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Loop-and-allied QG bibliography

  1. Jan 7, 2017 #2481
    Quantum gravity and Standard-Model-like fermions
    Astrid Eichhorn, Stefan Lippoldt
    (Submitted on 17 Nov 2016)
    We discover that chiral symmetry does not act as an infrared attractor of the renormalization group flow under the impact of quantum gravity fluctuations. Thus, observationally viable quantum gravity models must respect chiral symmetry. In our truncation, asymptotically safe gravity does, as a chiral fixed point exists. A second non-chiral fixed point with massive fermions provides a template for models with dark matter. This fixed point disappears for more than 10 fermions, suggesting that an asymptotically safe ultraviolet completion for the standard model plus gravity enforces chiral symmetry.
    Comments: 6 pages, 2 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1611.05878 [gr-qc]
    Quantum-gravity effects on a Higgs-Yukawa model
    Astrid Eichhorn, Aaron Held, Jan M. Pawlowski
    (Submitted on 7 Apr 2016 (v1), last revised 13 Sep 2016 (this version, v2))
    A phenomenologically viable theory of quantum gravity must accommodate all observed matter degrees of freedom and their properties. Here, we explore whether a toy model of the Higgs-Yukawa sector of the Standard Model is compatible with asymptotically safe quantum gravity. We discuss the phenomenological implications of our result in the context of the Standard Model. We analyze the quantum scaling dimension of the system, and find an irrelevant Yukawa coupling at a joint gravity-matter fixed point. Further, we explore the impact of gravity-induced couplings between scalars and fermions, which are non-vanishing in asymptotically safe gravity.
    Comments: 13 pages + appendix, 10 figures, simplified basis for induced couplings
    Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)
    Journal reference: Phys. Rev. D 94, 104027 (2016)
    DOI: 10.1103/PhysRevD.94.104027
    Cite as: arXiv:1604.02041 [hep-th]
    (or arXiv:1604.02041v2 [hep-th] for this version)
     
  2. Jan 7, 2017 #2482
    Quantum gravity on foliated spacetime - asymptotically safe and sound
    Jorn Biemans, Alessia Platania, Frank Saueressig
    (Submitted on 15 Sep 2016 (v1), last revised 17 Oct 2016 (this version, v2))
    Asymptotic Safety provides a mechanism for constructing a consistent and predictive quantum theory of gravity valid on all length scales. Its key ingredient is a non-Gaussian fixed point of the gravitational renormalization group flow which controls the scaling of couplings and correlation functions at high energy. In this work we use a functional renormalization group equation adapted to the ADM-formalism for evaluating the gravitational renormalization group flow on a cosmological Friedmann-Robertson-Walker background. Besides possessing the UV-non-Gaussian fixed point characteristic for Asymptotic Safety the setting exhibits a second non-Gaussian fixed point with a positive Newton's constant and real critical exponents. The new fixed point alters the phase diagram in such a way that all renormalization group trajectories connected to classical general relativity are well-defined on all length scales. In particular a positive cosmological constant is dynamically driven to zero in the deep infrared. Moreover, the scaling dimensions associated with the universality classes emerging within the causal setting exhibit qualitative agreement with results found within the ϵ-expansion around two dimensions, Monte Carlo simulations based on Lattice Quantum Gravity, and the discretized Wheeler-deWitt equation.
    Comments: 7 pages, 3 figures. Conclusions slightly modified
    Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1609.04813 [hep-th]
    (or arXiv:1609.04813v2 [hep-th] for this version)
     
  3. Jan 10, 2017 #2483

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    https://arxiv.org/abs/1701.02282
    Some Clarifications on the Duration of Inflation in Loop Quantum Cosmology
    Boris Bolliet, Aurélien Barrau, Killian Martineau, Flora Moulin
    (Submitted on 9 Jan 2017)
    The prediction of a phase of inflation whose number of e-folds is constrained is an important feature of loop quantum cosmology. This work aims at giving some elementary clarifications on the role of the different hypotheses leading to this conclusion. We show that the duration of inflation does not depend significantly on the modified background dynamics in the quantum regime.

