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Poll: Third quarter 2015 MIP (most important QG papers)

  1. Generalized effective description of loop quantum cosmology

  2. Recent results in CDT quantum gravity

  3. Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra

  4. A note on quantum supergravity and AdS/CFT

  5. Analog of the Peter-Weyl Expansion for Lorentz Group

  6. 4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block

  7. 4d Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks

  8. Quantum Cuboids and the EPRL-FK path integral for quantum gravity

  9. Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology

  10. Locality and entanglement in bandlimited quantum field theory

  11. Relational Quantum Cosmology

  12. Chaos, Dirac observables and constraint quantization

  13. Spin Foams Without Spins

  14. Running of the scalar spectral index in bouncing cosmologies

  15. Discrete Hamiltonian for General Relativity

  16. Phenomenology of bouncing black holes in quantum gravity: a closer look

  17. Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry

  18. Coherent State Operators in Loop Quantum Gravity

  19. New Hamiltonian constraint operator for loop quantum gravity

  20. Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for QG

Multiple votes are allowed.
Results are only viewable after voting.
  1. Oct 3, 2015 #1

    marcus

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    Dearly Missed

    Indicate the papers you think will prove most significant for future Loop-and-allied QG research. The poll is multiple choice, so it's possible to vote for several. Abstracts follow in the next post.

    http://arxiv.org/abs/1509.08899
    Generalized effective description of loop quantum cosmology
    Abhay Ashtekar, Brajesh Gupt

    http://arxiv.org/abs/1509.08788
    Recent results in CDT quantum gravity
    Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz

    http://arxiv.org/abs/1509.05693
    Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
    Ivan Agullo, Noah A. Morris

    http://arxiv.org/abs/1509.02036
    A note on quantum supergravity and AdS/CFT
    Norbert Bodendorfer

    http://arxiv.org/abs/1509.01312
    Analog of the Peter-Weyl Expansion for Lorentz Group
    Leonid Perlov

    http://arxiv.org/abs/1509.00466
    4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block
    Muxin Han

    http://arxiv.org/abs/1509.00458
    Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
    Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello

    http://arxiv.org/abs/1508.07961
    Quantum Cuboids and the EPRL-FK path integral for quantum gravity
    Benjamin Bahr, Sebastian Steinhaus

    http://arxiv.org/abs/1508.06786
    Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology
    Susanne Schander, Aurélien Barrau, Boris Bolliet, Linda Linsefors, Julien Grain

    http://arxiv.org/abs/1508.05953
    Locality and entanglement in bandlimited quantum field theory
    Jason Pye, William Donnelly, Achim Kempf

    http://arxiv.org/abs/1508.05543
    Relational Quantum Cosmology
    Francesca Vidotto

    http://arxiv.org/abs/1508.01947
    Chaos, Dirac observables and constraint quantization
    Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson

    http://arxiv.org/abs/1508.01416
    Spin Foams Without Spins
    Jeff Hnybida

    http://arxiv.org/abs/1507.08112
    Running of the scalar spectral index in bouncing cosmologies
    Jean-Luc Lehners, Edward Wilson-Ewing

    http://arxiv.org/abs/1507.07591
    Discrete Hamiltonian for General Relativity
    Jonathan Ziprick, Jack Gegenberg

    http://arxiv.org/abs/1507.05424
    Phenomenology of bouncing black holes in quantum gravity: a closer look
    Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer

    http://arxiv.org/abs/1507.04703
    Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
    Ivan Agullo

    http://arxiv.org/abs/1507.01153
    Coherent State Operators in Loop Quantum Gravity
    Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski

    http://arxiv.org/abs/1507.00986
    New Hamiltonian constraint operator for loop quantum gravity
    Jinsong Yang, Yongge Ma

    http://arxiv.org/abs/1507.00851
    Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
    Christoph Charles, Etera R. Livine
     
  2. jcsd
  3. Oct 3, 2015 #2

    marcus

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    Dearly Missed

    http://arxiv.org/abs/1509.08899
    Generalized effective description of loop quantum cosmology
    Abhay Ashtekar, Brajesh Gupt
    (Submitted on 29 Sep 2015)
    The effective description of loop quantum cosmology (LQC) has proved to be a convenient platform to study phenomenological implications of the quantum bounce that resolves the classical big-bang singularity. Originally, this description was derived using Gaussian quantum states with small dispersions. In this paper we present a generalization to incorporate states with large dispersions. Specifically, we derive the generalized effective Friedmann and Raychaudhuri equations and propose a generalized effective Hamiltonian which are being used in an ongoing study of the phenomenological consequences of a broad class of quantum geometries. We also discuss an interesting interplay between the physics of states with larger dispersions in standard LQC, and of sharply peaked states in (hypothetical) LQC theories with larger area gap.
    21 pages, 4 figures

    http://arxiv.org/abs/1509.08788
    Recent results in CDT quantum gravity
    Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
    (Submitted on 29 Sep 2015)
    We review some recent results from the causal dynamical triangulation (CDT) approach to quantum gravity. We review recent observations of dimensional reduction at a number of previously undetermined points in the parameter space of CDT, and discuss their possible relevance to the asymptotic safety scenario. We also present an updated phase diagram of CDT, discussing properties of a newly discovered phase and its possible relation to a signature change of the metric.
    6 pages, 3 figures, 1 table. Contribution to the proceedings of MG14, Rome July 2015

    http://arxiv.org/abs/1509.05693
    Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
    Ivan Agullo, Noah A. Morris
    (Submitted on 18 Sep 2015)
    We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
    24 pages, 5 figures

    http://arxiv.org/abs/1509.02036
    A note on quantum supergravity and AdS/CFT
    Norbert Bodendorfer
    (Submitted on 7 Sep 2015)
    We note that the non-perturbative quantisation of supergravity as recently investigated using loop quantum gravity techniques provides an opportunity to probe an interesting sector of the AdS/CFT correspondence, which is usually not considered in conventional treatments. In particular, assuming a certain amount of convergence between the quantum supergravity sector of string theory and quantum supergravity constructed via loop quantum gravity techniques, we argue that the large quantum number expansion in loop quantum supergravity corresponds to the 1/Nc2 expansion in the corresponding gauge theory. In order to argue that we are indeed dealing with an appropriate quantum supergravity sector of string theory, high energy (α′) corrections are being neglected, leading to a gauge theory at strong coupling, yet finite Nc. The arguments given in this paper are mainly of qualitative nature, with the aim of serving as a starting point for a more in depth interaction between the string theory and loop quantum gravity communities.
    8 pages.

    http://arxiv.org/abs/1509.01312
    Analog of the Peter-Weyl Expansion for Lorentz Group
    Leonid Perlov
    (Submitted on 4 Sep 2015)
    SS perlov.png
    19pages.

