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Symmetry
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Symmetry in Gravity Theories and Cosmology
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Symmetry in Gravity Theories and Cosmology

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

Deadline for manuscript submissions: closed (31 October 2024) | Viewed by 10456

Special Issue Editors

Dr. Sunil Kumar Tripathy

E-Mail Website
Guest Editor
Associate Professor, Department of Physics, Indira Gandhi Institute of Technology, Sarang, Dhenkanal 759146, India
Interests: gravitation and cosmology; astrophysics, nuclear equation of state
Dr. Dipanjali Behera

E-Mail Website
Guest Editor
Department of Physics, Indira Gandhi Institute of Technology, Sarang, Dhenkanal 759146, Odisha, India
Interests: theoretical nuclear physics; cosmology
Dr. Hooman Moradpour

E-Mail Website
Guest Editor
Associate Professor, Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), University of Maragheh, Maragheh, Iran
Interests: gravity; cosmology; quantum mechanics; gravity analogue; astrophysics

Special Issue Information

Dear Colleagues,

The late-time cosmic speed-up phenomenon, as witnessed in recent observations, has triggered the development of new ideas and concepts. Einstein’s general relativity (GR) theory is able to explain such a phenomenon through the incorporation of additional dynamical degrees of freedom, such as quintessence, tachyons, phantom fields, etc., usually dubbed as dark energy candidates. However, geometrically modified theories of gravity can handle this issue without the need for any dark energy candidates. On the other hand, theoretical shortcomings and tensions between different cosmological observations have raised questions about GR, at least at the large energy scale. Additionally, questions about the symmetrical expansion of the universe have arisen. In this context, symmetry plays an important role in addressing many issues arising in the fields of cosmology and astrophysics. Usually, for a dynamical system, different symmetries, such Noether symmetry and the non-local conservation laws, help to simplify the system of equations, allowing physical systems to be studied in an analytic manner.

This Special Issue aims to present the role played by symmetries and conservation laws in addressing issues in gravitation and cosmology concerning recent research challenges.

Dr. Sunil Kumar Tripathy
Dr. Dipanjali Behera
Dr. Hooman Moradpour
Guest Editors

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

  • wormholes
  • Casimir wormholes
  • modified theories of gravity
  • bouncing cosmological models
  • observational constraints on modified gravity theories
  • dark energy models
  • Noether symmetry
  • scalar field cosmology
  • exact solutions

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Published Papers (6 papers)