    https://arxiv.org/abs/1701.02037
    (3+1)-dimensional topological phases and self-dual quantum geometries encoded on Heegard surfaces
    Bianca Dittrich
    (Submitted on 8 Jan 2017)
    We apply the recently suggested strategy to lift state spaces and operators for (2+1)-dimensional topological quantum field theories to state spaces and operators for a (3+1)-dimensional TQFT with defects. We start from the (2+1)-dimensional Turaev-Viro theory and obtain the state space for the Crane-Yetter model with line defects.
    This work has important applications for quantum gravity as well as the theory of topological phases in (3+1) dimensions. It provides a self-dual quantum geometry realization based on a vacuum state peaked on a homogeneously curved geometry. The state spaces and operators we construct here provide also an improved version of the Walker-Wang model, and simplify its analysis considerably.
    We in particular show that the fusion bases of the (2+1)-dimensional theory lead to a rich set of bases for the (3+1)-dimensional theory. This includes a quantum deformed spin network basis, which in a loop quantum gravity context diagonalizes spatial geometry operators. We also obtain a dual curvature basis, that diagonalizes the Walker-Wang Hamiltonian.
    Furthermore, the construction presented here can be generalized to provide state spaces for the recently introduced dichromatic four--dimensional manifold invariants.
     
  4. Jan 11, 2017 #2484

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    https://arxiv.org/abs/1701.02311
    Hypercuboidal renormalization in spin foam quantum gravity
    Benjamin Bahr, Sebastian Steinhaus
    (Submitted on 9 Jan 2017)
    In this article we apply background-independent renormalization group methods to spin foam quantum gravity. It is aimed at extending and elucidating the analysis of a companion letter, in which the existence of a fixed point in the truncated RG flow for the model was reported. Here we repeat the analysis with various modifications, and find that both qualitative and quantitative features of the fixed point are robust in this setting. We also go into details about the various approximation schemes employed in the analysis.

    https://arxiv.org/abs/1701.02439
    Discretization of 3d gravity in different polarizations
    Maïté Dupuis, Laurent Freidel, Florian Girelli
    (Submitted on 10 Jan 2017)
    We study the discretization of 3d gravity with Λ=0 following the loop quantum gravity framework. In the process, we realize that different choices of polarization are possible. This allows to introduce a new discretization based on the triad as opposed to the connection as in the standard loop quantum gravity framework. We also identify the classical non-trivial symmetries of discrete gravity, namely the Drinfeld double, given in terms of momentum maps. Another choice of polarization is given by the Chern-Simons formulation of gravity. Our framework also provides a new discretization scheme of Chern-Simons, which keeps track of the link between the continuum variables and the discrete ones. We show how the Poisson bracket we recover between the Chern-Simons holonomies allows to recover the Goldman bracket. There is also a transparent link between the discrete Chern-Simons formulation and the discretization of gravity based on the connection (loop gravity) or triad variables (dual loop gravity).
     
  5. Jan 11, 2017 #2485

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    https://arxiv.org/abs/1701.03029
    Flowing to the continuum in discrete tensor models for quantum gravity
    Astrid Eichhorn, Tim Koslowski
    (Submitted on 11 Jan 2017)
    Tensor models provide a way to access the path-integral for discretized quantum gravity in d dimensions. As in the case of matrix models for two-dimensional quantum gravity, the continuum limit can be related to a Renormalization Group fixed point in a setup where the tensor size N serves as the Renormalization Group scale. We develop functional Renormalization Group tools for tensor models with a main focus on a rank-3 model for three-dimensional quantum gravity. We rediscover the double-scaling limit and provide an estimate for the scaling exponent. Moreover, we identify two additional fixed points with a second relevant direction in a truncation of the Renormalization Group flow. The new relevant direction might hint at the presence of additional degrees of freedom in the corresponding continuum limit.
     
