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Symmetry
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Cosmology and Extragalactic Astronomy
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Cosmology and Extragalactic Astronomy

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

Deadline for manuscript submissions: closed (31 May 2022) | Viewed by 10193

Special Issue Editor

Prof. Dr. Vasilis K. Oikonomou

E-Mail Website
Guest Editor
Physics Department, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
Interests: cosmology; inflationary cosmology; modified theories of gravity; physics of the early universe; dark energy; dark matter; supersymmetry; mathematical physics; high energy physics; theoretical physics; epistemic game theory; game theory
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The striking observational data coming from the Planck mission and from the LIGO-VIRGO experiments have stimulated theoretical astrophysicists and cosmologists to focus more on the realistic models that describe our Universe on a local and global scale. The theoretical tools available offer a fertile ground for model building, and the quest is to find a viable description of our Universe. For this Special Issue, we invite specialists that are active in the fields of theoretical cosmology, theoretical astrophysics, and galactic-extra-galactic astronomy to contribute their latest insights in the form of a review or original research paper, in order to provide up to date theoretical presentations of realist models of our Universe, at both large and local scales.

Prof. Dr. Vasileios Oikonomou
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

  • modified gravity
  • cosmology
  • theoretical astrophysics
  • extragalactic astronomy
  • astronomy

Published Papers (5 papers)