    http://arxiv.org/abs/1509.00466
    4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block
    Muxin Han
    (Submitted on 31 Aug 2015)
    A class of 3d N=2 supersymmetric gauge theories are constructed and shown to encode the simplicial geometries in 4-dimensions. The gauge theories are defined by applying the Dimofte-Gaiotto-Gukov construction in 3d/3d correspondence to certain graph complement 3-manifolds. Given a gauge theory in this class, the massive supersymmetric vacua of the theory contain the classical geometries on a 4d simplicial complex. The corresponding 4d simplicial geometries are locally constant curvature (either dS or AdS), in the sense that they are made by gluing geometrical 4-simplices of the same constant curvature. When the simplicial complex is sufficiently refined, the simplicial geometries can approximate all possible smooth geometries on 4-manifold. At the quantum level, we propose that a class of holomorphic blocks defined in arXiv:1211.1986 from the 3d N=2 gauge theories are wave functions of quantum 4d simplicial geometries. In the semiclassical limit, the asymptotic behavior of holomorphic block reproduces the classical action of 4d Einstein-Hilbert gravity in the simplicial context.
    40+7 pages, 9 figures

    http://arxiv.org/abs/1509.00458
    Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
    Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
    (Submitted on 1 Sep 2015)
    Prominent approaches to quantum gravity struggle when it comes to incorporating a positive cosmological constant in their models. Using quantization of a complex SL(2,ℂ) Chern-Simons theory we include a cosmological constant, of either sign, into a model of quantum gravity.
    5 pages and 2 figures

    http://arxiv.org/abs/1508.07961
    Quantum Cuboids and the EPRL-FK path integral for quantum gravity
    Benjamin Bahr, Sebastian Steinhaus
    (Submitted on 31 Aug 2015)
    In this work, we investigate the 4d path integral for Euclidean quantum gravity on a hypercubic lattice, as given by the EPRL-FK model. To tackle the problem, we restrict to a set of quantum geometries that reflects the large amount of lattice symmetries. In particular, the sum over intertwiners is restricted to quantum cuboids, i.e. coherent intertwiners which describe a cuboidal geometry in the large-j limit.
    Using asymptotic expressions for the vertex amplitude, we find several interesting properties of the state sum. First of all, the value of coupling constants in the amplitude functions determines whether geometric or non-geometric configurations dominate the path integral. Secondly, there is a critical value of the coupling constant α, which separates two phases. In one, the main contribution comes from very irregular and crumpled states. In the other, the dominant contribution comes from a highly regular configuration, which can be interpreted as flat Euclidean space, with small non-geometric perturbations around it.
    Thirdly, we use the state sum to compute the physical norm of kinematical states, i.e. their norm in the physical Hilbert space. We find that states which describe boundary geometry with high torsion have exponentially suppressed physical norm. We argue that this allows one to exclude them from the state sum in calculations.
    15 pages, 15 figures

    http://arxiv.org/abs/1508.06786
    Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology
    Susanne Schander, Aurélien Barrau, Boris Bolliet, Linda Linsefors, Julien Grain
    (Submitted on 27 Aug 2015)
    In effective models of loop quantum cosmology, the holonomy corrections lead to a deformed algebra of constraints. Among other consequences of this new spacetime structure is the emergence of an Euclidean phase around the bounce. In this article, we explicitly compute the resulting primordial power spectrum for scalar modes by setting initial conditions in the contracting phase.
    10 pages, 4 figures

    http://arxiv.org/abs/1508.05953
    Locality and entanglement in bandlimited quantum field theory
    Jason Pye, William Donnelly, Achim Kempf
    (Submitted on 24 Aug 2015)
    We consider a model for a Planck scale ultraviolet cutoff which is based on Shannon sampling. Shannon sampling originated in information theory, where it expresses the equivalence of continuous and discrete representations of information. When applied to quantum field theory, Shannon sampling expresses a hard ultraviolet cutoff in the form of a bandlimitation. This introduces nonlocality at the cutoff scale in a way that is more subtle than a simple discretization of space: quantum fields can then be represented as either living on continuous space or, entirely equivalently, as living on any one lattice whose average spacing is sufficiently small. We explicitly calculate vacuum entanglement entropies in 1+1 dimension and we find a transition between logarithmic and linear scaling of the entropy, which is the expected 1+1 dimensional analog of the transition from an area to a volume law. We also use entanglement entropy and mutual information as measures to probe in detail the localizability of the field degrees of freedom. We find that, even though neither translation nor rotation invariance are broken, each field degree of freedom occupies an incompressible volume of space, indicating a finite information density.
    23 pages, 13 figures.

    http://arxiv.org/abs/1508.05543
    Relational Quantum Cosmology
    Francesca Vidotto
    (Submitted on 22 Aug 2015)
    The application of quantum theory to cosmology raises a number of conceptual questions, such as the role of the quantum-mechanical notion of "observer" or the absence of a time variable in the Wheeler-DeWitt equation. I point out that a relational formulation of quantum mechanics, and more in general the observation that evolution is always relational, provides a coherent solution to this tangle of problems.
    20 pages, 4 figures. Contribution to the forthcoming book on Philosophy of Cosmology edited by K. Chamcham, J. Barrow, J. Silk and S. Saunders for Cambridge University Press

    http://arxiv.org/abs/1508.01947
    Chaos, Dirac observables and constraint quantization
    Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
    (Submitted on 8 Aug 2015)
    There is good evidence that full general relativity is non-integrable or even chaotic. We point out the severe repercussions: differentiable Dirac observables and a reduced phase space do not exist in non-integrable constrained systems and are thus unlikely to occur in a generic general relativistic context. Instead, gauge invariant quantities generally become discontinuous, thus not admitting Poisson-algebraic structures and posing serious challenges to a quantization. Non-integrability also renders the paradigm of relational dynamics cumbersome, thereby straining common interpretations of the dynamics. We illustrate these conceptual and technical challenges with simple toy models. In particular, we exhibit reparametrization invariant models which fail to be integrable and, as a consequence, can either not be quantized with standard methods or lead to sick quantum theories without a semiclassical limit. These troubles are qualitatively distinct from semiclassical subtleties in unconstrained quantum chaos and can be directly traced back to the scarcity of Dirac observables. As a possible resolution, we propose to change the method of quantization by refining the configuration space topology until the generalized observables become continuous in the new topology and can acquire a quantum representation. This leads to the polymer quantization method underlying loop quantum cosmology and gravity. Remarkably, the polymer quantum theory circumvents the problems of the quantization with smooth topology, indicating that non-integrability and chaos, while a challenge, may not be a fundamental obstruction for quantum gravity.
    48 pages, 9 figures, lots of discussion