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Research

Jump to: Review

19 pages, 2473 KiB  
Article
SU(∞) Quantum Gravity and Cosmology
by Houri Ziaeepour
Symmetry 2024, 16(12), 1672; https://doi.org/10.3390/sym16121672 - 17 Dec 2024
Cited by 1 | Viewed by 889
Abstract
In this letter, we highlight the structure and main properties of an abstract approach to quantum cosmology and gravity, dubbed SU(∞)-QGR. Beginning from the concept of the Universe as an isolated quantum system, the main axiom of the [...] Read more.
In this letter, we highlight the structure and main properties of an abstract approach to quantum cosmology and gravity, dubbed SU(∞)-QGR. Beginning from the concept of the Universe as an isolated quantum system, the main axiom of the model is the existence of an infinite number of mutually commuting observables. Consequently, the Hilbert space of the Universe represents SU(∞) symmetry. This Universe as a whole is static and topological. Nonetheless, quantum fluctuations induce local clustering in its quantum state and divide it into approximately isolated subsystems representing G×SU(∞), where G is a generic finite-rank internalsymmetry. Due to the global SU(∞) each subsystem is entangled to the rest of the Universe. In addition to parameters characterizing the representation of G, quantum states of subsystems depend on four continuous parameters: two of them characterize the representation of SU(∞), a dimensionful parameter arises from the possibility of comparing representations of SU(∞) by different subsystems, and the fourth parameter is a measurable used as time registered by an arbitrary subsystem chosen as a quantum clock. It introduces a relative dynamics for subsystems, formulated by a symmetry-invariant effective Lagrangian defined on the (3+1)D space of the continuous parameters. At lowest quantum order, the Lagrangian is a Yang–Mills field theory for both SU(∞) and internal symmetries. We identify the common SU(∞) symmetry and its interaction with gravity. Consequently, SU(∞)-QGR predicts a spin-1 mediator for quantum gravity (QGR). Apparently, this is in contradiction with classical gravity. Nonetheless, we show that an observer who is unable to detect the quantumness of gravity perceives its effect as curvature of the space of average values of the continuous parameters. We demonstrate Lorentzian geometry of this emergent classical spacetime. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
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10 pages, 398 KiB  
Article
Topological Classes of BTZ Black Holes
by Yongbin Du, Haida Li and Xiangdong Zhang
Symmetry 2024, 16(12), 1577; https://doi.org/10.3390/sym16121577 - 26 Nov 2024
Cited by 31 | Viewed by 807
Abstract
In a recent paper, black holes were viewed as topological thermodynamic defects using generalized off-shell free energy. The aforementioned work indicates that all black hole solutions in the pure Einstein–Maxwell gravity theory could be classified into three different topological classes for four and [...] Read more.
In a recent paper, black holes were viewed as topological thermodynamic defects using generalized off-shell free energy. The aforementioned work indicates that all black hole solutions in the pure Einstein–Maxwell gravity theory could be classified into three different topological classes for four and higher spacetime dimensions. In this paper, we investigate the topological number of BTZ black holes in distinct theories with different charges (Q) and rotational parameters (J). Using generalized free energy and Duan’s ϕ-mapping topological current theory, we found only two topological classes for BTZ spacetime. Particularly, for a BTZ black hole with rotation or in the Einstein–Power–Maxwell theory, there is only one zero point and the total topological number is 1. While for a BTZ black hole in new massive gravity, the global topological charge depends on the value of the specific parameter m, which provides a counter-example for the conjecture that the topological number is independent of the black hole’s parameters. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
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16 pages, 515 KiB  
Article
Evaporation of Primordial Charged Black Holes: Timescale and Evolution of Thermodynamic Parameters
by José Antonio de Freitas Pacheco
Symmetry 2024, 16(7), 895; https://doi.org/10.3390/sym16070895 - 13 Jul 2024
Cited by 1 | Viewed by 1035
Abstract
The evolution of primordial black holes formed during the reheating phase is revisited. For reheating temperatures in the range of 1012–1013 GeV, the initial masses are respectively of the order of 1010–108MP, [...] Read more.
The evolution of primordial black holes formed during the reheating phase is revisited. For reheating temperatures in the range of 1012–1013 GeV, the initial masses are respectively of the order of 1010–108MP, where MP is the Planck mass. These newborn black holes have a small charge-to-mass ratio of the order of 10−3, a consequence of statistical fluctuations present in the plasma constituting the collapsing matter. Charged black holes can be rapidly discharged by the Schwinger mechanism, but one expects that, for very light black holes satisfying the condition M/MP<<MP/mW (mW is the mass of the heaviest standard model charged W-boson), the pair production process is probably strongly quenched. Under these conditions, these black holes evaporate until attaining extremality with final masses of about 107–105MP. Timescales to reach extremality as a function of the initial charge excess were computed, as well as the evolution of the horizon temperature and the charge-to-mass ratio. The behavior of the horizon temperature can be understood in terms of the well-known discontinuity present in the heat capacity for a critical charge-to-mass ratio Q/GM=3/2. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
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16 pages, 303 KiB  
Article
Extension of Buchdahl’s Theorem on Reciprocal Solutions
by David S. Pereira, José Pedro Mimoso and Francisco S. N. Lobo
Symmetry 2024, 16(7), 881; https://doi.org/10.3390/sym16070881 - 11 Jul 2024
Cited by 1 | Viewed by 1372
Abstract
Since the development of Brans–Dicke gravity, it has become well-known that a conformal transformation of the metric can reformulate this theory, transferring the coupling of the scalar field from the Ricci scalar to the matter sector. Specifically, in this new frame, known as [...] Read more.
Since the development of Brans–Dicke gravity, it has become well-known that a conformal transformation of the metric can reformulate this theory, transferring the coupling of the scalar field from the Ricci scalar to the matter sector. Specifically, in this new frame, known as the Einstein frame, Brans–Dicke gravity is reformulated as General Relativity supplemented by an additional scalar field. In 1959, Hans Adolf Buchdahl utilized an elegant technique to derive a set of solutions for the vacuum field equations within this gravitational framework. In this paper, we extend Buchdahl’s method to incorporate the cosmological constant and to the scalar-tensor cases beyond the Brans–Dicke archetypal theory, thereby, with a conformal transformation of the metric, obtaining solutions for a version of Brans–Dicke theory that includes a quadratic potential. More specifically, we obtain synchronous solutions in the following contexts: in scalar-tensor gravity with massless scalar fields, Brans–Dicke theory with a quadratic potential, where we obtain specific synchronous metrics to the Schwarzschild–de Sitter metric, the Nariai solution, and a hyperbolically foliated solution. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
22 pages, 330 KiB  
Article
The Post-Quasi-Static Approximation: An Analytical Approach to Gravitational Collapse
by Luis Herrera, Alicia Di Prisco and Justo Ospino
Symmetry 2024, 16(3), 341; https://doi.org/10.3390/sym16030341 - 12 Mar 2024
Cited by 4 | Viewed by 1146
Abstract
A seminumerical approach proposed many years ago for describing gravitational collapse in the post-quasi-static approximation is modified in order to avoid the numerical integration of the basic differential equations the approach is based upon. For doing that we have to impose some restrictions [...] Read more.
A seminumerical approach proposed many years ago for describing gravitational collapse in the post-quasi-static approximation is modified in order to avoid the numerical integration of the basic differential equations the approach is based upon. For doing that we have to impose some restrictions on the fluid distribution. More specifically, we shall assume the vanishing complexity factor condition, which allows for analytical integration of the pertinent differential equations and leads to physically interesting models. Instead, we show that neither the homologous nor the quasi-homologous evolution are acceptable since they lead to geodesic fluids, which are unsuitable for being described in the post-quasi-static approximation. Also, we prove that, within this approximation, adiabatic evolution also leads to geodesic fluids, and therefore, we shall consider exclusively dissipative systems. Besides the vanishing complexity factor condition, additional information is required for a full description of models. We shall propose different strategies for obtaining such an information, which are based on observables quantities (e.g., luminosity and redshift), and/or heuristic mathematical ansatz. To illustrate the method, we present two models. One model is inspired in the well-known Schwarzschild interior solution, and another one is inspired in Tolman VI solution. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)