  6. Jan 14, 2017 #2486
    Naturalness of asymptotically safe Higgs
    Giulio Maria Pelaggi, Francesco Sannino, Alessandro Strumia, Elena Vigiani
    (Submitted on 5 Jan 2017)
    We introduce a model that contains a Higgs-like scalar with gauge, Yukawa and quartic interactions that enter a perturbative asymptotically safe regime at energies above a scale Λ. The model serves as a concrete example to test whether scalars masses unavoidably receive quantum correction of order Λ. The answer is that scalars can be naturally lighter. Although we do not have an answer to whether the Standard Model hypercharge coupling growth towards a Landau pole around Λ∼1040 GeV can be tamed by non-perturbative asymptotic safety, our toy-SM shows that such a possibility is worth exploring. In fact, if successful, it might also offer an explanation for the unbearable lightness of the Higgs.
    Comments: 16 pages
    Subjects: High Energy Physics - Phenomenology (hep-ph)
    Report number: CERN-PH-TH-2017-001, CP3-Origins-2017-001, IFUP-TH/2017
    Cite as: arXiv:1701.01453 [hep-ph]
    (or arXiv:1701.01453v1 [hep-ph] for this versio
     
  7. Jan 16, 2017 #2487
    Detailed investigation of the duration of inflation in loop quantum cosmology for a Bianchi-I universe with different inflaton potentials and initial conditions
    Killian Martineau, Aurélien Barrau, Susanne Schander
    (Submitted on 10 Jan 2017)
    There is a wide consensus on the correct dynamics of the background in loop quantum cosmology. In this article we make a systematic investigation of the duration of inflation by varying what we think to be the most important "unknowns" of the model: the way to set initial conditions, the amount of shear at the bounce and the shape of the inflaton potential.
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph)
    Cite as: arXiv:1701.02703 [gr-qc]
     
  8. Jan 23, 2017 #2488
    Numerical simulations of loop quantum Bianchi-I spacetimes
    Peter Diener, Anton Joe, Miguel Megevand, Parampreet Singh
    (Submitted on 20 Jan 2017)
    Due to the numerical complexities of studying evolution in an anisotropic quantum spacetime, in comparison to the isotropic models, the physics of loop quantized anisotropic models has remained largely unexplored. In particular, robustness of bounce and the validity of effective dynamics have so far not been established. Our analysis fills these gaps for the case of vacuum Bianchi-I spacetime. To efficiently solve the quantum Hamiltonian constraint we perform an implementation of the Cactus framework which is conventionally used for applications in numerical relativity. Using high performance computing, numerical simulations for a large number of initial states with a wide variety of fluctuations are performed. Big bang singularity is found to be replaced by anisotropic bounces for all the cases. We find that for initial states which are sharply peaked at the late times in the classical regime and bounce at a mean volume much greater than the Planck volume, effective dynamics is an excellent approximation to the underlying quantum dynamics. Departures of the effective dynamics from the quantum evolution appear for the states probing deep Planck volumes. A detailed analysis of the behavior of this departure reveals a non-monotonic and subtle dependence on fluctuations of the initial states. We find that effective dynamics in almost all of the cases underestimates the volume and hence overestimates the curvature at the bounce, a result in synergy with earlier findings in isotropic case. The expansion and shear scalars are found to be bounded throughout the evolution.
    Comments: 27 pages, 22 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1701.05824 [gr-qc]
    (or arXiv:1701.05824v1 [gr-qc] for this version)
    Submission history
     
  9. Jan 27, 2017 #2489

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    https://arxiv.org/abs/1701.07420
    Shape Dynamical Loop Gravity from a Conformal Immirzi Parameter
    Patrick J. Wong
    (Submitted on 25 Jan 2017)
    The Immirzi parameter of loop quantum gravity is a one parameter ambiguity of the theory whose precise interpretation is not universally agreed upon. It is an inherent characteristic of the quantum theory as it appears in the spectra of geometric operators, despite being irrelevant at the classical lever. The parameter's appearance in the area and volume spectra to the same power as the Planck area suggest that it plays a role in determining the fundamental length scale of space. In fact, a consistent interpretation is that it represents a constant rescaling of the kinematical spatial geometry. An interesting realization is that promoting the Immirzi parameter to be a general conformal transformation leads to a system which can be identified as analogous to the linking theory of shape dynamics. A three-dimensional gravitational gauge connection is then constructed within the linking theory in a manner analogous to loop quantum gravity, thereby facilitating the application of the established procedure of loop quantization.