    http://arxiv.org/abs/1508.01416
    Spin Foams Without Spins
    Jeff Hnybida
    (Submitted on 6 Aug 2015)
    We formulate the spin foam representation of discrete SU(2) gauge theory as a product of vertex amplitudes each of which is the spin network generating function of the boundary graph dual to the vertex. Thus the sums over spins have been carried out. We focus on the character expansion of Yang-Mills theory which is an approximate heat kernel regularization of BF theory. The boundary data of each n-valent node is an element of the Grassmannian Gr(2,n) which carries a coherent representation of U(n) and a geometrical interpretation as a framed polyhedron of fixed total area. Ultimately, sums over spins are traded for contour integrals over simple poles and recoupling theory is avoided using generating functions.
    21 pages, 2 figures

    http://arxiv.org/abs/1507.08112
    Running of the scalar spectral index in bouncing cosmologies
    Jean-Luc Lehners, Edward Wilson-Ewing
    (Submitted on 29 Jul 2015)
    We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10−4 and up to 10−2, leading in some cases to interesting expectations for near-future observations.
    6 pages

    http://arxiv.org/abs/1507.07591
    Discrete Hamiltonian for General Relativity
    Jonathan Ziprick, Jack Gegenberg
    (Submitted on 27 Jul 2015)
    Beginning from canonical general relativity written in terms of Ashtekar variables, we derive a discrete phase space with a physical Hamiltonian for gravity. The key idea is to define the gravitational fields within a complex of three-dimensional cells such that the dynamics is completely described by discrete boundary variables, and the full theory is recovered in the continuum limit. Canonical quantization is attainable within the loop quantum gravity framework, and we believe this will lead to a promising candidate for quantum gravity.
    6 pages

    http://arxiv.org/abs/1507.05424
    Phenomenology of bouncing black holes in quantum gravity: a closer look
    Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer
    (Submitted on 20 Jul 2015)
    It was recently shown that black holes could be bouncing stars as a consequence of quantum gravity. We investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. We consider different possible bounce times and study the integrated diffuse emission.
    8 pages, 8 figures

    http://arxiv.org/abs/1507.04703
    Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
    Ivan Agullo
    (Submitted on 16 Jul 2015)
    We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
    7 pages, 3 figure

    http://arxiv.org/abs/1507.01153
    Coherent State Operators in Loop Quantum Gravity
    Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski
    (Submitted on 4 Jul 2015)
    We present a new method for constructing operators in loop quantum gravity. The construction is an application of the general idea of "coherent state quantization", which allows one to associate a unique quantum operator to every function on a classical phase space. Using the heat kernel coherent states of Hall and Thiemann, we show how to construct operators corresponding to functions depending on holonomies and fluxes associated to a fixed graph. We construct the coherent state versions of the fundamental holonomy and flux operators, as well as the basic geometric operators of area, angle and volume. Our calculations show that the corresponding canonical operators are recovered from the coherent state operators in the limit of large spins.
    34 pages, 4 figures

    http://arxiv.org/abs/1507.00986
    New Hamiltonian constraint operator for loop quantum gravity
    Jinsong Yang, Yongge Ma
    (Submitted on 3 Jul 2015)
    A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity, which is well defined in the Hilbert space of diffeomorphism invariant states up to non-planar vertices. On one hand, it inherits the advantage of the original regularization method, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is less ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
    5 pages

    http://arxiv.org/abs/1507.00851
    Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
    Christoph Charles, Etera R. Livine
    (Submitted on 3 Jul 2015)
    In the context of the geometrical interpretation of the spin network states of Loop Quantum Gravity, we look at the holonomies of the Ashtekar-Barbero connection on loops embedded in space-like hyperboloids. We use this simple setting to illustrate two points. First, the Ashtekar-Barbero connection is not a space-time connection, its holonomies depend on the spacetime embedding of the canonical hypersurface. This fact is usually interpreted as an inconvenience, but we use it to extract the extrinsic curvature from the holonomy and separate it from the 3d intrinsic curvature. Second, we show the limitations of this reconstruction procedure, due to a periodicity of the holonomy in the Immirzi parameter, which underlines the role of a real Immirzi parameter as a cut-off for general relativity at the quantum level in contrast with its role of a mere coupling constant at the classical level.
    8 pages
     
  4. Nov 22, 2015 #3

    marcus

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    Dearly Missed

    Time to start gathering papers for the fourth quarter MIP poll:

    http://arxiv.org/abs/1511.07332
    Towards self dual Loop Quantum Gravity
    Jibril Ben Achour
    (Submitted on 23 Nov 2015)
    In this PhD thesis, we introduced a new strategy to investigate the kinematical and physical predictions of self dual Loop Quantum Gravity (LQG) and by-passed the old problem of implementing quantum mechanically the so called reality conditions inherent to the self dual Ashtekar's phase space.
    We first review the loop quantization of the spherically isolated horizon and the computation of its micro-canonical entropy. Then we present the so called gas of punctures model for the quantum horizon, discussing its results in the canonical and grand-canonical ensembles and its limits.
    The fourth chapter is devoted to studying to what extend the loop quantization based on the self dual variables could cure those problems. We introduce a new strategy, based on an analytic continuation of the degeneracy from γ∈R to γ=±i. We review in details the construction of the procedure, and present the results. At the leading term, we recover exactly the Bekenstein-Hawking area law.
    The fifth chapter is devoted to understanding more precisely the interplay between the status of the Immirzi parameter. In order to do this, we introduce from a new toy model describing 2+1 gravity which depends explicitly on the Immirzi parameter.
    Finally, the sixth chapter is devoted to applying our procedure to the simplest Loop Quantum Cosmology model. By first constructing the LQC dynamics for any arbitrary spin j and then implementing our analytic continuation, we show that our procedure preserves the key features of the LQC models, i.e. we obtain a bouncing universe which admits the right semi classical limit after the bounce.
    PhD Thesis - 254 pages

    http://arxiv.org/abs/1511.05441
    3D holography: from discretum to continuum
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 17 Nov 2015)
    We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
    42 pages

    http://arxiv.org/abs/1511.03644
    Hessian and graviton propagator of the proper vertex
    Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
    (Submitted on 11 Nov 2015)
    The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the `continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
    20 pages

    http://arxiv.org/abs/1510.08766
    Observational Exclusion of a Consistent Quantum Cosmological Scenario
    Boris Bolliet, Aurelien Barrau, Julien Grain, Susanne Schander
    (Submitted on 29 Oct 2015)
    It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very ad hocscenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the deformed algebra approach to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.
    5 pages, 1 figure

