Fri - 09/10/21

There is no new related paper today

There is no new related paper today

  1. [2109.03838] - A Euclidean Perspective on Completeness and Weak Gravity - Daniel Harlow, Hirosi Ooguri

  2. [2109.04414] - The cylinder amplitude in the Hard Dimer model on 2D Causal Dynamical Triangulations - John F. Wheater, P.D. Xavier

Wed - 09/08/21

Extended geometry of Gambini-Olmedo-Pullin polymer black hole and its quasinormal spectrum

  • Authors: Yu-Chen Liu, Jia-Xi Feng, Fu-Wen Shu, Anzhong Wang
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2109.02861
  • Abstract In this paper we systematically study a model of spherically symmetric polymer black holes recently proposed by Gambini, Olmedo, and Pullin (GOP). Within the framework of loop quantum gravity, the quantum parameters in the GOP model depend on the minimal area gap and the size of the discretization of the physical states. In this model, a spacelike transition surface takes place of the classical singularity. By means of coordinate transformations, we first extend the metric to the white hole region, and find that the geometric structure of the quantum black hole is similar to the wormhole structure, and the radius of the most quantum region is equal to the wormhole radius. In addition, we show that the energy conditions are violated not only at throat, but also at horizons and the spatial infinities. In order to show how the quantum effects affect the spacetimes, we calculate the Ricci and Kretschmann scalars at different places. It turns out that, as expected, the most quantum region is at the throat. Finally, we consider the quasinormal modes (QNMs) of massless scalar field perturbations, electromagnetic field perturbations, and axial gravitational perturbations. QNMs in the Eikonal limits are also considered. As anticipated, the spectrum of QNMs deviates from that of the classical case due to quantum effects. Interestingly, our results show that the quasinormal frequencies of the perturbations share the same qualitative tendency while setting quantum parameters with various values in this effective model, even if the potential deviations are different with different spins.

There is no new related paper today

  1. [2109.02660] - Unitarity, clock dependence and quantum recollapse in quantum cosmology - Steffen Gielen, Lucía Menéndez-Pidal

  2. [2109.03176] - Thermodynamic geometry of static and rotating regular black holes in conformal massive gravity - Saheb Soroushfar, Reza Saffari, Amare Abebe, Haidar Sheikhahmadi

  3. [2109.02649] - Negativity Spectra in Random Tensor Networks and Holography - Jonah Kudler-Flam, Vladimir Narovlansky, Shinsei Ryu

Tue - 09/07/21

There is no new related paper today

There is no new related paper today

Fri - 09/03/21

There is no new related paper today

Lorentzian quantum cosmology goes simplicial

  • Authors: Bianca Dittrich, Steffen Gielen, Susanne Schander
  • Subjects: General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2109.00875
  • Abstract We employ the methods of discrete (Lorentzian) Regge calculus for analysing Lorentzian quantum cosmology models with a special focus on discrete analogues of the no-boundary proposal for the early universe. We use a simple 4-polytope, a subdivided 4-polytope and shells of discrete 3-spheres as triangulations to model a closed universe with cosmological constant, and examine the semiclassical path integral for these different choices. We find that the shells give good agreement with continuum results for small values of the scale factor and in particular for finer discretisations of the boundary 3-sphere, while the simple and subdivided 4-polytopes can only be compared with the continuum in certain regimes, and in particular are not able to capture a transition from Euclidean geometry with small scale factor to a large Lorentzian one. Finally, we consider a closed universe filled with dust particles and discretised by shells of 3-spheres. This model can approximate the continuum case quite well. Our results embed the no-boundary proposal in a discrete setting where it is possibly more naturally defined, and prepare for its discussion within the realm of spin foams.

