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

Deadline for manuscript submissions: closed (31 August 2021) | Viewed by 9544

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Prof. Dr. Artyom Astashenok

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Mathematics and IT, Institute of Physics, Immanuel Kant Baltic Federal University, Nevskogo str., 14, 236041 Kaliningrad, Russia
Interests: electrodynamics; vector and tensor analysis; philosophical problems of modern science and thechnology; physics of fundamental interactions; quantum field theory in mesophysics

Special Issue Information

Dear colleagues,

Cosmology is nowadays one of the most rapidly developing areas of theoretical physics. The discovery of the accelerated expansion of the universe in 1998 led to a number of new ideas and cosmological models to describe this phenomenon. Various models of so-called dark energy with negative pressure have been proposed. An alternative explanation of accelerated expansion can be obtained within so-called modified gravity, including f(R) theories, Gauss–Bonnet gravity, scalar-tensor theories, etc. The nature of the proposed fields and theories of gravity is unclear from the physical point of view. Its explanation often requires quantum field theory and quantum gravity. Therefore modern cosmology is closely related to microphysics.

Another branch of investigation is describing the early expansion the of universe in terms of the inflation paradigm. On the basis of quantum field theory, dozens of inflation models have been proposed. However, the recent PLANCK observations allow us to restrict the values of the parameters of these models.

Tremendous technical progress in the field of astronomical observations has allowed us to better understand our Universe. But many puzzles remain unsolved from a fundamental theoretical point of view, with explanations to be found at the phenomenological level.

The purpose of the present Special Issue, entitled "Cosmology", is the presentation of new fundamental ideas and methods of describing the observable universe and various models for early inflation, late acceleration and another topics in modern cosmology including possible interdisciplinary questions.

Prof. Dr. Artyom Astashenok
Guest Editor

Manuscript Submission Information

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Keywords

Dark Energy
Modified Gravity
Inflation

Published Papers (5 papers)

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Research

13 pages, 10120 KiB

Open AccessArticle

Geodesic Structure of the Accelerated Stephani Universe

by Irina Bormotova, Elena Kopteva and Zdeněk Stuchlík

Symmetry 2021, 13(6), 1001; https://0-doi-org.brum.beds.ac.uk/10.3390/sym13061001 - 3 Jun 2021

Viewed by 1600

Abstract

For the spherically symmetric Stephani cosmological model with an accelerated expansion, we investigate the main scenarios of the test particle and photon motion. We show that a comoving observer sees an appropriate picture. In the case of purely radial motion, the radial velocity decreases slightly with time due to the universe expansion. Both particles and photons spiral out of the center when the radial coordinate is constant. In the case of the motion with arbitrary initial velocity, the observable radial distance to the test particle can increase under negative observable radial velocity. Full article

(This article belongs to the Special Issue Cosmology)

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8 pages, 242 KiB

Open AccessFeature PaperArticle

Particles with Negative Energies in Nonrelativistic and Relativistic Cases

by Andrey A. Grib and Yuri V. Pavlov

Symmetry 2020, 12(4), 528; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12040528 - 3 Apr 2020

Cited by 4 | Viewed by 1783

Abstract

States of particles with negative energies are considered for the nonrelativistic and relativistic cases. In the nonrelativistic case it is shown that the decay close to the attracting center can lead to the situation similar to the Penrose effect for a rotating black hole when the energy of one of the fragments is larger than the energy of the initial body. This is known as the Oberth effect in the theory of the rocket movement. The realizations of the Penrose effect in the non-relativistic case in collisions near the attracting body and in the evaporation of stars from star clusters are indicated. In the relativistic case similar to the well known Penrose process in the ergosphere of the rotating black hole it is shown that the same situation as in ergosphere of the black hole occurs in rotating coordinate system in Minkowski space-time out of the static limit due to existence of negative energies. In relativistic cases differently from the nonrelativistic ones, the mass of the fragment can be larger than the mass of the decaying body. Negative energies for particles are possible in the relativistic case in cosmology of the expanding space when the coordinate system is used with a nondiagonal term in metrical tensor of the space-time. Friedmann metrics for three cases: open, close and quasieuclidian, are analyzed. The De Sitter space-time is shortly discussed. Full article

(This article belongs to the Special Issue Cosmology)