    http://arxiv.org/abs/1510.05593
    Asymptotics with a positive cosmological constant: III. The quadrupole formula
    Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
    (Submitted on 19 Oct 2015)
    Almost a century ago, Einstein used a weak field approximation around Minkowski space-time to calculate the energy carried away by gravitational waves emitted by a time changing mass-quadrupole. However, by now there is strong observational evidence for a positive cosmological constant, Λ. To incorporate this fact, Einstein's celebrated derivation is generalized by replacing Minkowski space-time with de Sitter space-time. The investigation is motivated by the fact that, because of the significant differences between the asymptotic structures of Minkowski and de Sitter space-times, many of the standard techniques, including the standard 1/r expansions, can not be used for Λ>0. Furthermore since, e.g., the energy carried by gravitational waves is always positive in Minkowski space-time but can be arbitrarily negative in de Sitter space-time irrespective of how small Λ is, the limit Λ→0 can fail to be continuous. Therefore, a priori it is not clear that a small Λ would introduce only negligible corrections to Einstein's formula. We show that, while even a tiny cosmological constant does introduce qualitatively new features, in the end, corrections to Einstein's formula are negligible for astrophysical sources currently under consideration by gravitational wave observatories.
    31 pages, 2 figures

    http://arxiv.org/abs/1510.04990
    Gravitational waves from isolated systems: The phantom menace of a positive cosmological constant
    Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
    (Submitted on 16 Oct 2015)
    There is a deep tension between the well-developed theory of gravitational waves from isolated systems and the presence of a positive cosmological constant Λ, however tiny. In particular, even the post-Newtonian quadrupole formula, derived by Einstein in 1918, has not been generalized to include a positive Λ. We first explain the principal difficulties and then show that it is possible to overcome them in the weak field limit. These results also provide concrete hints for constructing the Λ>0 generalization of the Bondi-Sachs framework for full, non-linear general relativity.
    5 pages, 1 Figure

    http://arxiv.org/abs/1510.04896
    Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
    Béatrice Bonga, Brajesh Gupt
    (Submitted on 16 Oct 2015)
    We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum gravity. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.
    30 pages, 11 figures

    http://arxiv.org/abs/1510.04701
    Timelike information broadcasting in cosmology
    Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
    (Submitted on 15 Oct 2015)
    We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology.
    16 pages, 10 figures.

    http://arxiv.org/abs/1510.03858
    The thermodynamics of quantum spacetime histories
    Lee Smolin
    (Submitted on 13 Oct 2015)
    We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories.
    To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez and Bianchi. This allows us to apply a recent argument of Jacobson to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations.
    These results suggest also a proposal for a quantum equivalence principle.
    39 pages, 6 figures

    http://arxiv.org/abs/1510.03855
    Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
    Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu
    (Submitted on 13 Oct 2015)
    We derive the primordial power spectra, spectral indices and runnings of both cosmological scalar perturbations and gravitational waves in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous treatment for σ being integers to the case with any given value of σ. For this purpose, we adopt a new calculational strategy in the uniform asymptotic approximation, by expanding the involved integrals first in terms of the inverse-volume correction parameter to its first-order, a consistent requirement of the approximation of the inverse-volume corrections. In this way, we calculate explicitly the quantum gravitational corrections to the standard inflationary spectra and spectral indices to the second-order of the slow-roll parameters, and obtain the observational constraints on the inverse-volume corrections from Planck 2015 data for various values of σ. Using these constraints we discuss whether these quantum gravitational corrections lead to measurable signatures in the cosmological observations. We show that the scale-dependent contributions to inflationary spectra from the inverse-volume corrections could be well within the range of the detectability of the forthcoming generation of experiments.
    19 pages, 4 figures, and 2 tables

    http://arxiv.org/abs/1510.03135
    Loop quantum cosmology: the horizon problem and the probability of inflation
    Long Chen, Jian-Yang Zhu
    (Submitted on 12 Oct 2015)
    Anomaly-free perturbations of loop quantum cosmology reveal a deformed space-time structure, in which the signature changes when the energy density is ρ=ρc/2. Furthermore, in loop quantum cosmology, one can obtain an effective causal structure only for a low density region (ρ≤ρc/2), which gives a natural initial condition to consider the horizon problem. Choosing the initial value at ρ(0)=ρc/2 in this paper, we investigate the horizon problem and the probability of inflation in the framework of loop quantum cosmology. Two models are considered: the quadratic inflation and the natural inflation. We use the Liouville measure to calculate the probability of inflation which solves the horizon problem, and find that, for the quadratic inflation model, the probability is very close to unity, while for the natural inflation model, the probability is about 35%.
    7 pages, 2 figures

    http://arxiv.org/abs/1510.01926
    Projective Limits of State Spaces IV. Fractal Label Sets
    Suzanne Lanéry, Thomas Thiemann
    (Submitted on 7 Oct 2015)
    Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski [Kijowski 1977] to represent quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. One can thus bypass the need to select a vacuum state for the theory, and still be provided with an explicit and constructive description of the quantum state space, at least as long as the label set indexing the projective structure is countable. Because uncountable label sets are much less practical in this context, we develop in the present article a general procedure to trim an originally uncountable label set down to countable cardinality. In particular, we investigate how to perform this tightening of the label set in a way that preserves both the physical content of the algebra of observables and its symmetries. This work is notably motivated by applications to the holonomy-flux algebra underlying Loop Quantum Gravity. Building on earlier work by Okolow [arXiv:1304.6330], a projective state space was introduced for this algebra in [arXiv:1411.3592]. However, the non-trivial structure of the holonomy-flux algebra prevents the construction of satisfactory semi-classical states. Implementing the general procedure just mentioned in the case of a one-dimensional version of this algebra, we show how a discrete subalgebra can be extracted without destroying universality nor diffeomorphism invariance. On this subalgebra, states can then be constructed whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
    42 pages, 1 figure

    http://arxiv.org/abs/1510.01925
    Projective Loop Quantum Gravity II. Searching for Semi-Classical States
    Suzanne Lanéry, Thomas Thiemann
    (Submitted on 7 Oct 2015)
    In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a `no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
    51 pages, 1 figure

    http://arxiv.org/abs/1510.00699
    A Perfect Bounce
    Steffen Gielen, Neil Turok
    (Submitted on 2 Oct 2015)
    We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
    5 pages.
     