    https://arxiv.org/abs/1701.07519
    SO*(2N) coherent states for loop quantum gravity
    Florian Girelli, Giuseppe Sellaroli
    (Submitted on 25 Jan 2017)
    A SU(2) intertwiner with N legs can be interpreted as the quantum state of a convex polyhedron with N faces (when working in 3d). We show that the intertwiner Hilbert space carries a representation of the non-compact group SO*(2N). This group can be viewed as the subgroup of the symplectic group Sp(4N,R) which preserves the SU(2) invariance. We construct the associated Perelomov coherent states and discuss the notion of semi-classical limit, which is more subtle that we could expect. Our work completes the work by Freidel and Livine which focused on the U(N) subgroup of SO*(2N).
     
  10. Jan 30, 2017 #2490
    The black hole quantum atmosphere
    Ramit Dey, Stefano Liberati, Daniele Pranzetti
    (Submitted on 22 Jan 2017)
    Ever since the discovery of black hole evaporation, the region of origin of the radiated quanta has been a topic of debate. Recently it was argued by Giddings that the Hawking quanta originate from a region well outside the black hole horizon by calculating the effective radius of a radiating body via the Stefan--Boltzmann law. In this paper we try to further explore this issue and end up corroborating this claim, using both a heuristic argument and a detailed study of the stress energy tensor. We show that the Hawking quanta originate from what might be called a quantum atmosphere around the black hole with energy density and fluxes of particles peaked at about 4M, running contrary to the popular belief that these originate from the ultra high energy excitations very close to the horizon. This long distance origin of Hawking radiation could have a profound impact on our understanding of the information and transplanckian problems.
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1701.06161 [gr-qc]
     
  11. Jan 31, 2017 #2491
    research papers on the topic of MOND as a low energy 4D QG effect esp in Verlinde emergent gravity theory

    Anisotropic Distribution of High Velocity Galaxies in the Local Group

    Indranil Banik, Hongsheng Zhao
    (Submitted on 21 Jan 2017)
    We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than in a 3D dynamical model of it based on ΛCDM, the standard cosmological paradigm (MNRAS, stx151). 5 out of these 6 galaxies are located very close to a plane with root mean square thickness of only 88.2 kpc despite a radial extent of almost 1 Mpc. This plane also passes within 140 kpc of both the Milky Way (MW) and M31 and just 6 kpc from their mid-point. The orientation of the plane is such that the MW-M31 line is only 20∘ from lying within it.
    We develop a basic model in which a past MW-M31 flyby encounter forms tidal dwarf galaxies that later settle into the recently discovered planes of satellites around the MW and M31. The MW-M31 orbital plane required by this scenario is oriented similarly to that of the LG dwarfs with anomalously high RVs. The fast relative motion of the MW and M31 at one time would lead to LG dwarfs being flung out via gravitational slingshot encounters. These encounters would likely be most efficient for objects flung out close to the MW-M31 orbital plane. This suggests a possible dynamical reason for our findings, which are otherwise difficult to explain as a chance alignment of isotropically distributed galaxies (probability < 0.01).
    Comments: 13 pages, 8 figures, 7 tables. Submitted to the Monthly Notices of the Royal Astronomical Society in this form
    Subjects: Astrophysics of Galaxies (astro-ph.GA)
    Cite as: arXiv:1701.06559 [astro-ph.GA]

