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Symmetry
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Relativistic Gravity, Cosmology and Physics of Compact Stars
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Relativistic Gravity, Cosmology and Physics of Compact Stars

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

Deadline for manuscript submissions: closed (30 January 2022) | Viewed by 8546

Special Issue Editor

Prof. Dr. Aram Saharian

E-Mail Website
Guest Editor
Department of Physics, Yerevan State University, Alex Manoogian Street, 0025 Yerevan, Armenia
Interests: quantum effects in external fields; the Casimir effect; string effective gravity and cosmology; alternative theories of gravity
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The proposed Special Issue will cover actively pursued subjects of modern relativistic astrophysics and cosmology. It will include articles touching on the most diverse aspects of the field, such as modern problems in Einstein’s classical theory of gravity and its alternatives, string theory-motivated cosmological models, quantum field theory in a curved background, quantum effects in cosmological models with nontrivial topology, quantum effects in a braneworld scenario, the properties of superdense matter and the relativistic structure of compact stellar objects, the vast diversity of the manifestations of compact stars, including the modern aspects of the equation of state of superdense matter, its magnetic and thermal properties, rotational dynamics, superfluidity and superconductivity, and phase transition from hadronic to quark matter.

Prof. Dr. Aram Saharian
Guest Editor

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Keywords

  • relativistic gravity
  • cosmology
  • compact stars
  • quantum effects in curved space–time

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Published Papers (5 papers)