    Last edited: Nov 27, 2015
  5. Dec 13, 2015 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Possible candidates for the fourth quarter MIP poll. Currently 19 papers are under consideration:

    http://arxiv.org/abs/1512.05331
    A simpler way of imposing simplicity constraints
    Andrzej Banburski, Lin-Qing Chen
    (Submitted on 16 Dec 2015)
    We investigate a way of imposing simplicity constraints in a holomorphic Spin Foam model that we recently introduced. Rather than imposing the constraints on the boundary spin network, as is usually done, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour, with differences appearing at higher order. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit.
    16 pages

    http://arxiv.org/abs/1512.04993
    Complexity, Action, and Black Holes
    Adam Brown, Daniel A. Roberts, Leonard Susskind, Brian Swingle, Ying Zhao
    (Submitted on 15 Dec 2015)
    Our earlier paper "Complexity Equals Action" conjectured that the quantum computational complexity of a holographic state is given by the classical action of a region in the bulk (the `Wheeler-DeWitt' patch). We provide calculations for the results quoted in that paper, explain how it fits into a broader (tensor) network of ideas, and elaborate on the hypothesis that black holes are fastest computers in nature.
    55+14 pages, many figures

    http://arxiv.org/abs/1512.04566
    Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
    Tommaso De Lorenzo, Alejandro Perez
    (Submitted on 14 Dec 2015)
    A new scenario for gravitational collapse has been recently proposed by Haggard and Rovelli. Presenting the model under the name of black hole fireworks, they claimed that the accumulation of quantum gravitational effects outside the horizon can cause the tunneling of geometry from a black hole to a white hole, allowing a bounce of the collapsing star which can eventually go back to infinity. In this paper we discuss the instabilities of this model and propose a simple minimal modification which eliminates them, as well as other related instabilities discussed in the literature. The new scenario is a time-asymmetric version of the original model with a time-scale for the final explosion that is shorter than m log m in Planck units. Our analysis highlights the importance of irreversibility in gravitational collapse which, in turn, uncovers important issues that cannot be addressed in detail without a full quantum gravity treatment.
    18 Pages, 6 Figures

    http://arxiv.org/abs/1512.03684
    Anisotropic loop quantum cosmology with self-dual variables
    Edward Wilson-Ewing
    (Submitted on 11 Dec 2015)
    A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big-bang and big-crunch singularities are resolved in the sense that singular and non-singular states decouple under the action of the Hamiltonian constraint operator.
    16 pages

    http://arxiv.org/abs/1512.02083
    Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
    Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
    (Submitted on 7 Dec 2015)
    Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
    5 pages, 1 figure

    http://arxiv.org/abs/1512.01589
    Asymptotic safety in an interacting system of gravity and scalar matter
    Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci
    (Submitted on 4 Dec 2015)
    Asymptotic safety is an attractive scenario for the dynamics of quantum spacetime. Here, we work from a phenomenologically motivated point of view and emphasize that a viable dynamics for quantum gravity in our universe must account for the existence of matter. In particular, we explore the scale-dependence of a scalar matter-gravity-vertex, and investigate whether an interacting fixed point exists for the so-defined Newton coupling. We find a viable fixed point in the pure-gravity system, disregarding scalar quantum fluctuations. We explore its extensions to the case with dynamical scalars, and find indications of asymptotic safety in the matter-gravity system. We moreover examine the anomalous dimensions for different components of the metric fluctuations, and find significant differences between the transverse traceless and scalar component.
    15 pages, 14 figures

    http://arxiv.org/abs/1512.00713
    An embedding of loop quantum cosmology in (b, v) variables into a full theory context
    Norbert Bodendorfer
    (Submitted on 2 Dec 2015)
    Loop quantum cosmology in (b, v) variables, which is governed by a unit step size difference equation, is embedded into a full theory context based on similar variables. From the full theory perspective, the symmetry reduction is characterised by the vanishing of certain phase space functions which are implemented as operator equations in the quantum theory. The loop quantum cosmology dynamics arise as the action of the full theory Hamiltonian on maximally coarse states in the kernel of the reduction constraints. An application of this reduction procedure to spherical symmetry is also sketched, with similar results, but only one canonical pair in (b, v) form.
    17 pages

    http://arxiv.org/abs/1512.00221
    On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
    Norbert Bodendorfer, Antonia Zipfel
    (Submitted on 1 Dec 2015)
    Building on a recent proposal for a quantum reduction to spherical symmetry from full loop quantum gravity, we investigate the relation between a quantisation of spherically symmetric general relativity and a reduction at the quantum level. To this end, we generalise the previously proposed quantum reduction by dropping the gauge fixing condition on the radial diffeomorphisms, thus allowing to make direct contact between previous work on reduced quantisation. A dictionary between spherically symmetric variables and observables with respect to the reduction constraints in the full theory is discussed, as well as an embedding of reduced quantum states to a sub sector of the quantum symmetry reduced full theory states. On this full theory sub sector, the quantum algebra of the mentioned observables is computed and shown to qualitatively reproduce the quantum algebra of the reduced variables in the large quantum number limit for a specific choice of regularisation. Insufficiencies in recovering the reduced algebra quantitatively from the full theory are attributed to the oversimplified full theory quantum states we use.
    34 pages

    http://arxiv.org/abs/1511.05441
    3D holography: from discretum to continuum
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 17 Nov 2015)
    We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
    42 pages

    http://arxiv.org/abs/1511.03644
    Hessian and graviton propagator of the proper vertex
    Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
    (Submitted on 11 Nov 2015)
    The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the `continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
    20 pages

    http://arxiv.org/abs/1510.08766
    Observational Exclusion of a Consistent Quantum Cosmological Scenario
    Boris Bolliet, Aurelien Barrau, Julien Grain, Susanne Schander
    (Submitted on 29 Oct 2015)
    It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very ad hocscenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the deformed algebra approach to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.
    5 pages, 1 figure

    http://arxiv.org/abs/1510.05593
    Asymptotics with a positive cosmological constant: III. The quadrupole formula
    Abhay Ashtekar, Béatrice Bonga, Aruna Kesavan
    (Submitted on 19 Oct 2015)
    Almost a century ago, Einstein used a weak field approximation around Minkowski space-time to calculate the energy carried away by gravitational waves emitted by a time changing mass-quadrupole. However, by now there is strong observational evidence for a positive cosmological constant, Λ. To incorporate this fact, Einstein's celebrated derivation is generalized by replacing Minkowski space-time with de Sitter space-time. The investigation is motivated by the fact that, because of the significant differences between the asymptotic structures of Minkowski and de Sitter space-times, many of the standard techniques, including the standard 1/r expansions, can not be used for Λ>0. Furthermore since, e.g., the energy carried by gravitational waves is always positive in Minkowski space-time but can be arbitrarily negative in de Sitter space-time irrespective of how small Λ is, the limit Λ→0 can fail to be continuous. Therefore, a priori it is not clear that a small Λ would introduce only negligible corrections to Einstein's formula. We show that, while even a tiny cosmological constant does introduce qualitatively new features, in the end, corrections to Einstein's formula are negligible for astrophysical sources currently under consideration by gravitational wave observatories.
    31 pages, 2 figures

    http://arxiv.org/abs/1510.04896
    Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
    Béatrice Bonga, Brajesh Gupt
    (Submitted on 16 Oct 2015)
    We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum gravity. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.
    30 pages, 11 figures

    http://arxiv.org/abs/1510.04701
    Timelike information broadcasting in cosmology
    Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
    (Submitted on 15 Oct 2015)
    We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology.
    16 pages, 10 figures.