    Perspective on MOND emergence from Verlinde's "emergent gravity" and its recent test by weak lensing
    Mordehai Milgrom (Weizmann Institute), Robert H. Sanders (Kapteyn Institute)
    (Submitted on 30 Dec 2016 (v1), last revised 17 Jan 2017 (this version, v2))
    We highlight phenomenological aspects of Verlinde's recent proposal to account for the mass anomalies in galactic systems without dark matter -- in particular in their relation to MOND. Welcome addition to the MOND lore as it is, this approach have reproduced, so far, only a small fraction of MOND phenomenology, and is still rather tentative, both in its theoretical foundations and in its phenomenology. What Verlinde has extracted from this approach, so far, is a formula -- of rather limited applicability, and with no road to generalization in sight -- for the effective gravitational field of a spherical, isolated, static baryonic system. This formula cannot be used to calculate the gravitational field of disk galaxies, with their rich MOND phenomenology. Notably, it cannot predict their rotation curves, except asymptotically. It does not apply to the few-, or many-body problem; so, it cannot give, e.g., the two-body force between two galaxies, or be used to conduct N-body calculations of galaxy formation, evolution, and interactions. The formula cannot be applied to the internal dynamics of a system embedded in an external field, where MOND predicts important consequences. etc. MOND is backed by full-fledged, Lagrangian theories that can be, and are, routinely applied to all the above phenomena, and more. Verlinde's formula, as it now stands, strongly conflicts with solar-system and possibly earth-surface constraints, and cannot fully account for the mass anomalies in the cores of galaxy clusters (a standing conundrum in MOND). The recent weak-lensing test of the formula is, in fact, testing a cornerstone prediction of MOND, one that the formula does reproduce, and which has been tested before in the very same way.
    Comments: 6 pages; added discussion of earth-surface effects
    Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1612.09582 [astro-ph.GA]

    A universal velocity dispersion profile for pressure supported systems: evidence for MONDian gravity across 7 orders of magnitude in mass
    R. Durazo, X. Hernandez, B. Cervantes Sodi, S. F. Sanchez
    (Submitted on 12 Jun 2015 (v1), last revised 2 Dec 2016 (this version, v2))
    For any MONDian extended theory of gravity where the rotation curves of spiral galaxies are explained through a change in physics rather than the hypothesis of dark matter, a generic dynamical behavior is expected for pressure supported systems: an outer flattening of the velocity dispersion profile occurring at a characteristic radius, where both the amplitude of this flat velocity dispersion and the radius at which it appears are predicted to show distinct scalings with the total mass of the system. By carefully analyzing the dynamics of globular clusters and elliptical galaxies, we are able to significantly extend the astronomical diversity of objects in which MONDian gravity has been tested, from spiral galaxies, to the much larger mass range covered by pressure supported systems. We show that a universal projected velocity dispersion profile accurately describes various classes of pressure supported systems, and further, that the expectations of extended gravity are met, across seven orders of magnitude in mass. These observed scalings are not expected under dark matter cosmology, and would require particular explanations tuned at the scales of each distinct astrophysical system.
    Comments: 12 pages, 15 figures 1 table
    Subjects: Astrophysics of Galaxies (astro-ph.GA)
    Cite as: arXiv:1506.04099 [astro-ph.GA]

    dℓ(z) and BAO in the emergent gravity and the dark universe
    Ding-fang Zeng
    (Submitted on 3 Jan 2017)
    We illustrate that ΛMOND cosmology following from E. Verlinde's emergent gravity idea which contains only constant dark energy and baryonic matters governed by linear inverse gravitation forces at and beyond galaxy scales fit with the luminosity distance v.s. redshift relationship, i.e. dℓ(z) of type Ia supernovae equally well as the standard ΛCDM cosmology does. But in a rather broad and reasonable parameter space, ΛMOND gives too strong baryon acoustic oscillation, i.e. BAO signals on the matter power spectrum contradicting with observations from various galaxy survey and counting experiments.
    Comments: 4.2 two column style pages, 4 figures, version asking for comments
    Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1701.00690 [astro-ph.CO]
    (or arXiv:1701.00690v1 [astro-ph.CO] for this version)

