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Loop-and-allied QG bibliography

  1. Mar 13, 2017 #2501
    Repulsive gravity induced by a conformally coupled scalar field implies a bouncing radiation-dominated universe
    Vicente Antunes, Mario Novello
    (Submitted on 8 Mar 2017)
    In the present work we revisit a model consisting of a scalar field with a quartic self-interaction potential non-minimally (conformally) coupled to gravity [1]. When the scalar field vacuum is in a broken symmetry state, an effective gravitational constant emerges which, in certain regimes, can lead to gravitational repulsive effects when only ordinary radiation is coupled to gravity. In this case, a bouncing universe is shown to be the only cosmological solution admissible by the field equations when the scalar field is in such a broken symmetry state.
    Comments: 10 pages, 1 figure
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1703.03060 [gr-qc]

    Poincaré gauge gravity: an emergent scenario
    J.L. Chkareuli
    (Submitted on 4 Mar 2017)
    The Poincar\'e gauge gravity (PGG) with the underlying vector fields of tetrads and spin-connections is perhaps the best theory candidate for gravitation to be unified with the other three elementary forces of nature. There is a clear analogy between local frame in PGG and local internal symmetry space in the Standard Model. As a result, the spin-connection fields, gauging the local frame Lorentz symmetry group SO(1,3)_{LF}, appear in PGG much as photons and gluons appear in SM. We propose that such an analogy may follow from their common emergent nature allowing to derive PGG in the same way as conventional gauge theories. In essence, we start with an arbitrary theory of some vector and fermion fields which possesses only global spacetime symmetries, such as Lorentz and translational invariance, in flat Minkowski space. The two vector field multiplets involved are proposed to belong, respectively, to the adjoint (A_{{\mu}}^{ij}) and vector (e_{{\mu}}^{i}) representations of the starting global Lorentz symmetry. We show that if these prototype vector fields are covariantly constrained, A_{{\mu}}^{ij}A_{ij}^{{\mu}}=M_{A} and e_{{\mu}}^{i}e_{i}^{{\mu}}=M_{e}, thus causing a spontaneous violation of the accompanying global symmetries (M_{A,e} are their proposed violation scales), then the only possible theory compatible with these length-preserving constraints is turned out to be the gauge invariant PGG, while the corresponding massless (pseudo)Goldstone modes are naturally collected in the emergent gauge fields of tetrads and spin-connections. In a minimal theory case being linear in a curvature we unavoidably come to the Einstein-Cartan theory. The extending theories with propagating spin-connection and tetrad modes are also considered and their possible unification with the Standard Model is briefly discussed.
    Comments: 33 pages, submitted to Physical Review D. arXiv admin note: text overlap with arXiv:1305.6898
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1703.01492 [gr-qc]
    Gravastars in f(R,T) gravity
    Amit Das, Shounak Ghosh, Swapan Das, B.K. Guha, Saibal Ray
    (Submitted on 26 Feb 2017)
    We propose a unique stellar model under the f(R,T) gravity by using the conjecture of Mazur-Mottola~\cite{Mazur2001,Mazur2004} which is known as gravastar and a viable alternative to the black hole as available in literature. This gravastar is described by the three different regions, viz., (I) Interior core region, (II) Intermediate thin shell, and (III) Exterior spherical region. The pressure within the interior region is equal to the constant negative matter density which provides a repulsive force over the thin spherical shell. This thin shell is assumed to be formed by a fluid of ultra relativistic plasma and the pressure, which is directly proportional to the matter-energy density according to Zel'dovich's conjecture of stiff fluid~\cite{zeldovich1972}, does counter balance the repulsive force exerted by the interior core region. The exterior spherical region is completely vacuum and assumed to be de Sitter spacetime which can be described by the Schwarzschild solution. Under this specification we find out a set of exact and singularity-free solution of the collapsing star which presents several other physically valid features within the framework of alternative gravity.
