Special Issue "Cosmology and Extragalactic Astronomy"

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

Deadline for manuscript submissions: 31 May 2021.

Special Issue Editor

Prof. Dr. Vasileios Oikonomou
E-Mail Website
Guest Editor
Physics Department, Aristotle University of Thessaloniki, Thessaloniki 541 24, 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
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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 papers will be 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 1800 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 (4 papers)

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Research

Open AccessArticle
The d-Dimensional Cosmological Constant and the Holographic Horizons
Symmetry 2021, 13(2), 237; https://doi.org/10.3390/sym13020237 - 31 Jan 2021
Viewed by 360
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 D3 models. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
Open AccessArticle
Non-Extensive Thermodynamics Effects in the Cosmology of f(T) Gravity
Symmetry 2021, 13(1), 75; https://doi.org/10.3390/sym13010075 - 04 Jan 2021
Viewed by 346
Abstract
Using f(T) gravitational theory, we construct modified cosmological models via the first law of thermodynamics by using the non-extensive thermodynamics framework, the effects of which are captured by the parameter δ. The resulting cosmological equations are modified compared to the standard Einstein-Hilbert ones, with the modifications coming from the f(T) gravitational theory and from the non-extensive parameter which quantifies the non-extensive thermodynamics effects quantified by the parameter δ, which when is set equal to unity, one recovers the field equations of f(T) gravity. We study in detail the cosmological evolution of the model in the presence of collisionless non-relativistic matter case, and we derive the exact forms of the dark energy density parameter and of the dark energy equation of state parameter, from which we impose constraints on the non-extensive thermodynamics parameter, δ, by using the Planck 2018 data on cosmological parameters. Accordingly, we repeat our calculations after including the relativistic matter along with the non-relativistic one, and we derive the new forms of the dark energy density parameter and of the dark energy equation of state parameter. Our study shows that the inclusion of non-extensive thermodynamic effects, quantified by the parameter δ, for a flat Friedmann-Robertson-Walker Universe, has measurable differences compared with the normal thermodynamics case. We confront our results with Type Ia supernovae observations for z0.4 and we obtain reasonably agreement with the observational data. Full article
(This article belongs to the Special Issue Cosmology and Extragalactic Astronomy)
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Open AccessArticle
The Gross–Pitaevskii Equation with a Nonlocal Interaction in a Semiclassical Approximation on a Curve
Symmetry 2020, 12(2), 201; https://doi.org/10.3390/sym12020201 - 01 Feb 2020
Viewed by 613
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)
Open AccessArticle
Gravitational Instability Caused by the Weight of Heat
Symmetry 2019, 11(12), 1435; https://doi.org/10.3390/sym11121435 - 21 Nov 2019
Cited by 4 | Viewed by 638
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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