes by assessing the underlying stochastic processes. Pointing out that manifestly different diffusion processes lead to the same spectral dimension, we propose the probability distribution of the diffusion process as a refined probe of quantum spacetime.
    14 pages, 5 figures

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan
    (Submitted on 24 Apr 2013)
    We examine spinfoam cosmology by use of a simple graph adapted to homogeneous cosmological models. We calculate dynamics in the isotropic limit, and provide the framework for the aniostropic case. The dynamical behaviour is calculating transition amplitudes between holomorphic coherent states on a single node graph. The resultant dynamics is peaked on solutions which have no support on the zero volume state, indicating that big bang type singularities are avoided within such models.
    19 pages, 4 figures

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 22 Apr 2013)
    The diffeomorphism symmetry of general relativity leads in the canonical formulation to constraints, which encode the dynamics of the theory. These constraints satisfy a complicated algebra, known as Dirac's hypersurface deformation algebra. This algebra has been a long standing challenge for quantization. One reason is that discretizations, on which many quantum gravity approaches rely, generically break diffeomorphism symmetry. In this work we find a representation for the Dirac constraint algebra of hypersurface deformations in a formulation of discrete 3D gravity and for the flat as well as homogeneously curved sector of discrete 4D gravity. In these cases diffeomorphism symmetry can be preserved. Furthermore we present different versions of the hypersurface deformation algebra for the boundary of a simplex in arbitrary dimensions.
    30 pages

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski
    (Submitted on 20 Apr 2013)
    A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
    36 pages
     
  17. Jun 2, 2013 #16

    marcus

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    We'll need a compressed form of the above list (with abstracts removed):

    http://arxiv.org/abs/1305.6714
    Black hole entropy from KMS-states of quantum isolated horizons
    Daniele Pranzetti

    http://arxiv.org/abs/1305.6680
    CDT and the Search for a Theory of Quantum Gravity
    J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll

    http://arxiv.org/abs/1305.6315
    Why gravity codes the renormalization of conformal field theories
    Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin

    http://arxiv.org/abs/1305.3326
    A Discrete and Coherent Basis of Intertwiners
    Laurent Freidel, Jeff Hnybida

    http://arxiv.org/abs/1305.2207
    The imaginary part of the gravitational action at asymptotic boundaries and horizons
    Yasha Neiman

    http://arxiv.org/abs/1305.0822
    On the Origin of Gravitational Lorentz Covariance
    Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

    http://arxiv.org/abs/1305.1487
    Shape Dynamics and Effective Field Theory
    Tim Koslowski

    http://arxiv.org/abs/1305.0310
    A Birkhoff theorem for Shape Dynamics
    Henrique Gomes

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski
     
  18. Jun 20, 2013 #17

    marcus

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    With the 2nd quarter poll coming up in little over a week, two more to add to the list--making it 15 papers in all:

    http://arxiv.org/abs/1306.2987
    Coarse graining of spin net models: dynamics of intertwiners
    Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

    http://arxiv.org/abs/1306.0861
    Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
    Emanuele Alesci, Klaus Liegener, Antonia Zipfel

    http://arxiv.org/abs/1305.6714
    Black hole entropy from KMS-states of quantum isolated horizons
    Daniele Pranzetti

    http://arxiv.org/abs/1305.6680
    CDT and the Search for a Theory of Quantum Gravity
    J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll

    http://arxiv.org/abs/1305.6315
    Why gravity codes the renormalization of conformal field theories
    Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin

    http://arxiv.org/abs/1305.3326
    A Discrete and Coherent Basis of Intertwiners
    Laurent Freidel, Jeff Hnybida

    http://arxiv.org/abs/1305.2207
    The imaginary part of the gravitational action at asymptotic boundaries and horizons
    Yasha Neiman

    http://arxiv.org/abs/1305.0822
    On the Origin of Gravitational Lorentz Covariance
    Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

    http://arxiv.org/abs/1305.1487
    Shape Dynamics and Effective Field Theory
    Tim Koslowski

    http://arxiv.org/abs/1305.0310
    A Birkhoff theorem for Shape Dynamics
    Henrique Gomes