    Comments: 7 pages, 4 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1702.08873 [gr-qc]
    Last edited: Mar 13, 2017
  2. Mar 13, 2017 #2502
    Poincaré Gauge Gravity Cosmology
    Hongchao Zhang, Lixin Xu
    (Submitted on 2 Mar 2017)
    In this work, we construct the logical framework of the Poincar\'e gauge gravity cosmology based on five postulations, and introduce the modified redshift relation within this framework. Then we solve a system with quadratic action and some other assumptions to get an analytic solution on background level. The evolution of the Universe on background can be reproduced from this solution without hypothesizing dark energy. Further, we use the type Ia supernova data set JLA to test the effect of the modified redshift relation under the constraints of system parameters. The results show that the constraint on some parameters are compact.
    Comments: 5 pages, 3 figures, 2 tables
    Subjects: General Relativity and Quantum Cosmology (gr-qc)
    Cite as: arXiv:1703.00639 [gr-qc]
    (or arXiv:1703.00639v1 [gr-qc] for this version)
  3. Mar 15, 2017 #2503
    Analogue Gravity Models From Conformal Rescaling
    Sabine Hossenfelder, Tobias Zingg
    (Submitted on 13 Mar 2017)
    Analogue gravity is based on a mathematical identity between quantum field theory in curved space-time and the propagation of perturbations in certain condensed matter systems. But not every curved space-time can be simulated in such a way, because one does not only need a condensed matter system that generates the desired metric tensor, but that system then also has to obey its own equations of motion. And specifying the metric tensor that one wishes to realize usually overdetermines the underlying condensed matter system, such that its equations of motion are in general not fulfilled, in which case the desired metric does not have an analogue.
    Here, we show that the class of metrics that have an analogue is bigger than what a first cursory consideration might suggest. This is due to the analogue metric only being defined up to a choice of parametrization of the perturbation in the underlying condensed matter system. In this way, the class of analogue gravity models can be vastly expanded. In particular, we demonstrate how this freedom of choice can be used to insert an intermediary conformal factor. Then, as a corollary, we find that any metric conformal to a Painlev\'e--Gullstrand type line element can, potentially, result as an analogue of a perturbation propagating in a non-viscous, barotropic fluid.
    Comments: 12 pages
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
    MSC classes: 83-XX
    Cite as: arXiv:1703.04462 [gr-qc]
    (or arXiv:1703.04462v1 [gr-qc] for this version)
  4. Mar 15, 2017 #2504
    Gravitational echoes from macroscopic quantum gravity effects
    Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
    (Submitted on 31 Jan 2017 (v1), last revised 1 Feb 2017 (this version, v2))
    New theoretical approaches developed in the last years predict that macroscopic quantum gravity effects in black holes should lead to modifications of the gravitational wave signals expected in the framework of classical general relativity, with these modifications being characterized by the existence of dampened repetitions of the primary signal. Here we use the fact that non-perturbative corrections to the near-horizon external geometry of black holes are necessary for these modifications to exist, in order to classify different proposals and paradigms with respect to this criterion and study in a neat and systematic way their phenomenology. Proposals that lead naturally to the existence of echoes in the late-time ringdown of gravitational wave signals from black hole mergers must share the replacement of black holes by horizonless configurations with a physical surface showing reflective properties in the relevant range of frequencies. On the other hand, proposals or paradigms that restrict quantum gravity effects on the external geometry to be perturbative, such as black hole complementarity or the closely related firewall proposal, do not display echoes. For the sake of completeness we exploit the interplay between the timescales associated with the formation of firewalls and the mechanism behind the existence of echoes in order to conclude that even unconventional distortions of the firewall concept (such as naked firewalls) do not lead to this phenomenon.
    Comments: V1: 21 pages + references, 1 figure. V2: updated references
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1701.09156 [gr-qc]
    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.