    On the Davies-Unruh effect in a wide range of temperatures
    Carlos E. Navia
    (Submitted on 12 Jan 2017)
    The Debye model of the specific heat of solid at low temperatures is incorporate in the Entropic Gravity Theory (EGT). Rather of a smooth surface, the holographic screen is considered as an oscillating elastic membrane, with a continuous range of frequencies, that cuts off at a maximum (Debye) temperature, TD. We show that at low temperatures T<TD, the conservation of the equivalence principle in EGT requires a modification of the Davies-Unruh effect. While the maintenance of Davies-Unruh effect requires a violation of the equivalence principle. These two possibilities are equivalents, because both can emulate the same quantity of dark matter. However, in both cases, the central mechanism is the Davies-Unruh effect, this seems to indicate that the modification of the Davies-Unruh effect emulates dark matter which in turn can be see as a violation of the equivalence principle. This scenario is promising to explain why MOND theory works at very low temperatures (accelerations) regime, i. e., the galaxies sector. We also show that in the intermediate region, for temperatures slightly lower or slightly higher than Debye temperature, EGT predicts the mass-temperature relation of hot X-ray galaxy clusters.
    Comments: 4 pages, 4 figures
    Subjects: Astrophysics of Galaxies (astro-ph.GA)
    Cite as: arXiv:1701.03442 [astro-ph.GA]
    (or arXiv:1701.03442v1 [astro-ph.GA] for this version)
     
  12. Feb 11, 2017 #2492
    general Relativity and Quantum Cosmology
    Time evolution in deparametrized models of loop quantum gravity

    Mehdi Assanioussi, Jerzy Lewandowski, Ilkka Mäkinen
    (Submitted on 6 Feb 2017)
    An important aspect in understanding the dynamics in the context of deparametrized models of LQG is to obtain a sufficient control on the quantum evolution generated by a given Hamiltonian operator. More specifically, we need to be able to compute the evolution of relevant physical states and observables with a relatively good precision. In this article, we introduce an approximation method to deal with the physical Hamiltonian operators in deparametrized LQG models, and apply it to models in which a free Klein-Gordon scalar field or a non-rotational dust field is taken as the physical time variable. This method is based on using standard time-independent perturbation theory of quantum mechanics to define a perturbative expansion of the Hamiltonian operator, the small perturbation parameter being determined by the Barbero-Immirzi parameter β. This method allows us to define an approximate spectral decomposition of the Hamiltonian operators and hence to compute the evolution over a certain time interval. As a specific example, we analyze the evolution of expectation values of the volume and curvature operators starting with certain physical initial states, using both the perturbative method and a straightforward expansion of the expectation value in powers of the time variable. This work represents a first step towards achieving the goal of understanding and controlling the new dynamics developed in [25, 26].
    Comments: 23 pages, 18 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1702.01688 [gr-qc]
    (or arXiv:1702.01688v1 [gr-qc] for this version)
     
  13. Feb 14, 2017 #2493
    Loop-Quantum-Gravity Simplicity Constraint as Surface Defect in Complex Chern-Simons Theory
    Muxin Han, Zichang Huang
    (Submitted on 10 Feb 2017)
    The simplicity constraint is studied in the context of 4d spinfoam models with cosmological constant. We find that the quantum simplicity constraint is realized as the 2d surface defect in SL(2,C) Chern-Simons theory in the construction of spinfoam amplitudes. By this realization of simplicity constraint in Chern-Simons theory, we are able to construct the new spinfoam amplitude with cosmological constant for arbitrary simplicial complex (with many 4-simplices). The semiclassical asymptotics of the amplitude is shown to reproduce correctly the 4-dimensional Einstein-Regge action with cosmological constant term.
    Comments: 16 pages, 3 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Mathematical Physics (math-ph)
    Cite as: arXiv:1702.03285 [gr-qc]
    (or arXiv:1702.03285v1 [gr-qc] for this version)
     