    http://arxiv.org/abs/1510.03858
    The thermodynamics of quantum spacetime histories
    Lee Smolin
    (Submitted on 13 Oct 2015)
    We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories.
    To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez and Bianchi. This allows us to apply a recent argument of Jacobson to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations.
    These results suggest also a proposal for a quantum equivalence principle.
    39 pages, 6 figures

    http://arxiv.org/abs/1510.03855
    Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
    Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu
    (Submitted on 13 Oct 2015)
    We derive the primordial power spectra, spectral indices and runnings of both cosmological scalar perturbations and gravitational waves in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous treatment for σ being integers to the case with any given value of σ. For this purpose, we adopt a new calculational strategy in the uniform asymptotic approximation, by expanding the involved integrals first in terms of the inverse-volume correction parameter to its first-order, a consistent requirement of the approximation of the inverse-volume corrections. In this way, we calculate explicitly the quantum gravitational corrections to the standard inflationary spectra and spectral indices to the second-order of the slow-roll parameters, and obtain the observational constraints on the inverse-volume corrections from Planck 2015 data for various values of σ. Using these constraints we discuss whether these quantum gravitational corrections lead to measurable signatures in the cosmological observations. We show that the scale-dependent contributions to inflationary spectra from the inverse-volume corrections could be well within the range of the detectability of the forthcoming generation of experiments.
    19 pages, 4 figures, and 2 tables

    http://arxiv.org/abs/1510.01926
    Projective Limits of State Spaces IV. Fractal Label Sets
    Suzanne Lanéry, Thomas Thiemann
    (Submitted on 7 Oct 2015)
    Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski [Kijowski 1977] to represent quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. One can thus bypass the need to select a vacuum state for the theory, and still be provided with an explicit and constructive description of the quantum state space, at least as long as the label set indexing the projective structure is countable. Because uncountable label sets are much less practical in this context, we develop in the present article a general procedure to trim an originally uncountable label set down to countable cardinality. In particular, we investigate how to perform this tightening of the label set in a way that preserves both the physical content of the algebra of observables and its symmetries. This work is notably motivated by applications to the holonomy-flux algebra underlying Loop Quantum Gravity. Building on earlier work by Okolow [arXiv:1304.6330], a projective state space was introduced for this algebra in [arXiv:1411.3592]. However, the non-trivial structure of the holonomy-flux algebra prevents the construction of satisfactory semi-classical states. Implementing the general procedure just mentioned in the case of a one-dimensional version of this algebra, we show how a discrete subalgebra can be extracted without destroying universality nor diffeomorphism invariance. On this subalgebra, states can then be constructed whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
    42 pages, 1 figure

    http://arxiv.org/abs/1510.01925
    Projective Loop Quantum Gravity II. Searching for Semi-Classical States
    Suzanne Lanéry, Thomas Thiemann
    (Submitted on 7 Oct 2015)
    In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a `no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
    51 pages, 1 figure

    http://arxiv.org/abs/1510.00699
    A Perfect Bounce
    Steffen Gielen, Neil Turok
    (Submitted on 2 Oct 2015)
    We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
    5 pages.
     
    Last edited: Dec 17, 2015
  6. Dec 17, 2015 #5

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Currently 20 papers are under consideration:

    http://arxiv.org/abs/1512.09010
    Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
    Anna Ijjas, Paul J. Steinhardt
    (Submitted on 30 Dec 2015)
    The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions.
    9 pages and 2 figures. Invited review article for the focus issue "Planck and the fundamentals of cosmology" in Classical and Quantum Gravity. Accepted for publication.

    http://arxiv.org/abs/1512.08959
    Entanglement time in the primordial universe
    Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo
    (Submitted on 30 Dec 2015)
    We investigate the behavior of the entanglement entropy of space in the primordial phase of the universe before the beginning of cosmic inflation. We argue that in this phase the entanglement entropy of a region of space grows from a zero-law to an area-law. This behavior provides a quantum version of the classical BKL conjecture that spatially separated points decouple in the approach to a cosmological singularity. We show that the relational growth of the entanglement entropy with the scale factor provides a new statistical notion of arrow of time in quantum gravity. The growth of entanglement in the pre-inflationary phase provides a mechanism for the production of the quantum correlations present at the beginning of inflation and imprinted in the CMB sky.
    12 pages, 3 figures, Honorable Mention in the Gravity Research Foundation 2015 Essay Competition

    http://arxiv.org/abs/1512.08346
    Quantum black hole without singularity
    Claus Kiefer
    (Submitted on 28 Dec 2015)
    We discuss the quantization of a spherical dust shell in a rigorous manner. Classically, the shell can collapse to form a black hole with a singularity. In the quantum theory, we construct a well-defined self-adjoint extension for the Hamilton operator. As a result, the evolution is unitary and the singularity is avoided. If we represent the shell initially by a narrow wave packet, it will first contract until it reaches the region where classically a black hole would form, but then re-expands to infinity. In a way, the state can be interpreted as a superposition of a black hole with a white hole.
    5 pages, invited contribution to the BH6 session at the Marcel Grossmann Conference MG14

    http://arxiv.org/abs/1512.07690
    SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
    Hal M. Haggard, Muxin Han, Wojciech Kaminski, Aldo Riello
    (Submitted on 24 Dec 2015)
    The present paper analyze SL(2,ℂ) Chern-Simons theory on a class of graph complement 3-manifolds, and its relation with classical and quantum geometries on 4-dimensional manifolds. In classical theory, we explain the correspondence between a class of SL(2,ℂ) flat connections on 3-manifold and the Lorentzian simplicial geometries in 4 dimensions. The class of flat connections on the graph complement 3-manifold is specified by a certain boundary condition. The corresponding simplicial 4-dimensional geometries are made by constant curvature 4-simplices. The quantization of 4d simplicial geometry can be carried out via the quantization of flat connection on 3-manifold in Chern-Simons theory. In quantum SL(2,ℂ) Chern-Simons theory, a basis of physical wave functions is the class of (holomorphic) 3d block, defined by analytically continued Chern-Simons path integral over Lefschetz thimbles. Here we propose that the (holomorphic) 3d block with the proper boundary condition imposed gives the quantization of simplicial 4-dimensional geometry. Interestingly in the semiclassical asymptotic expansion of (holomorphic) 3d block, the leading contribution gives the classical action of simplicial Einstein-Hilbert gravity in 4 dimensions, i.e. Lorentzian 4d Regge action on constant curvature 4-simplices with a cosmological constant. Such a result suggests a relation between SL(2,ℂ) Chern-Simons theory on a class of 3-manifolds and simplicial quantum gravity on 4-dimensional manifolds. This paper presents the details for the results reported in arXiv:1509.00458.
    43+11 pages, 9 figures