14 pages, 2668 KiB

Open AccessArticle

Fingerprints of the Cosmological Constant: Folds in the Profiles of the Axionic Dark Matter Distribution in a Dyon Exterior

by Alexander Balakin and Dmitry Groshev

Symmetry 2020, 12(3), 455; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12030455 - 13 Mar 2020

Cited by 5 | Viewed by 1929

Abstract

We consider the magnetic monopole in the axionic dark matter environment (axionic dyon) in the framework of the Reissner-Nordström-de Sitter model. Our aim is to study the distribution of the pseudoscalar (axion) and electric fields near the so-called folds, which are characterized by the profiles with the central minimum, the barrier on the left, and the maximum on the right of this minimum. The electric field in the fold-like zones is shown to change the sign twice, i.e., the electric structure of the near zone of the axionic dyon contains the domain similar to a double electric layer. We have shown that the described fold-like structures in the profile of the gravitational potential, and in the profiles of the electric and axion fields can exist, when the value of the dyon mass belongs to the interval enclosed between two critical masses, which depend on the cosmological constant. Full article

(This article belongs to the Special Issue Cosmology)

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24 pages, 513 KiB

Open AccessArticle

Exponential Cosmological Solutions with Three Different Hubble-Like Parameters in (1 + 3 + k₁ + k₂)-Dimensional EGB Model with a Λ-Term

by K. K. Ernazarov and V. D. Ivashchuk

Symmetry 2020, 12(2), 250; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12020250 - 6 Feb 2020

Cited by 3 | Viewed by 1715

Abstract

A D-dimensional Einstein–Gauss–Bonnet model with a cosmological term

Λ

, governed by two non-zero constants:

α_{1}

and

α_{2}

, is considered. By restricting the metrics to diagonal ones, we study a class of solutions with the exponential time dependence of [...] Read more.

A D-dimensional Einstein–Gauss–Bonnet model with a cosmological term

Λ

, governed by two non-zero constants:

α_{1}

and

α_{2}

, is considered. By restricting the metrics to diagonal ones, we study a class of solutions with the exponential time dependence of three scale factors, governed by three non-coinciding Hubble-like parameters:

H > 0

h_{1}

, and

h_{2}

, obeying

3 H + k_{1} h_{1} + k_{2} h_{2} \neq 0

and corresponding to factor spaces of dimensions: 3,

k_{1} > 1

, and

k_{2} > 1

, respectively, with

D = 4 + k_{1} + k_{2}

. The internal flat factor spaces of dimensions

k_{1}

and

k_{2}

have non-trivial symmetry groups, which depend on the number of compactified dimensions. Two cases: (i)

3 < k_{1} < k_{2}

and (ii)

1 < k_{1} = k_{2} = k

k \neq 3

, are analyzed. It is shown that in both cases, the solutions exist if

α = α_{2} / α_{1} > 0

and

α Λ > 0

obey certain restrictions, e.g., upper and lower bounds. In Case (ii), explicit relations for exact solutions are found. In both cases, the subclasses of stable and non-stable solutions are singled out. Case (i) contains a subclass of solutions describing an exponential expansion of

3 d

subspace with Hubble parameter

H > 0

and zero variation of the effective gravitational constant G. Full article

(This article belongs to the Special Issue Cosmology)

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15 pages, 283 KiB

Open AccessArticle

Modifications of Gravity Via Differential Transformations of Field Variables

by Anton Sheykin, Dmitry Solovyev, Vladimir Sukhanov and Sergey Paston

Symmetry 2020, 12(2), 240; https://0-doi-org.brum.beds.ac.uk/10.3390/sym12020240 - 5 Feb 2020

Cited by 11 | Viewed by 1733

Abstract

We discuss field theories appearing as a result of applying field transformations with derivatives (differential field transformations, DFTs) to a known theory. We begin with some simple examples of DFTs to see the basic properties of the procedure. In this process, the dynamics of the theory might either change or be conserved. After that, we concentrate on the theories of gravity which appear as a result of various DFTs applied to general relativity, namely the mimetic gravity and Regge–Teitelboim embedding theory. We review the main results related to the extension of dynamics in these theories, as well as the possibility to write down the action of a theory after DFTs as the action of the original theory before DFTs plus an additional term. Such a term usually contains some constraints with Lagrange multipliers and can be interpreted as an action of additional matter, which might be of use in cosmological applications, e.g., for the explanation of the effects of dark matter. Full article

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