  14. Feb 25, 2017 #2494
    Hybrid loop quantum cosmology and predictions for the cosmic microwave background
    Laura Castelló Gomar, Daniel Martín de Blas, Guillermo A. Mena Marugán, Javier Olmedo
    (Submitted on 20 Feb 2017)
    We investigate the consequences of the hybrid quantization approach for primordial perturbations in loop quantum cosmology, obtaining predictions for the cosmic microwave background and comparing them with data collected by the Planck mission. In this work, we complete previous studies about the scalar perturbations and incorporate tensor modes. We compute their power spectrum for a variety of vacuum states. We then analyze the tensor-to-scalar ratio and the consistency relation between this quantity and the spectral index of the tensor power spectrum. We also compute the temperature-temperature, electric-electric, temperature-electric, and magnetic-magnetic correlation functions. Finally, we discuss the effects of the quantum geometry in these correlation functions and confront them with observations.
    Comments: 33 pages, 23 figures and 1 table
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Report number: IGC-17|2-1
    Cite as: arXiv:1702.06036 [gr-qc]
    (or arXiv:1702.06036v1 [gr-qc] for this version)
     
  15. Feb 27, 2017 #2495
    Gravity as an SU(1,1) gauge theory in four dimensions
    Hongguang Liu, Karim Noui
    (Submitted on 22 Feb 2017)
    We start with the Hamiltonian formulation of the first order action of pure gravity with a full sl(2,C) internal gauge symmetry. We make a partial gauge-fixing which reduces sl(2,C) to its sub-algebra su(1,1). This case corresponds to a splitting of the space-time M=Σ×R where Σ inherits an arbitrary Lorentzian metric of signature (−,+,+). Then, we find a parametrization of the phase space in terms of an su(1,1) commutative connection and its associated conjugate electric field. Following the techniques of Loop Quantum Gravity, we start the quantization of the theory and we consider the kinematical Hilbert space on a given fixed graph Γ whose edges are colored with unitary representations of su(1,1). We compute the spectrum of area operators acting of the kinematical Hilbert space: we show that space-like areas have discrete spectra, in agreement with usual su(2) Loop Quantum Gravity, whereas time-like areas have continuous spectra. We conclude on the possibility to make use of this formulation of gravity to construct a holographic description of black holes in the framework of Loop Quantum Gravity.
    Comments: 19 pages, 2 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1702.06793 [gr-qc]
    (or arXiv:1702.06793v1 [gr-qc] for this version)
     
  16. Mar 6, 2017 #2496

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    https://arxiv.org/abs/1702.04137
    Asymptotically safe cosmology - a status report
    Alfio Bonanno, Frank Saueressig
    (Submitted on 14 Feb 2017)
    Asymptotic Safety, based on a non-Gaussian fixed point of the gravitational renormalization group flow, provides an elegant mechanism for completing the gravitational force at sub-Planckian scales. At high energies the fixed point controls the scaling of couplings such that unphysical divergences are absent while the emergence of classical low-energy physics is linked to a crossover between two renormalization group fixed points. These features make Asymptotic Safety an attractive framework for cosmological model building. The resulting scenarios may naturally give rise to a quantum gravity driven inflationary phase in the very early universe and an almost scale-free fluctuation spectrum. Moreover, effective descriptions arising from an renormalization group improvement permit a direct comparison to cosmological observations as, e.g. Planck data.

    https://arxiv.org/abs/1702.04439
    General Relativity and Quantum Cosmology
    Black Hole Entropy from BMS Symmetry at the Horizon

    S. Carlip
    (Submitted on 15 Feb 2017 (v1), last revised 1 Mar 2017 (this version, v2))
    Near the horizon, the obvious symmetries of a black hole spacetime---the horizon-preserving diffeomorphisms---are enhanced to a larger symmetry group with a BMS3 algebra. Using dimensional reduction and covariant phase space techniques, I investigate this augmented symmetry, and show that it is strong enough to determine the black hole entropy.
     
