Fri - 12/22/23

There is no new related paper today

Area metric gravity revisited

  • Authors: Johanna N. Borissova, Bianca Dittrich, Kirill Krasnov
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2312.13935
  • Abstract Area metrics are an intriguing generalization of length metrics which appears in several quantum-gravity approaches. We describe the space of diffeomorphism-invariant area-metric actions quadratic in fluctuations and derivatives. A general theory is found to be specified by four parameters, two of which are mass parameters for the non-length degrees of freedom. We find that a two-parameter subclass of theories exhibits an additional shift symmetry of the kinetic term, and leads to a ghost-free graviton propagator for the effective theory obtained after integrating out the non-length degrees of freedom. One of the two parameters determines the strength of parity violations, the other defines a mass parameter for the non-length degrees of freedom. The same type of action has been found to appear from modified Plebanski theory and in the continuum limit of (effective) spin foams. We moreover find that area-metric actions in Lorentzian (but not in Euclidean) signature feature wrong-sign kinetic and mass terms for the non-length degrees of freedom. Nevertheless, despite a coupling of these degrees of freedom to the length metric, the linearized dynamics turns out to be stable for the above subclass of actions.

  1. [2312.13552] - Holographic study on Einstein ring for a charged black hole in conformal gravity - Xin-Yun Hu, Xiao-Xiong Zeng, Li-Fang Li, Peng Xu

  2. [2312.13336] - Graviton Mass and Entanglement Islands in Low Spacetime Dimensions - Hao Geng

  3. [2312.13850] - On AdS/CFT duality in the twisted sector of string theory on $AdS_5 \times S^5/\mathbb{Z}_2$ orbifold background - Torben Skrzypek, Arkady A. Tseytlin

  4. [2312.14108] - A Smooth Horizon without a Smooth Horizon - Vaibhav Burman, Suchetan Das, Chethan Krishnan

Thu - 12/21/23

Stability of the de-Sitter universe: One-loop nonlocal $f(R)$ gravity

  • Authors: Haiyuan Feng, Yi Liao, Rong-Jia Yang
  • Subjects: General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2312.11908
  • Abstract With the method of the background field expansion, we investigate the one-loop quantization of the Euclidean nonlocal $f(R)$ model in the de-Sitter universe. We obtain the ghost-free condition (GFC) based on the transformation from the Jordan frame to the Einstein frame and the classical stability condition (CSC) satisfied $\frac{f^{(0)}{RR}}{F^{(0)}{RR}}<\frac{R_0f^{(0)}_R-2f(R_0)}{R_0 F(R_0)}$. We present the on-shell and off-shell one-loop effective action and quantum stability condition (QSC) by utilizing the generalized zeta-function. We have established the equivalence between the QSC and the CSC. This implies that the nonlocal $f(R)$ model remains ghost-free and stability conditions in both classical and quantum situation.

Geodetic precession and shadow of quantum extended black holes

  • Authors: Reza Saadati, Fatimah Shojai
  • Subjects: General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2312.12087
  • Abstract We study the circular motion of massive and massless particles in a recently proposed quantum-corrected Schwarzschild black hole in loop quantum gravity. This solution is supposed to introduce small but non-zero quantum corrections in the low curvature limit. In this paper, we confine our attention to the shadow of the black hole and the geodetic precession of a freely falling gyroscope in a circular orbit. Despite the mathematical complexity of the metric, our results are exact and show that the black hole shadow decreases slightly in this solution while the quantum corrections introduce a non-trivial term in the geodetic precession frequency of the gyroscope.

Cosmology in Lorentzian Regge calculus: causality violations, massless scalar field and discrete dynamics

  • Authors: Alexander F. Jercher, Sebastian Steinhaus
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2312.11639
  • Abstract We develop a model of spatially flat, homogeneous and isotropic cosmology in Lorentzian Regge calculus, employing 4-dimensional Lorentzian frusta as building blocks. By examining the causal structure of the discrete spacetimes obtained by gluing such 4-frusta in spatial and temporal direction, we find causality violations if the sub-cells connecting spatial slices are spacelike. A Wick rotation to the Euclidean theory can be defined globally by a complexification of the variables and an analytic continuation of the action. Introducing a discrete free massless scalar field, we study its equations of motion and show that it evolves monotonically. Furthermore, in a continuum limit, we obtain the equations of a homogeneous scalar field on a spatially flat Friedmann background. Vacuum solutions to the causally regular Regge equations are static and flat and show a restoration of time reparametrisation invariance. In the presence of a scalar field, the height of a frustum is a dynamical variable that has a solution if causality violations are absent and if an inequality relating geometric and matter boundary data is satisfied. Edge lengths of cubes evolve monotonically, yielding a contracting or an expanding branch of the universe. In a small deficit angle expansion, the system can be deparametrised via the scalar field and a continuum limit of the discrete theory can be defined which we show to yield the relational Friedmann equation. These properties are obstructed if higher orders of the deficit angle are taken into account. Our results suggest that the inclusion of timelike sub-cells is necessary for a causally regular classical evolution in this symmetry restricted setting. Ultimately, this works serves as a basis for forthcoming investigations on the cosmological path integral within the framework of effective spin foams.