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Research

26 pages, 883 KiB  
Article
Anisotropic Compact Stars in D → 4 Limit of Gauss–Bonnet Gravity
by Gamal G. L. Nashed, Sergei D. Odintsov and Vasillis K. Oikonomou
Symmetry 2022, 14(3), 545; https://doi.org/10.3390/sym14030545 - 07 Mar 2022
Cited by 14 | Viewed by 1638
Abstract
In the frame of Gauss–Bonnet gravity and in the limit of D→4, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory, we derive a new interior [...] Read more.
In the frame of Gauss–Bonnet gravity and in the limit of D→4, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory, we derive a new interior spherically symmetric solution assuming specific forms of the metric potentials that have two constants. Using the junction condition we determine these two constants. By using the data of the star EXO 1785-248, whose mass is M=1.3±0.2M⊙ and radius l=8.849±0.4 km, we calculate the numerical values of these constants, in terms of the dimensionful coupling parameter of the Gauss–Bonnet term, and eventually, we get real values for these constants. In this regard, we show that the components of the energy–momentum tensor have a finite value at the center of the star as well as a smaller value to the surface of the star. Moreover, we show that the equations of the state behave in a non-linear way due to the impact of the Gauss–Bonnet term. Using the Tolman–Oppenheimer–Volkoff equation, the adiabatic index, and stability in the static state we show that the model under consideration is always stable. Finally, the solution of this study is matched with observational data of other pulsars showing satisfactory results. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
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15 pages, 1017 KiB  
Article
Geodesics of a Static Charged Black Hole Spacetime in f(R) Gravity
by Prateek Sharma, Hemwati Nandan, Gamal G. L. Nashed, Shobhit Giri and Amare Abebe
Symmetry 2022, 14(2), 309; https://doi.org/10.3390/sym14020309 - 03 Feb 2022
Cited by 3 | Viewed by 1448
Abstract
In recent years, the modification of general relativity (GR) through f(R) gravity is widely used to study gravity in a variety of scenarios. In this article, we study various physical properties of a black hole (BH) that emerged in the [...] Read more.
In recent years, the modification of general relativity (GR) through f(R) gravity is widely used to study gravity in a variety of scenarios. In this article, we study various physical properties of a black hole (BH) that emerged in the linear Maxwell f(R) gravity to constrain the values of different BH parameters, i.e., c and α. In particular, we study those values of the defining α and c for which the particles around the above-mentioned BH behave like other astrophysical BH in GR. The main motivation of the present research is to study the geodesics equations and discuss the possible orbits for c=0.5 in detail. Furthermore, the frequency shift of a photon emitted by a timelike particle orbiting around the BH is studied given different values of α and c. The stability of both timelike and null geodesics is discussed via Lyapunov’s exponent. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
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14 pages, 315 KiB  
Article
The d-Dimensional Cosmological Constant and the Holographic Horizons
by Artyom V. Yurov and Valerian A. Yurov
Symmetry 2021, 13(2), 237; https://doi.org/10.3390/sym13020237 - 31 Jan 2021
Cited by 1 | Viewed by 1586
Abstract
This article is dedicated to establishing a novel approach to the cosmological constant, in which it is treated as an eigenvalue of a certain Sturm–Liouville problem. The key to this approach lies in the proper formulation of physically relevant boundary conditions. Our suggestion [...] Read more.
This article is dedicated to establishing a novel approach to the cosmological constant, in which it is treated as an eigenvalue of a certain Sturm–Liouville problem. The key to this approach lies in the proper formulation of physically relevant boundary conditions. Our suggestion in this regard is to utilize the “holographic boundary condition”, under which the cosmological horizon can only bear a natural (i.e., non-fractional) number of bits of information. Under this framework, we study the general d-dimensional problem and derive the general formula for the discrete spectrum of a positive energy density of vacuum. For the particular case of two dimensions, the resultant problem can be analytically solved in the degenerate hypergeometric functions, so it is possible to define explicitly a self-action potential, which determines the fields of matter in the model. We conclude the article by taking a look at the d-dimensional model of a fractal horizon, where the Bekenstein’s formula for the entropy gets replaced by the Barrow entropy. This gives us a chance to discuss a recently realized problem of possible existence of naked singularities in the D≠3 models. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
25 pages, 420 KiB  
Article
The Gross–Pitaevskii Equation with a Nonlocal Interaction in a Semiclassical Approximation on a Curve
by Alexander V. Shapovalov, Anton E. Kulagin and Andrey Yu. Trifonov
Symmetry 2020, 12(2), 201; https://doi.org/10.3390/sym12020201 - 01 Feb 2020
Cited by 6 | Viewed by 2423
Abstract
We propose an approach to constructing semiclassical solutions for the generalized multidimensional Gross–Pitaevskii equation with a nonlocal interaction term. The key property of the solutions is that they are concentrated on a one-dimensional manifold (curve) that evolves over time. The approach reduces the [...] Read more.
We propose an approach to constructing semiclassical solutions for the generalized multidimensional Gross–Pitaevskii equation with a nonlocal interaction term. The key property of the solutions is that they are concentrated on a one-dimensional manifold (curve) that evolves over time. The approach reduces the Cauchy problem for the nonlocal Gross–Pitaevskii equation to a similar problem for the associated linear equation. The geometric properties of the resulting solutions are related to Maslov’s complex germ, and the symmetry operators of the associated linear equation lead to the approximation of the symmetry operators for the nonlocal Gross–Pitaevskii equation. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
12 pages, 697 KiB  
Article
Gravitational Instability Caused by the Weight of Heat
by Zacharias Roupas
Symmetry 2019, 11(12), 1435; https://doi.org/10.3390/sym11121435 - 21 Nov 2019
Cited by 10 | Viewed by 2034
Abstract
Thermal energy points towards a disordered, completely uniform state act to counter gravity’s tendency to generate order and structure through gravitational collapse. It is, therefore, expected to contribute to the stabilization of a self-gravitating, classical ideal gas over collapse. However, I identified an [...] Read more.
Thermal energy points towards a disordered, completely uniform state act to counter gravity’s tendency to generate order and structure through gravitational collapse. It is, therefore, expected to contribute to the stabilization of a self-gravitating, classical ideal gas over collapse. However, I identified an instability that always occurs at sufficiently high energies: the high-energy or relativistic gravothermal instability. I argue here that this instability presents an analogous core–halo structure as its Newtonian counterpart, the Antonov instability. The main difference is that in the former case the core is dominated by the gravitation of thermal energy and not rest mass energy. A relativistic generalization of Antonov’s instability—the low-energy gravothermal instability—also occurs. The two turning points, which make themselves evident as a double spiral of the caloric curve, approach each other as relativistic effects become more intense and eventually merge in a single point. Thus, the high and low-energy cases may be realized as two aspects of a single phenomenon—the gravothermal instability—which involves a core–halo separation and an intrinsic heat flow. Finally, I argue that the core formed during a core-collapse supernova is subject to the relativistic gravothermal instability if it becomes sufficiently hot and compactified at the time of the bounce. In this case, it will continue to collapse towards the formation of a black hole. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
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