Review

Jump to: Research

29 pages, 429 KiB  
Review
A Review of Stable, Traversable Wormholes in f(R) Gravity Theories
by Ramesh Radhakrishnan, Patrick Brown, Jacob Matulevich, Eric Davis, Delaram Mirfendereski and Gerald Cleaver
Symmetry 2024, 16(8), 1007; https://doi.org/10.3390/sym16081007 - 7 Aug 2024
Cited by 6 | Viewed by 4397
Abstract
It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in [...] Read more.
It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in any quantum field. Even reasonable classical scalar fields violate the energy conditions. In the case of the Casimir effect and squeezed vacuum states, these violations have been experimentally proven. It is advantageous to investigate methods to minimize the use of exotic matter. One such area of interest is extended theories of Einstein gravity. It has been claimed that in some extended theories, stable traversable wormholes solutions can be found without the use of exotic matter. There are many extended theories of gravity, and in this review paper, we first explore f(R) theories and then explore some wormhole solutions in f(R) theories, including Lovelock gravity and Einstein Dilaton Gauss–Bonnet (EdGB) gravity. For completeness, we have also reviewed ‘Other wormholes’ such as Casimir wormholes, dark matter halo wormholes, thin-shell wormholes, and Nonlocal Gravity (NLG) wormholes, where alternative techniques are used to either avoid or reduce the amount of exotic matter that is required. Full article
(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)
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