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Symmetry
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Recent Advances in Quantum Gravity
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Recent Advances in Quantum Gravity

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (31 May 2021) | Viewed by 12242

Special Issue Editor

Prof. Dr. Roberto Casadio

E-Mail Website
Guest Editor
Department of Physics and Astronomy, Alma Mater Studiorum Università di Bologna, Bologna, Italy
Interests: theoretical physics; cosmology; generalised theories of gravity

Special Issue Information

Dear Colleagues,

This Special Issue is devoted to new and recent theoretical investigations aiming at understanding physical aspects of the fundamental gravitational interaction which should be relevant at the quantum level, both from top–down (mathematical methods and general theory to phenomenology) as well as bottom–up (studies of specific systems to general theory) approaches, and with applications to the astrophysics of very compact and exotic objects, black hole physics, and the issue of space-time singularities, gravitational wave production in black hole mergers, and cosmology of the early and present Universe.

Prof. Dr. Roberto Casadio
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Symmetry is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Black holes
  • Binary mergers
  • Compact astrophysical objects
  • Dark energy
  • Dark matter
  • Early cosmology
  • Effective field theory of gravity

Published Papers (6 papers)

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Research

8 pages, 260 KiB  
Article
Bootstrapped Newtonian Cosmology and the Cosmological Constant Problem
by Roberto Casadio and Andrea Giusti
Symmetry 2021, 13(2), 358; https://0-doi-org.brum.beds.ac.uk/10.3390/sym13020358 - 22 Feb 2021
Cited by 4 | Viewed by 1480
Abstract
Bootstrapped Newtonian gravity was developed with the purpose of estimating the impact of quantum physics in the nonlinear regime of the gravitational interaction, akin to corpuscular models of black holes and inflation. In this work, we set the ground for extending the bootstrapped [...] Read more.
Bootstrapped Newtonian gravity was developed with the purpose of estimating the impact of quantum physics in the nonlinear regime of the gravitational interaction, akin to corpuscular models of black holes and inflation. In this work, we set the ground for extending the bootstrapped Newtonian picture to cosmological spaces. We further discuss how such models of quantum cosmology can lead to a natural solution to the cosmological constant problem. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
13 pages, 425 KiB  
Article
A Conceptual Model for the Origin of the Cutoff Parameter in Exotic Compact Objects
by Wilson Alexander Rojas Castillo and Jose Robel Arenas Salazar
Symmetry 2020, 12(12), 2072; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12122072 - 14 Dec 2020
Cited by 1 | Viewed by 1847
Abstract
A Black Hole (BH) is a spacetime region with a horizon and where geodesics converge to a singularity. At such a point, the gravitational field equations fail. As an alternative to the problem of the singularity arises the existence of Exotic Compact Objects [...] Read more.
A Black Hole (BH) is a spacetime region with a horizon and where geodesics converge to a singularity. At such a point, the gravitational field equations fail. As an alternative to the problem of the singularity arises the existence of Exotic Compact Objects (ECOs) that prevent the problem of the singularity through a transition phase of matter once it has crossed the horizon. ECOs are characterized by a closeness parameter or cutoff, ϵ, which measures the degree of compactness of the object. This parameter is established as the difference between the radius of the ECO’s surface and the gravitational radius. Thus, different values of ϵ correspond to different types of ECOs. If ϵ is very big, the ECO behaves more like a star than a black hole. On the contrary, if ϵ tends to a very small value, the ECO behaves like a black hole. It is considered a conceptual model of the origin of the cutoff for ECOs, when a dust shell contracts gravitationally from an initial position to near the Schwarzschild radius. This allowed us to find that the cutoff makes two types of contributions: a classical one governed by General Relativity and one of a quantum nature, if the ECO is very close to the horizon, when estimating that the maximum entropy is contained within the material that composes the shell. Such entropy coincides with the Bekenstein–Hawking entropy. The established cutoff corresponds to a dynamic quantity dependent on coordinate time that is measured by a Fiducial Observer (FIDO). Without knowing the details about quantum gravity, parameter ϵ is calculated, which, in general, allows distinguishing the ECOs from BHs. Specifically, a black shell (ECO) is undistinguishable from a BH. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