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Research

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26 pages, 659 KiB  
Article
Topological Effects in a Fermionic Condensate Induced by a Cosmic String and Compactification on the AdS Bulk
by Stefano Bellucci, Wagner Oliveira dos Santos, Eugenio R. Bezerra de Mello and Aram A. Saharian
Symmetry 2022, 14(3), 584; https://doi.org/10.3390/sym14030584 - 16 Mar 2022
Cited by 6 | Viewed by 1785
Abstract
In this paper, we analyzed the fermionic condensate (FC) associated with a massive fermionic field on a five-dimensional anti-de Sitter (AdS) spacetime in the presence of a cosmic string taking into account a magnetic flux running along its core. In addition, a compactified [...] Read more.
In this paper, we analyzed the fermionic condensate (FC) associated with a massive fermionic field on a five-dimensional anti-de Sitter (AdS) spacetime in the presence of a cosmic string taking into account a magnetic flux running along its core. In addition, a compactified dimension was considered. Due to this compactification, the FC is expressed in terms of two distinct contributions: the first one corresponds to the geometry without compactification, and the second one is induced by the compactification. Depending on the values of the physical parameters, the total FC can be positive or negative. As a limiting case, the expression for the FC on locally Minkowski spacetime was derived. It vanishes for a massless fermionic field, and the nonzero FC on the AdS background space in the massless case is an effect induced by gravitation. This shows that the gravitational field may essentially influence the parameter space for phase transitions. For a massive field, the FC diverges on the string as the inverse cube of the proper distance from the string. In the case of a massless field, depending on the magnetic flux along the string and planar angle deficit, the limiting value of the FC on the string can be either finite or infinite. At large distances, the decay of the FC as a function of the distance from the string is a power law for both cases of massive and massless fields. For a cosmic string on the Minkowski bulk and for a massive field, the decay is exponential. The topological part in the FC vanishes on the AdS boundary. We show that the FCs coincide for the fields realizing two inequivalent irreducible representations of the Clifford algebra. In the special case of the zero planar angle deficit, the results presented in this paper describe Aharonov–Bohm-type effects induced by magnetic fluxes in curved spacetime. Full article
(This article belongs to the Special Issue Relativistic Gravity, Cosmology and Physics of Compact Stars)
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16 pages, 2012 KiB  
Article
Hadron–Quark Phase Transition in the SU (3) Local Nambu–Jona-Lasinio (NJL) Model with Vector Interaction
by Grigor Alaverdyan
Symmetry 2021, 13(1), 124; https://doi.org/10.3390/sym13010124 - 13 Jan 2021
Cited by 14 | Viewed by 2985
Abstract
We study the hadron–quark hybrid equation of state (EOS) of compact-star matter. The Nambu–Jona-Lasinio (NJL) local SU (3) model with vector-type interaction is used to describe the quark matter phase, while the relativistic mean field (RMF) theory with the scalar-isovector δ-meson effective [...] Read more.
We study the hadron–quark hybrid equation of state (EOS) of compact-star matter. The Nambu–Jona-Lasinio (NJL) local SU (3) model with vector-type interaction is used to describe the quark matter phase, while the relativistic mean field (RMF) theory with the scalar-isovector δ-meson effective field is adopted to describe the hadronic matter phase. It is shown that the larger the vector coupling constant GV, the lower the threshold density for the appearance of strange quarks. For a sufficiently small value of the vector coupling constant, the functions of the mass dependence on the baryonic chemical potential have regions of ambiguity that lead to a phase transition in nonstrange quark matter with an abrupt change in the baryon number density. We show that within the framework of the NJL model, the hypothesis on the absolute stability of strange quark matter is not realized. In order to describe the phase transition from hadronic matter to quark matter, Maxwell’s construction is applied. It is shown that the greater the vector coupling, the greater the stiffness of the EOS for quark matter and the phase transition pressure. Our results indicate that the infinitesimal core of the quark phase, formed in the center of the neutron star, is stable. Full article
(This article belongs to the Special Issue Relativistic Gravity, Cosmology and Physics of Compact Stars)
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16 pages, 327 KiB  
Article
Rotating Melvin-like Universes and Wormholes in General Relativity
by Kirill A. Bronnikov, Vladimir G. Krechet and Vadim B. Oshurko
Symmetry 2020, 12(8), 1306; https://doi.org/10.3390/sym12081306 - 5 Aug 2020
Cited by 17 | Viewed by 2261
Abstract
We find a family of exact solutions to the Einstein–Maxwell equations for rotating cylindrically symmetric distributions of a perfect fluid with the equation of state p=wρ (|w|<1), carrying a circular electric current in the [...] Read more.
We find a family of exact solutions to the Einstein–Maxwell equations for rotating cylindrically symmetric distributions of a perfect fluid with the equation of state p=wρ (|w|<1), carrying a circular electric current in the angular direction. This current creates a magnetic field along the z axis. Some of the solutions describe geometries resembling that of Melvin’s static magnetic universe and contain a regular symmetry axis, while some others (in the case w>0) describe traversable wormhole geometries which do not contain a symmetry axis. Unlike Melvin’s solution, those with rotation and a magnetic field cannot be vacuum and require a current. The wormhole solutions admit matching with flat-space regions on both sides of the throat, thus forming a cylindrical wormhole configuration potentially visible for distant observers residing in flat or weakly curved parts of space. The thin shells, located at junctions between the inner (wormhole) and outer (flat) regions, consist of matter satisfying the Weak Energy Condition under a proper choice of the free parameters of the model, which thus forms new examples of phantom-free wormhole models in general relativity. In the limit w→1, the magnetic field tends to zero, and the wormhole model tends to the one obtained previously, where the source of gravity is stiff matter with the equation of state p=ρ. Full article
(This article belongs to the Special Issue Relativistic Gravity, Cosmology and Physics of Compact Stars)
30 pages, 681 KiB  
Article
The Casimir Densities for a Sphere in the Milne Universe
by Aram A. Saharian and Tigran A. Petrosyan
Symmetry 2020, 12(4), 619; https://doi.org/10.3390/sym12040619 - 14 Apr 2020
Cited by 5 | Viewed by 1977
Abstract
The influence of a spherical boundary on the vacuum fluctuations of a massive scalar field is investigated in the background of a ( D + 1 ) -dimensional Milne universe, assuming that the field obeys Robin boundary conditions on the sphere. The normalized [...] Read more.
The influence of a spherical boundary on the vacuum fluctuations of a massive scalar field is investigated in the background of a ( D + 1 ) -dimensional Milne universe, assuming that the field obeys Robin boundary conditions on the sphere. The normalized mode functions are derived for the regions inside and outside the sphere and different vacuum states are discussed. For the conformal vacuum, the Hadamard function is decomposed into boundary-free and sphere-induced contributions and an integral representation is obtained for the latter in both the interior and exterior regions. As important local characteristics of the vacuum state, the vacuum expectation values (VEVs) of the field squared and of the energy-momentum tensor are investigated. It is shown that the vacuum energy-momentum tensor has an off-diagonal component that corresponds to the energy flux along the radial direction. Depending on the coefficient in Robin boundary conditions, the sphere-induced contribution to the vacuum energy and the energy flux can be either positive or negative. At late stages of the expansion and for a massive field the decay of the sphere-induced VEVs, as functions of time, is damping oscillatory. The geometry under consideration is conformally related to that for a static spacetime with negative constant curvature space and the sphere-induced contributions in the corresponding VEVs are compared. Full article
(This article belongs to the Special Issue Relativistic Gravity, Cosmology and Physics of Compact Stars)
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11 pages, 1436 KiB  
Conference Report
Transport Properties in Dense QCD Matter
by Toshitaka Tatsumi and Hiroaki Abuki
Symmetry 2020, 12(3), 366; https://doi.org/10.3390/sym12030366 - 2 Mar 2020
Cited by 1 | Viewed by 2084
Abstract
Transport properties of dense QCD matter are discussed. Using the Kubo formula for conductivity, we discuss some topological aspects of quark matter during the chiral transition. The close relation to Weyl semimetal is pointed out and anomalous Hall effect is demonstrated to be [...] Read more.
Transport properties of dense QCD matter are discussed. Using the Kubo formula for conductivity, we discuss some topological aspects of quark matter during the chiral transition. The close relation to Weyl semimetal is pointed out and anomalous Hall effect is demonstrated to be possible. In particular, it is shown that the spectral asymmetry of the quasi-particles plays an important role for the Hall conductivity in the magnetic field. Full article
(This article belongs to the Special Issue Relativistic Gravity, Cosmology and Physics of Compact Stars)
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