  1. [2312.11611] - Non-minimal Elliptic Threefolds at Infinite Distance II: Asymptotic Physics - Rafael Álvarez-García, Seung-Joo Lee, Timo Weigand

  2. [2312.11622] - Mixmaster chaos in an AdS black hole interior - Marine De Clerck, Sean A. Hartnoll, Jorge E. Santos

  3. [2312.11634] - String Theory in Rolling Tachyon Vacuum - Ashoke Sen

Wed - 12/20/23

Analysis of improved dynamics of non-rotating charged black holes

  • Authors: Florencia Benitez, Esteban Mato, Javier Olmedo
  • Subjects: General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2312.11450
  • Abstract We provide the quantization of a charged black hole. We consider a redefinition of the scalar constraint in order to render the algebra of constraints as a Lie algebra. We apply loop quantum gravity techniques adhered to a novel improved dynamics scheme. We show that the model is solvable in closed form. We compute effective geometries, and show that the resulting effective space-times replace the inner horizon with a transition surface that connects trapped and antitrapped regions within the charged black hole interior. Quantum effects therefore stabilize the classical inner Cauchy horizons, as long as the charge is small compared to the mass. We further discuss the properties of these effective geometries by defining an effective stress-energy tensor out of the Einstein tensor, concluding that the null energy condition is violated.

There is no new related paper today

  1. [2312.10272] - Covariant canonical quantization and the problem of time - S. Carlip, Weixuan Hu

  2. [2312.10530] - Coloured combinatorial maps and quartic bi-tracial 2-matrix ensembles from noncommutative geometry - Masoud Khalkhali, Nathan Pagliaroli

Tue - 12/19/23

Reconstructing the metric in group field theory

  • Authors: Steffen Gielen, Lisa Mickel
  • 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/2312.10016
  • Abstract We study a group field theory (GFT) for quantum gravity coupled to four massless scalar fields, using these matter fields to define a (relational) coordinate system. We exploit symmetries of the GFT action, in particular under shifts in the values of the scalar fields, to derive a set of classically conserved currents, and show that the same conservation laws hold exactly at the quantum level regardless of the choice of state. We propose a natural interpretation of the conserved currents which implies that the matter fields always satisfy the Klein-Gordon equation in GFT. We then observe that in our matter reference frame, the same conserved currents can be used to extract all components of an effective GFT spacetime metric. Finally, we apply this construction to the simple example of a spatially flat homogeneous and isotropic universe. Our proposal goes substantially beyond the GFT literature in which only specific geometric quantities such as the total volume or volume perturbations could be defined, opening up the possibility to study more general geometries as emerging from GFT.

There is no new related paper today

Mon - 12/18/23

Reconstructing the metric in group field theory

  • Authors: Steffen Gielen, Lisa Mickel
  • 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/2312.10016
  • Abstract We study a group field theory (GFT) for quantum gravity coupled to four massless scalar fields, using these matter fields to define a (relational) coordinate system. We exploit symmetries of the GFT action, in particular under shifts in the values of the scalar fields, to derive a set of classically conserved currents, and show that the same conservation laws hold exactly at the quantum level regardless of the choice of state. We propose a natural interpretation of the conserved currents which implies that the matter fields always satisfy the Klein-Gordon equation in GFT. We then observe that in our matter reference frame, the same conserved currents can be used to extract all components of an effective GFT spacetime metric. Finally, we apply this construction to the simple example of a spatially flat homogeneous and isotropic universe. Our proposal goes substantially beyond the GFT literature in which only specific geometric quantities such as the total volume or volume perturbations could be defined, opening up the possibility to study more general geometries as emerging from GFT.

There is no new related paper today

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