Causal structure in spin-foams

  • Authors: Eugenio Bianchi, Pierre Martin-Dussaud
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); Mathematical Physics (math-ph); Quantum Physics (quant-ph)
  • Arxiv link: https://arxiv.org/abs/2109.00986
  • Abstract The metric field of general relativity is almost fully determined by its causal structure. Yet, in spin-foam models for quantum gravity, the role played by the causal structure is still largely unexplored. The goal of this paper is to clarify how causality is encoded in such models. The quest unveils the physical meaning of the orientation of the two-complex and its role as a dynamical variable. We propose a causal version of the EPRL spin-foam model and discuss the role of the causal structure in the reconstruction of a semiclassical spacetime geometry.

Higher Dimensional Polytopal Universe in Regge Calculus

  • Authors: Ren Tsuda, Takanori Fujiwara
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Lattice (hep-lat); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2109.01075
  • Abstract Higher dimensional closed Friedmann-Lema\^itre-Robertson-Walker (FLRW) universe with positive cosmological constant is investigated by Regge calculus. A Cauchy surface of discretized FLRW universe is replaced by a regular polytope in accordance with the Collins-Williams (CW) formalism. Polytopes in an arbitrary dimensions can be systematically dealt with by a set of five integers integrating the Schl"afli symbol of the polytope. Regge action in continuum time limit is given. It possesses reparameterization invariance of the time variable. Variational principle for edge lengths and struts yields Hamiltonian constraint and evolution equation. They describe oscillating universe in dimensions larger than three. To go beyond the approximation by regular polytopes, we propose pseudo-regular polytopes with fractional Schl"afli symbols as a substitute for geodesic domes in higher dimensions. We examine the pseudo-regular polytope model as an effective theory of Regge calculus for the geodesic domes. In the infinite frequency limit, the pseudo-regular polytope model reduces to the continuum FLRW universe.
  1. [2109.00564] - Curved accretion disks around rotating black holes without reflection symmetry - Che-Yu Chen, Hsiang-Yi Karen Yang

Thu - 09/02/21

There is no new related paper today

Four-dimensional Spinfoam Quantum Gravity with Cosmological Constant: Finiteness and Semiclassical Limit

  • Authors: Muxin Han
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Geometric Topology (math.GT)
  • Arxiv link: https://arxiv.org/abs/2109.00034
  • Abstract We present an improved formulation of 4-dimensional Lorentzian spinfoam quantum gravity with cosmological constant. The construction of spinfoam amplitudes uses the state-integral model of PSL(2,$\mathbb{C}$) Chern-Simons theory and the implementation of simplicity constraint. The formulation has 2 key features: (1) spinfoam amplitudes are all finite, and (2) With suitable boundary data, the semiclassical asymptotics of the vertex amplitude has two oscillatory terms, with phase plus or minus the 4-dimensional Lorentzian Regge action with cosmological constant for the constant curvature 4-simplex.
  1. [2109.00017] - Integrable deformations of AdS/CFT - Marius de Leeuw, Anton Pribytok, Ana L. Retore, Paul Ryan

Wed - 09/01/21

There is no new related paper today

There is no new related paper today

  1. [2108.13427] - Circuit Complexity in Topological Quantum Field Theory - Josiah Couch, Yale Fan, Sanjit Shashi

Tue - 08/31/21

Quantum gravity might restrict a cyclic evolution

  • Authors: Bao-Fei Li, Parampreet Singh
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2108.12553
  • Abstract It is generally expected that in a non-singular cosmological model a cyclic evolution is straightforward to obtain on introduction of a suitable choice of a scalar field with a negative potential or a negative cosmological constant which causes a recollapse at some time in the evolution. We present a counter example to this conventional wisdom. Working in the realm of loop cosmological models with non-perturbative quantum gravity modifications we show that a modified version of standard loop quantum cosmology based on Thiemann’s regularization of the Hamiltonian constraint while generically non-singular does not allow a cyclic evolution unless some highly restrictive conditions hold. Irrespective of the energy density of other matter fields, a recollapse and hence a cyclic evolution is only possible if one chooses an almost Planck sized negative potential of the scalar field or a negative cosmological constant. Further, cycles when present do not occur in the classical regime. Surprisingly, a necessary condition for a cyclic evolution, not singularity resolution, turns out to be a violation of the weak energy condition. These results are in a striking contrast to standard loop quantum cosmology where obtaining a recollapse at large volumes and a cyclic evolution is straightforward, and, there is no violation of weak energy condition. On one hand our work shows that some quantum cosmological models even though non-singular and bouncing are incompatible with a cyclic evolution, and on the other hand demonstrates that differences in various quantization prescriptions in loop cosmology need not be faint and buried in the pre-bounce regime, but can be striking and profound even in the post-bounce regime.