    http://arxiv.org/abs/1512.05743
    Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
    Edward Wilson-Ewing
    (Submitted on 17 Dec 2015)
    I review how the separate universe approach to cosmological perturbation theory can be used to study the dynamics of long wavelength perturbations in loop quantum cosmology, and use it to calculate how the long wavelength perturbations evolve across the bounce assuming a constant equation of state. An interesting result is that the tensor-to-scalar ratio can be damped or amplified by quantum gravity effects during the bounce, depending on the equation of state of the matter field dominating the dynamics. In particular, if the equation of state is greater than -1/3 the value of the tensor-to-scalar ratio will be damped during the bounce, in some cases significantly.
    12 pages. Prepared for submission to International Journal of Modern Physics series D, special issue on Loop Quantum Cosmology

    http://arxiv.org/abs/1512.05331
    A simpler way of imposing simplicity constraints
    Andrzej Banburski, Lin-Qing Chen
    (Submitted on 16 Dec 2015)
    We investigate a way of imposing simplicity constraints in a holomorphic Spin Foam model that we recently introduced. Rather than imposing the constraints on the boundary spin network, as is usually done, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour, with differences appearing at higher order. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit.
    16 pages

    http://arxiv.org/abs/1512.04566
    Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
    Tommaso De Lorenzo, Alejandro Perez
    (Submitted on 14 Dec 2015)
    A new scenario for gravitational collapse has been recently proposed by Haggard and Rovelli. Presenting the model under the name of black hole fireworks, they claimed that the accumulation of quantum gravitational effects outside the horizon can cause the tunneling of geometry from a black hole to a white hole, allowing a bounce of the collapsing star which can eventually go back to infinity. In this paper we discuss the instabilities of this model and propose a simple minimal modification which eliminates them, as well as other related instabilities discussed in the literature. The new scenario is a time-asymmetric version of the original model with a time-scale for the final explosion that is shorter than m log m in Planck units. Our analysis highlights the importance of irreversibility in gravitational collapse which, in turn, uncovers important issues that cannot be addressed in detail without a full quantum gravity treatment.
    18 Pages, 6 Figures

    http://arxiv.org/abs/1512.03684
    Anisotropic loop quantum cosmology with self-dual variables
    Edward Wilson-Ewing
    (Submitted on 11 Dec 2015)
    A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big-bang and big-crunch singularities are resolved in the sense that singular and non-singular states decouple under the action of the Hamiltonian constraint operator.
    16 pages

    http://arxiv.org/abs/1512.02083
    Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
    Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
    (Submitted on 7 Dec 2015)
    Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
    5 pages, 1 figure

    http://arxiv.org/abs/1512.01589
    Asymptotic safety in an interacting system of gravity and scalar matter
    Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci
    (Submitted on 4 Dec 2015)
    Asymptotic safety is an attractive scenario for the dynamics of quantum spacetime. Here, we work from a phenomenologically motivated point of view and emphasize that a viable dynamics for quantum gravity in our universe must account for the existence of matter. In particular, we explore the scale-dependence of a scalar matter-gravity-vertex, and investigate whether an interacting fixed point exists for the so-defined Newton coupling. We find a viable fixed point in the pure-gravity system, disregarding scalar quantum fluctuations. We explore its extensions to the case with dynamical scalars, and find indications of asymptotic safety in the matter-gravity system. We moreover examine the anomalous dimensions for different components of the metric fluctuations, and find significant differences between the transverse traceless and scalar component.
    15 pages, 14 figures

    http://arxiv.org/abs/1512.00713
    An embedding of loop quantum cosmology in (b, v) variables into a full theory context
    Norbert Bodendorfer
    (Submitted on 2 Dec 2015)
    Loop quantum cosmology in (b, v) variables, which is governed by a unit step size difference equation, is embedded into a full theory context based on similar variables. From the full theory perspective, the symmetry reduction is characterised by the vanishing of certain phase space functions which are implemented as operator equations in the quantum theory. The loop quantum cosmology dynamics arise as the action of the full theory Hamiltonian on maximally coarse states in the kernel of the reduction constraints. An application of this reduction procedure to spherical symmetry is also sketched, with similar results, but only one canonical pair in (b, v) form.
    17 pages

    http://arxiv.org/abs/1512.00221
    On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
    Norbert Bodendorfer, Antonia Zipfel
    (Submitted on 1 Dec 2015)
    Building on a recent proposal for a quantum reduction to spherical symmetry from full loop quantum gravity, we investigate the relation between a quantisation of spherically symmetric general relativity and a reduction at the quantum level. To this end, we generalise the previously proposed quantum reduction by dropping the gauge fixing condition on the radial diffeomorphisms, thus allowing to make direct contact between previous work on reduced quantisation. A dictionary between spherically symmetric variables and observables with respect to the reduction constraints in the full theory is discussed, as well as an embedding of reduced quantum states to a sub sector of the quantum symmetry reduced full theory states. On this full theory sub sector, the quantum algebra of the mentioned observables is computed and shown to qualitatively reproduce the quantum algebra of the reduced variables in the large quantum number limit for a specific choice of regularisation. Insufficiencies in recovering the reduced algebra quantitatively from the full theory are attributed to the oversimplified full theory quantum states we use.
    34 pages

    http://arxiv.org/abs/1511.05441
    3D holography: from discretum to continuum
    Valentin Bonzom, Bianca Dittrich
    (Submitted on 17 Nov 2015)
    We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
    42 pages

    http://arxiv.org/abs/1511.03644
    Hessian and graviton propagator of the proper vertex
    Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
    (Submitted on 11 Nov 2015)
    The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the `continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
    20 pages

    http://arxiv.org/abs/1510.04896
    Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
    Béatrice Bonga, Brajesh Gupt
    (Submitted on 16 Oct 2015)
    We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum gravity. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.
    30 pages, 11 figures

    http://arxiv.org/abs/1510.04701
    Timelike information broadcasting in cosmology
    Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
    (Submitted on 15 Oct 2015)
    We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology.
    16 pages, 10 figures.

    http://arxiv.org/abs/1510.03858
    The thermodynamics of quantum spacetime histories
    Lee Smolin
    (Submitted on 13 Oct 2015)
    We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories.
    To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez and Bianchi. This allows us to apply a recent argument of Jacobson to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations.
    These results suggest also a proposal for a quantum equivalence principle.
    39 pages, 6 figures

    http://arxiv.org/abs/1510.03855
    Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
    Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu
    (Submitted on 13 Oct 2015)
    We derive the primordial power spectra, spectral indices and runnings of both cosmological scalar perturbations and gravitational waves in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous treatment for σ being integers to the case with any given value of σ. For this purpose, we adopt a new calculational strategy in the uniform asymptotic approximation, by expanding the involved integrals first in terms of the inverse-volume correction parameter to its first-order, a consistent requirement of the approximation of the inverse-volume corrections. In this way, we calculate explicitly the quantum gravitational corrections to the standard inflationary spectra and spectral indices to the second-order of the slow-roll parameters, and obtain the observational constraints on the inverse-volume corrections from Planck 2015 data for various values of σ. Using these constraints we discuss whether these quantum gravitational corrections lead to measurable signatures in the cosmological observations. We show that the scale-dependent contributions to inflationary spectra from the inverse-volume corrections could be well within the range of the detectability of the forthcoming generation of experiments.
    19 pages, 4 figures, and 2 tables

    http://arxiv.org/abs/1510.01925
    Projective Loop Quantum Gravity II. Searching for Semi-Classical States
    Suzanne Lanéry, Thomas Thiemann
    (Submitted on 7 Oct 2015)
    In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a `no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
    51 pages, 1 figure

    http://arxiv.org/abs/1510.00699
    A Perfect Bounce
    Steffen Gielen, Neil Turok
    (Submitted on 2 Oct 2015)
    We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
    5 pages.
     