    Comments: 16 pages
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
    Cite as: arXiv:1701.03029 [gr-qc]
    (or arXiv:1701.03029v1 [gr-qc] for this version)
  5. Mar 21, 2017 #2505
    Quantum Metric and Entanglement on Spin Networks
    Fabio M. Mele
    (Submitted on 19 Mar 2017)
    Motivated by the idea that, in the background-independent framework of a Quantum Theory of Gravity, entanglement is expected to play a key role in the reconstruction of spacetime geometry, we investigate the possibility of using the formalism of Geometric Quantum Mechanics (GQM) to give a tensorial characterization of entanglement on spin network states. Our analysis focuses on the simple case of a single link graph (Wilson line state) for which we define a dictionary to construct a Riemannian metric tensor and a symplectic structure on the space of states. The manifold of (pure) quantum states is then stratified in terms of orbits of equally entangled states and the block-coefficient matrices of the corresponding pulled-back tensors fully encode the information about separability and entanglement. In particular, the off-diagonal blocks define an entanglement monotone interpreted as a distance with respect to the separable state. As such, it provides a measure of graph connectivity. Finally, in the maximally entangled gauge-invariant case, the entanglement monotone is proportional to a power of the area of the surface dual to the link. This suggests a connection between the GQM formalism and the (simplicial) geometric properties of spin network states through entanglement.
    Comments: 162 pages, 11 figures, Master Thesis
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
    Cite as: arXiv:1703.06415 [gr-qc]

    Fisher Metric, Geometric Entanglement and Spin Networks
    Goffredo Chirco, Fabio M. Mele, Daniele Oriti, Patrizia Vitale
    (Submitted on 15 Mar 2017)
    We introduce the geometric formulation of Quantum Mechanics in the quantum gravity context, and we use it to give a tensorial characterization of entanglement on spin network states. Starting from the simplest case of a single-link graph (Wilson line), we define a dictionary to construct a Riemannian metric tensor and a symplectic structure on the space of spin network states, showing how they fully encode the information about separability and entanglement, and, in particular, an entanglement monotone interpreted as a distance with respect to the separable state. In the maximally entangled gauge-invariant case, the entanglement monotone is proportional to a power of the area of the surface dual to the link thus supporting a connection between entanglement and the (simplicial) geometric properties of spin network states. We extend then such analysis to the study of non-local correlations between two non-adjacent regions of a generic spin network. In the end, our analysis shows that the same spin network graph can be understood as an information graph whose connectivity encodes, both at the local and non-local level, the quantum correlations among its parts. This gives a further connection between entanglement and geometry.
    Comments: 51 pages, 3 figures
    Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
    Cite as: arXiv:1703.05231 [gr-qc]

    Holographic spin networks from tensor network states
    Sukhwinder Singh, Nathan A. McMahon, Gavin K. Brennen
    (Submitted on 1 Feb 2017)
    In the holographic correspondence of quantum gravity, a global onsite symmetry at the boundary generally translates to a local gauge symmetry in the bulk. In this paper, we extend the tensor network based toy model for holography introduced in [arXiv:1701.04778] to incorporate this feature. We lift the multi-scale renormalization ansatz (MERA) representation of the ground state of a one dimensional (1D) local Hamiltonian, which has a global onsite symmetry, to a dual quantum state of a 2D lattice on which the symmetry appears gauged. We show how the 2D bulk state decomposes in terms of spin network states, which label a basis in the gauge-invariant sector of the bulk lattice. This decomposition is instrumental to obtain expectation values of gauge-invariant observables in the bulk, and also reveals that the bulk state is generally entangled between the gauge and the remaining bulk degrees of freedom that are not fixed by the symmetry. (In analogy with the holographic correspondence, we interpret the latter to possibly include "gravitational" degrees of freedom.) We illustrate these features for a particular subset of bulk states referred to as copy bulk states. We present numerical results for ground states of several 1D critical spin chains to illustrate that: (i) entanglement in the dual copy bulk states potentially depends on the central charge of the underlying conformal field theories, and (ii) the spectrum of a bulk density matrix, obtained by tracing out the gauge degrees of freedom, exhibits degeneracies, possibly suggesting an emergent symmetry in the non-gauge sector of the bulk. We also illustrate the possibility of emergent topological order in the bulk. More broadly, our holographic model translates the MERA, a tensor network state, to a superposition of spin network states, as they appear in lattice gauge theories in one higher dimension.