  17. Mar 6, 2017 #2497

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    https://arxiv.org/abs/1703.01156
    Quantum Surface and Intertwiner Dynamics in Loop Quantum Gravity
    Alexandre Feller, Etera R. Livine
    (Submitted on 3 Mar 2017)
    We introduce simple generic models of surface dynamics in loop quantum gravity (LQG). A quantum surface is defined as a set of elementary patches of area glued together. We provide it with an extra structure of locality (nearest neighbors), thought of as induced by the whole spin network state defining the 3d bulk geometry around the quantum surface. Here, we focus on classical surface dynamics, using a spinorial description of surface degrees of freedom. We introduce two classes of dynamics, to be thought as templates for future investigation of LQG dynamics with in mind the dynamics of quantum black holes. The first defines global dynamics of the closure defect of the surface, with two basic toy-models, either a dissipative dynamics relaxing towards the closure constraint or a Hamiltonian dynamics precessing the closure defect. The second class of dynamics describes the isolated regime, when both area and closure defect are conserved throughout the evolution. The surface dynamics is implemented through U(N) transformations and generalizes to a Bose-Hubbard Hamiltonian with a local quadratic potential interaction. We briefly discuss the implications of modeling the quantum black hole dynamics by a surface Bose-Hubbard model.
     
  18. Mar 9, 2017 #2498
    A Covariant Version of Verlinde's Emergent Gravity
    S. Hossenfelder
    (Submitted on 4 Mar 2017)
    A generally covariant version of Erik Verlinde's emergent gravity model is proposed. The Lagrangian constructed here allows an improved interpretation of the underlying mechanism. It suggests that de-Sitter space is filled with a vector-field that couples to baryonic matter and, by dragging on it, creates an effect similar to dark matter. We solve the covariant equation of motion in the background of a Schwarzschild space-time and obtain correction terms to the non-covariant expression. Furthermore, we demonstrate that the vector field can also mimic dark energy.
    Comments: 14 pages, no figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1703.01415 [gr-q
     
  19. Mar 13, 2017 #2499

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    https://arxiv.org/abs/1703.03710
    The matter-ekpyrotic bounce scenario in Loop Quantum Cosmology
    Jaume Haro, Jaume Amorós, Llibert Aresté Saló
    (Submitted on 10 Mar 2017)
    We will perform a detailed study of the matter-ekpyrotic bouncing scenario in Loop Quantum Cosmology using the methods of the dynamical systems theory. We will show that when the background is driven by a single scalar field, at very late times, in the contracting phase, all orbits depict a matter dominated Universe, which evolves to an ekpyrotic phase. After the bounce the Universe enters in the expanding phase, where the orbits leave the ekpyrotic regime going to a kination (also named deflationary) regime. Moreover, this scenario supports the production of heavy massive particles conformally coupled with gravity, which reheats the universe at temperatures compatible with the nucleosynthesis bounds and also the production of massless particles non-conformally coupled with gravity leading to very high reheating temperatures but ensuring the nucleosynthesis success. Dealing with cosmological perturbations, these background dynamics produce a nearly scale invariant power spectrum for the modes that leave the Hubble radius, in the contracting phase, when the Universe is quasi-matter dominated, whose spectral index and corresponding running is compatible with the recent experimental data obtained by PLANCK's team.

    https://arxiv.org/abs/1703.03757
    Canonical LQG operators and kinematical states for plane gravitational waves
    F. Hinterleitner
    (Submitted on 10 Mar 2017)
    In a 1+1 dimensional model of plane gravitational waves the flux-holonomy algebra of loop quantum gravity is modified in such a way that the new basic operators satisfy canonical commutation relations. Thanks to this construction it is possible to find kinematical solutions for unidirectional plane gravitational waves with finite geometric expectation values and fluctuations, which was problematic in a more conventional approach in a foregoing paper by the author and coauthors.
     
  20. Mar 13, 2017 #2500

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    https://arxiv.org/abs/1703.03443
    Classical and quantum: a conflict of interest
    T. P. Singh
    (Submitted on 9 Mar 2017)
    We highlight three conflicts between quantum theory and classical general relativity, which make it implausible that a quantum theory of gravity can be arrived at by quantising classical gravity. These conflicts are: quantum nonlocality and space-time structure; the problem of time in quantum theory; and the quantum measurement problem. We explain how these three aspects bear on each other, and how they point towards an underlying noncommutative geometry of space-time.
     
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