Nonclassical cosmological dynamics in the low-energy limit of loop quantum scalar-tensor theory

  • Authors: Yu Han
  • Subjects: General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2108.12556
  • Abstract In previous work, we showed that in loop quantum cosmology of scalar-tensor theory (STT) with the holonomy correction the background equations of motion in the Jordan frame have two branches, i.e., the $b_{+}$ branch and the $b_{-}$ branch. In the low-energy limit, the $b_{+}$ branch of the equations of motion reproduce the equations of motion of classical STT while the $b_{-}$ branch of equations of motion do not reproduce the classical equations. In this paper, we investigate cosmological dynamics in an expanding Universe whose background is described by the the $b_{-}$ branch of equations of motion; and we especially focus on the dynamics of the perturbations in the low-energy limit because it is most relevant to the current observational range.First, we show that the low-energy limit of $b_{-}$ branch of equations of motion can be a stable attractor in the expansion phase of the Universe. Then, we find a low-energy effective Hamiltonian on the spatially flat Friedmann-Robertson-Walker background. The background part of this Hamiltonian can yield the low-energy limit of the $b_{-}$ branch of equations and this Hamiltonian consists of constraints whose constraint algebra is different from the classical case but also closed up to arbitrary order of perturbations. Remarkably, we find that this Hamiltonian can be transformed into the Hamiltonian of the Einstein frame by field redefinitions. Moreover, we also develop the linear cosmological perturbation theory and apply it to study the slow-roll inflation in this context. Finally, we study a specific model of STT in which the background of the expanding Universe is described by the $b_{-}$ branch of equations of motion. It is shown that the slow-roll inflation can take place in this case and the spectral indices agree well with the observations.

There is no new related paper today

  1. [2108.12668] - A semiclassical singularity theorem - Christopher J. Fewster, Eleni-Alexandra Kontou

Mon - 08/30/21

Glimmers of a post-geometric perspective

  • Authors: Federico Piazza
  • Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
  • Arxiv link: https://arxiv.org/abs/2108.12362
  • Abstract Quantum gravitational effects could become important at low energy if the wavefunction of the metric field fails to be peaked around a classical configuration. We try to understand such deviations from classicality within canonical quantum gravity by introducing a “fluid of observers” in the low energy theory and defining a distance operator “at equal time” among them. We find that, even in the presence of relevant fluctuations in the metric field, a “locally flat” limit is recovered in the neighbourhood of each observer. Deviations from classicality have no local consequences. However, at larger separations the expectation value of the distance operator behaves differently than a standard Riemannian distance. In particular, it is non-additive and thus cannot be obtained by the integral of a differential line element. This emerging beyond Riemannian geometry is a metric space similar to embedded Riemannian manifolds equipped with chord distances that “cut through” the ambient space. We study deviations from flat space by looking at triangles in the limit where their sizes go to zero. Beyond-Riemannian deviations with respect to flat space are of the same order as standard Riemannian ones, but qualitatively different. Some implications on the bulk picture of holographic theories and black hole evaporation are discussed.

There is no new related paper today

  1. [2108.12090] - Finite-$N$ superconformal index via the AdS/CFT correspondence - Yosuke Imamura

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