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski
     
  19. Jun 29, 2013 #18

    marcus

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    The 2nd quarter MIP poll should be ready this weekend.
    at this point the list is 18 papers in all. Here's how it will likely start off:
    ==============
    Of the eighteen candidates, please choose the one(s) you think will prove most significant for future research in Loop-and-allied quantum gravity. Since the poll is multiple choice, it's possible to vote for several papers. Abstract summaries follow in the next post.

    http://arxiv.org/abs/1306.6142
    Consistent probabilities in loop quantum cosmology
    David A. Craig, Parampreet Singh

    http://arxiv.org/abs/1306.6126
    The generator of spatial diffeomorphisms in the Koslowski-Sahlmann representation
    Madhavan Varadarajan

    http://arxiv.org/abs/1306.5697
    Dynamical Black Holes: Approach to the Final State
    Abhay Ashtekar, Miguel Campiglia, Samir Shah

    http://arxiv.org/abs/1306.5206
    The boundary is mixed
    Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

    http://arxiv.org/abs/1306.3021
    The Trace-Free Einstein Equations and inflation
    George F R Ellis

    http://arxiv.org/abs/1306.2987
    Coarse graining of spin net models: dynamics of intertwiners
    Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

    http://arxiv.org/abs/1306.0861
    Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
    Emanuele Alesci, Klaus Liegener, Antonia Zipfel

    http://arxiv.org/abs/1305.6714
    Black hole entropy from KMS-states of quantum isolated horizons
    Daniele Pranzetti

    http://arxiv.org/abs/1305.6315
    Why gravity codes the renormalization of conformal field theories
    Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin

    http://arxiv.org/abs/1305.3326
    A Discrete and Coherent Basis of Intertwiners
    Laurent Freidel, Jeff Hnybida

    http://arxiv.org/abs/1305.2207
    The imaginary part of the gravitational action at asymptotic boundaries and horizons
    Yasha Neiman

    http://arxiv.org/abs/1305.1487
    Shape Dynamics and Effective Field Theory
    Tim Koslowski

    http://arxiv.org/abs/1305.0822
    On the Origin of Gravitational Lorentz Covariance
    Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

    http://arxiv.org/abs/1304.7686
    A quantum gravitational inflationary scenario in Bianchi-I spacetime
    Brajesh Gupt, Parampreet Singh

    http://arxiv.org/abs/1304.7247
    Probing the quantum nature of spacetime by diffusion
    Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig

    http://arxiv.org/abs/1304.6688
    Towards Anisotropic Spinfoam Cosmology
    Julian Rennert, David Sloan

    http://arxiv.org/abs/1304.5983
    Dirac's discrete hypersurface deformation algebras
    Valentin Bonzom, Bianca Dittrich

    http://arxiv.org/abs/1304.5626
    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
    Muxin Han, Thomas Krajewski

    ================================
    Thread heading:
    Our picks for second quarter 2013 MIP (most important QG paper)

    Poll question and titles list:
    Which paper(s) will contribute most significantly to future research?

    Consistent probabilities in loop quantum cosmology

    The generator of spatial diffeomorphisms in the Koslowski-Sahlmann representation

    Dynamical Black Holes: Approach to the Final State

    The boundary is mixed

    The Trace-Free Einstein Equations and inflation

    Coarse graining of spin net models: dynamics of intertwiners

    Matrix Elements of Lorentzian Hamiltonian Constraint in LQG

    Black hole entropy from KMS-states of quantum isolated horizons

    Why gravity codes the renormalization of conformal field theories

    A Discrete and Coherent Basis of Intertwiners

    The imaginary part of the gravitational action at asymptotic boundaries and horizons

    Shape Dynamics and Effective Field Theory

    On the Origin of Gravitational Lorentz Covariance

    A quantum gravitational inflationary scenario in Bianchi-I spacetime

    Probing the quantum nature of spacetime by diffusion

    Towards Anisotropic Spinfoam Cosmology

    Dirac's discrete hypersurface deformation algebras

    Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
     
    Last edited: Jun 29, 2013
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