    Comments: 21 pages, 18 figures
    Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
    Cite as: arXiv:1702.00392 [cond-mat.str-el]
  6. Mar 21, 2017 #2506
    On the fate of the Hoop Conjecture in quantum gravity
    Fabio Anzà, Goffredo Chirco
    (Submitted on 15 Mar 2017 (v1), last revised 21 Mar 2017 (this version, v2))
    We consider a closed region R of 3d quantum space modeled by SU(2) spin-networks. Using the concentration of measure phenomenon we prove that, whenever the ratio between the boundary ∂R and the bulk edges of the graph overcomes a finite threshold, the state of the boundary is always thermal, with an entropy proportional to its area. The emergence of a thermal state of the boundary can be traced back to a large amount of entanglement between boundary and bulk degrees of freedom. Using the dual geometric interpretation provided by loop quantum gravity, we interprete such phenomenon as a pre-geometric analogue of Thorne's "Hoop conjecture", at the core of the formation of a horizon in General Relativity.
    Comments: 7 pages, 2 figures, minor improvements
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
    Cite as: arXiv:1703.05241 [gr-qc]
    Naturalizing Gravity of the Quantum Fields, and the Hierarchy Problem
    Durmus Demir
    (Submitted on 10 Mar 2017)
    It is shown that gravity can be incorporated into the Standard Model (SM) in a way solving the hierarchy problem. For this, the SM effective action in flat spacetime is adapted to curved spacetime via not only the general covariance but also the gauge invariance. For the latter, gauge field hard masses, induced by loops at the UV scale Λ, are dispelled by construing Λ as the constant value assigned to curvature. This gives way to an unprecedented mechanism for incorporating gravity into the SM in that the hierarchy problem is solved by transmutation of the Higgs boson Λ2--mass into the Higgs-curvature coupling, and the cosmological constant problem is alleviated by metamorphosis of the vacuum Λ4--energy into the Einstein-Hilbert term. Gravity emerges correctly if the SM is accompanied by a secluded dark sector sourcing non-interacting dark matter, dark energy and dark radiation. Physics beyond the SM, containing Higgs-phobic scalars that resolve the strong CP problem, flavor problem, baryogenesis and inflation, respects the hierarchy. Majorana neutrinos are naturally incorporated if Λ lies at the see-saw scale. This mechanism, in general, leaves no compelling reason to anticipate new particles at the LHC or higher-energy colliders.
    Comments: 27 pages, 2 tables
    Subjects: High Energy Physics - Phenomenology (hep-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  7. Mar 23, 2017 at 10:14 AM #2507


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    The Overview Chapter in Loop Quantum Gravity: The First 30 Years
    Abhay Ashtekar, Jorge Pullin
    (Submitted on 21 Mar 2017)
    This is the introductory Chapter in the monograph Loop Quantum Gravity: The First 30 Years, edited by the authors, that was just published in the series "100 Years of General Relativity. The 8 invited Chapters that follow provide fresh perspectives on the current status of the field from some of the younger and most active leaders who are currently shaping its development. The purpose of this Chapter is to provide a global overview by bridging the material covered in subsequent Chapters. The goal and scope of the monograph is described in the Preface which can be read by following the Front Matter link at the website listed below.

    Evolution of Universes in Causal Set Cosmology
    Fay Dowker, Stav Zalel
    (Submitted on 22 Mar 2017)
    The causal set approach to the problem of quantum gravity is based on the hypothesis that spacetime is fundamentally discrete. Spacetime discreteness opens the door to novel types of dynamical law for cosmology and the Classical Sequential Growth (CSG) models of Rideout and Sorkin form an interesting class of such laws. It has been shown that a renormalisation of the dynamical parameters of a CSG model occurs whenever the universe undergoes a Big Crunch-Big Bang bounce. In this paper we propose a way to model the creation of a new universe after the singularity of a black hole. We show that renormalisation of dynamical parameters occurs in a CSG model after such a creation event. We speculate that this could realise aspects of Smolin's Cosmological Natural Selection proposal.
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