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37 pages, 784 KiB  
Article
Glimpses on the Micro Black Hole Planck Phase
by Fabio Scardigli
Symmetry 2020, 12(9), 1519; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12091519 - 15 Sep 2020
Cited by 15 | Viewed by 2078
Abstract
Mass thresholds, lifetimes, entropy and heat capacity for micro black holes close to their late Schwarzschild phase are computed using two different generalized uncertainty principles, in the framework of models with extra spatial dimensions. Emissions of both photons and gravitons (in the bulk) [...] Read more.
Mass thresholds, lifetimes, entropy and heat capacity for micro black holes close to their late Schwarzschild phase are computed using two different generalized uncertainty principles, in the framework of models with extra spatial dimensions. Emissions of both photons and gravitons (in the bulk) are taken into account. Results are discussed and compared. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
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11 pages, 242 KiB  
Article
Long-Range Quantum Gravity
by Mariano Cadoni, Matteo Tuveri and Andrea P. Sanna
Symmetry 2020, 12(9), 1396; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12091396 - 21 Aug 2020
Cited by 14 | Viewed by 1663
Abstract
It is a tantalising possibility that quantum gravity (QG) states remaining coherent at astrophysical, galactic and cosmological scales could exist and that they could play a crucial role in understanding macroscopic gravitational effects. We explore, using only general principles of General Relativity, quantum [...] Read more.
It is a tantalising possibility that quantum gravity (QG) states remaining coherent at astrophysical, galactic and cosmological scales could exist and that they could play a crucial role in understanding macroscopic gravitational effects. We explore, using only general principles of General Relativity, quantum and statistical mechanics, the possibility of using long-range QG states to describe black holes. In particular, we discuss in a critical way the interplay between various aspects of long-range quantum gravity, such as the holographic bound, classical and quantum criticality and the recently proposed quantum thermal generalisation of Einstein’s equivalence principle. We also show how black hole thermodynamics can be easily explained in this framework. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
11 pages, 290 KiB  
Article
Unsettling Physics in the Quantum-Corrected Schwarzschild Black Hole
by Valerio Faraoni and Andrea Giusti
Symmetry 2020, 12(8), 1264; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12081264 - 01 Aug 2020
Cited by 23 | Viewed by 2348
Abstract
We study a quantum-corrected Schwarzschild black hole proposed recently in Loop Quantum Gravity. Prompted by the fact that corrections to the innermost stable circular orbit of Schwarzschild diverge, we investigate time-like and null radial geodesics. Massive particles moving radially outwards are confined, while [...] Read more.
We study a quantum-corrected Schwarzschild black hole proposed recently in Loop Quantum Gravity. Prompted by the fact that corrections to the innermost stable circular orbit of Schwarzschild diverge, we investigate time-like and null radial geodesics. Massive particles moving radially outwards are confined, while photons make it to infinity with infinite redshift. This unsettling physics, which deviates radically from both Schwarzschild (near the horizon) and Minkowski (at infinity) is due to repulsion by the negative quantum energy density that makes the quasilocal mass vanish as one approaches spatial infinity. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
13 pages, 451 KiB  
Article
MGD Dirac Stars
by Roldão da Rocha
Symmetry 2020, 12(4), 508; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12040508 - 01 Apr 2020
Cited by 57 | Viewed by 2012
Abstract
The method of geometric deformation (MGD) is here employed to study compact stellar configurations, which are solutions of the effective Einstein–Dirac coupled field equations on fluid branes. Non-linear, self-interacting, fermionic fields are then employed to derive MGD Dirac stars, whose properties are analyzed [...] Read more.
The method of geometric deformation (MGD) is here employed to study compact stellar configurations, which are solutions of the effective Einstein–Dirac coupled field equations on fluid branes. Non-linear, self-interacting, fermionic fields are then employed to derive MGD Dirac stars, whose properties are analyzed and discussed. The MGD Dirac star maximal mass is shown to increase as a specific function of the spinor self-interaction coupling constant, in a realistic model involving the most strict phenomenological current bounds for the brane tension. Full article
(This article belongs to the Special Issue Recent Advances in Quantum Gravity)
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