    Last edited: Dec 31, 2015
  7. Dec 20, 2015 #6

    marcus

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    Thanks to all who have voted in this poll so far! In particular, to Atty, Chronos, David H., Greg, Jkxzgyk, and Nonlinearity. Here is how the votes stack up at present. It's interesting that number #11, which there of us voted for, is by a longtime Physicsforums member, who was active in this BtSM forum's discussions of quantum gravity as a grad student some years back.

    Which paper(s) will contribute most significantly to future research?

    1. Generalized effective description of loop quantum cosmology
      1 vote(s)
    2. Recent results in CDT quantum gravity
      1 vote(s)
    3. Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
      2 vote(s)
    4. A note on quantum supergravity and AdS/CFT
      2 vote(s)
    5. Analog of the Peter-Weyl Expansion for Lorentz Group
      2 vote(s)
    6. 4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block
      0 vote(s)
    7. 4d Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
      2 vote(s)
    8. Quantum Cuboids and the EPRL-FK path integral for quantum gravity
      2 vote(s)
    9. Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology
      1 vote(s)
    10. Locality and entanglement in bandlimited quantum field theory
      1 vote(s)
    11. Relational Quantum Cosmology
      3 vote(s)
    12. Chaos, Dirac observables and constraint quantization
      1 vote(s)
    13. Spin Foams Without Spins
      1 vote(s)
    14. Running of the scalar spectral index in bouncing cosmologies
      0 vote(s)
    15. Discrete Hamiltonian for General Relativity
      1 vote(s)
    16. Phenomenology of bouncing black holes in quantum gravity: a closer look
      1 vote(s)
    17. Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
      0 vote(s)
    18. Coherent State Operators in Loop Quantum Gravity
      0 vote(s)
    19. New Hamiltonian constraint operator for loop quantum gravity
      0 vote(s)
    20. Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for QG
      2 vote(s)

    Actually #11 is an exceptionally interesting one. I'd recommend anyone interested in quantum cosmology (or QG in general) to have a look at it. Here's the abstract:

    http://arxiv.org/abs/1508.05543
    Relational Quantum Cosmology
    Francesca Vidotto
    (Submitted on 22 Aug 2015)
    The application of quantum theory to cosmology raises a number of conceptual questions, such as the role of the quantum-mechanical notion of "observer" or the absence of a time variable in the Wheeler-DeWitt equation. I point out that a relational formulation of quantum mechanics, and more in general the observation that evolution is always relational, provides a coherent solution to this tangle of problems.
    20 pages, 4 figures. Contribution to the forthcoming book on Philosophy of Cosmology edited by K. Chamcham, J. Barrow, J. Silk and S. Saunders for Cambridge University Press
     
    Last edited: Jan 12, 2016
  8. Jan 6, 2016 #7

    marcus

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    Candidate list for 4th quarter MIP poll

    http://arxiv.org/abs/1512.09010
    Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
    Anna Ijjas, Paul J. Steinhardt

    http://arxiv.org/abs/1512.08959
    Entanglement time in the primordial universe
    Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo

    http://arxiv.org/abs/1512.08346
    Quantum black hole without singularity
    Claus Kiefer

    http://arxiv.org/abs/1512.07690
    SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
    Hal M. Haggard, Muxin Han, Wojciech Kaminski, Aldo Riello

    http://arxiv.org/abs/1512.05743
    Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
    Edward Wilson-Ewing

    http://arxiv.org/abs/1512.05331
    A simpler way of imposing simplicity constraints
    Andrzej Banburski, Lin-Qing Chen

    http://arxiv.org/abs/1512.04566
    Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
    Tommaso De Lorenzo, Alejandro Perez

    http://arxiv.org/abs/1512.03684
    Anisotropic loop quantum cosmology with self-dual variables
    Edward Wilson-Ewing

    http://arxiv.org/abs/1512.02083
    Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
    Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan

    http://arxiv.org/abs/1512.01589
    Asymptotic safety in an interacting system of gravity and scalar matter
    Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci

    http://arxiv.org/abs/1512.00713
    An embedding of loop quantum cosmology in (b, v) variables into a full theory context
    Norbert Bodendorfer

    http://arxiv.org/abs/1512.00221
    On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
    Norbert Bodendorfer, Antonia Zipfel

    http://arxiv.org/abs/1511.05441
    3D holography: from discretum to continuum
    Valentin Bonzom, Bianca Dittrich

    http://arxiv.org/abs/1511.03644
    Hessian and graviton propagator of the proper vertex
    Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky

    http://arxiv.org/abs/1510.04896
    Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
    Béatrice Bonga, Brajesh Gupt

    http://arxiv.org/abs/1510.04701
    Timelike information broadcasting in cosmology
    Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

    http://arxiv.org/abs/1510.03858
    The thermodynamics of quantum spacetime histories
    Lee Smolin

    http://arxiv.org/abs/1510.03855
    Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
    Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu

    http://arxiv.org/abs/1510.01925
    Projective Loop Quantum Gravity II. Searching for Semi-Classical States
    Suzanne Lanéry, Thomas Thiemann

    http://arxiv.org/abs/1510.00699
    A Perfect Bounce
    Steffen Gielen, Neil Turok

    ========================
    List of titles:

    Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
    Entanglement time in the primordial universe
    Quantum black hole without singularity
    SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
    Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
    A simpler way of imposing simplicity constraints
    Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
    Anisotropic loop quantum cosmology with self-dual variables
    Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
    Asymptotic safety in an interacting system of gravity and scalar matter
    An embedding of loop quantum cosmology in (b, v) variables into a full theory context
    On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
    3D holography: from discretum to continuum
    Hessian and graviton propagator of the proper vertex
    Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
    Timelike information broadcasting in cosmology
    The thermodynamics of quantum spacetime histories
    Inflationary spectra in loop quantum cosmology and their observational constraints from Planck 2015 data
    Projective Loop Quantum Gravity II. Searching for Semi-Classical States
    A Perfect Bounce
     
    Last edited: Jan 6, 2016
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