Cosmological Models of the Universe

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Cosmology".

Deadline for manuscript submissions: 1 April 2025 | Viewed by 5390

Special Issue Editors


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Guest Editor
Department of Mathematics, National and Kapodistrian University of Athens, Athens, Greece
Interests: modified gravity; cosmology; gravitational waves; Finsler cosmology; extended Friedmann equations; dark matter; dark energy
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Guest Editor
1. National Observatory of Athens, Lofos Nymfon, 11852 Athens, Greece
2. CAS Key Laboratory for Researches in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
3. Departamento de Matemáticas, Universidad Católica del Norte, Avda. Angamos 0610, Casilla, Antofagasta 1280, Chile
Interests: ark energy formulation; modified theories of gravity; inflationary cosmology; brane cosmology; observational cosmology
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Cosmological models are described by mathematical formulas that attempt to explain the current behavior and evolution of the universe over time. They are based on observations making predictions that can be effective by resultant investigations and rely on general relativity because that theory describes the best way that the large-scale behavior of spacetime can be observed. Cosmological models extend the form of general relativity through various methods, leading to different field equations and thus to different cosmological implications. They play an essential role and contribute to modern cosmology, providing a foundation for the current understanding of the physical phenomena of the universe. A fundamental category of cosmological models is inflationary models which provide potential solutions to the fundamental problems of standard cosmological models on Big Bang cosmology, i.e., the Flatness and Horizon problem. Additionally, these models sufficiently describe energy density fluctuation mechanisms that investigate the large-scale structure of the universe. The measurement of CMB polarization and its anisotropies has a great significance to early universe physics, and the mechanisms of matter creation generate primordial metric fluctuations that affect the geometry of the universe (e.g., inflationary models). The abovementioned cosmological model provides observational tests which explain the origin of light and the existence of the cosmic microwave background and establishes the fact that the universe has a thermal history. On the other hand, the study of the large-scale structure of the universe and observational evidence demands the existence of dark matter and a nonzero cosmological constant; investigations in this direction introduce a model known as the ΛCDM model. As well, anisotropic cosmological models have been developed when the underline geometry of spacetime has a more generalized metric structure than the Riemannian one. Topics of interest for this Special Issue include, but are not limited to, the following: Standard cosmological models; Inflationary cosmological models; ΛCDM cosmological model; Anisotropic cosmological models; Finsler cosmology.

Prof. Dr. Panayiotis Stavrinos
Prof. Dr. Emmanuel N. Saridakis
Guest Editors

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Keywords

  • standard cosmological model
  • dark matter
  • dark energy
  • bounce cosmology
  • inflation cosmology
  • ΛCDM-cosmology
  • anisotropic cosmology

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Published Papers (7 papers)

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Research

10 pages, 289 KiB  
Article
Universes Emerging from Nothing and Disappearing into Nothing as an Endless Cosmological Process
by Leonid Marochnik
Universe 2024, 10(10), 388; https://doi.org/10.3390/universe10100388 - 3 Oct 2024
Viewed by 335
Abstract
The equation of state of quantum fluctuations of the gravitational field of the universe depends on H4, where H is the Hubble constant. This means that it is invariant with respect to the Wick rotation, i.e., the transition from Lorentzian space-time [...] Read more.
The equation of state of quantum fluctuations of the gravitational field of the universe depends on H4, where H is the Hubble constant. This means that it is invariant with respect to the Wick rotation, i.e., the transition from Lorentzian space-time to Euclidean space-time and vice versa. It is shown that the quantum birth of universes from Euclidean space-time, i.e., from nothing, and their quantum disappearance to nothing (return to Euclidean space-time) by the time the density of the matter filling the universe becomes negligible could be a likely cosmological scenario. On an infinite time axis, this is an endless process of birth and death of universes appearing and disappearing and replacing each other. Within this scenario, our current universe is going to disappear into nothing at z0.68, i.e., after 18.37 billion years, and the lifetime of our universe and similar universes is about 32 billion years. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
12 pages, 272 KiB  
Article
Ellipsoidal Universe and Cosmic Shear
by Luigi Tedesco
Universe 2024, 10(9), 363; https://doi.org/10.3390/universe10090363 - 10 Sep 2024
Viewed by 506
Abstract
We consider a Bianchi I geometry of the universe. We obtain a cosmic shear expression related to the eccentricity of the universe. In particular, we study the connections among cosmic shear, eccentricity, and CMB. The equations are self-contained, with only two parameters. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
19 pages, 452 KiB  
Article
From de Sitter to de Sitter: A Thermal Approach to Running Vacuum Cosmology and the Non-Canonical Scalar Field Description
by Pedro Eleuterio Mendonça Almeida, Rose Clivia Santos and Jose Ademir Sales Lima
Universe 2024, 10(9), 362; https://doi.org/10.3390/universe10090362 - 9 Sep 2024
Viewed by 508
Abstract
The entire classical cosmological history between two extreme de Sitter vacuum solutions is discussed based on Einstein’s equations and non-equilibrium thermodynamics. The initial non-singular de Sitter state is characterised by a very high energy scale, which is equal or smaller than the reduced [...] Read more.
The entire classical cosmological history between two extreme de Sitter vacuum solutions is discussed based on Einstein’s equations and non-equilibrium thermodynamics. The initial non-singular de Sitter state is characterised by a very high energy scale, which is equal or smaller than the reduced Planck mass. It is structurally unstable, and all of the continuous created matter, energy, and entropy of the material component comes from the irreversible flow powered by the primeval vacuum energy density. The analytical expression describing the running vacuum is obtained from the thermal approach. It opens a new perspective to solve the old puzzles and current observational challenges plaguing the cosmic concordance model driven by a rigid vacuum. Such a scenario is also modelled through a non-canonical scalar field. It is demonstrated that the resulting scalar field model is shown to be a step-by-step a faithful analytical representation of the thermal running vacuum cosmology. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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14 pages, 563 KiB  
Article
Constraint on the Cosmic Curvature in a Model with the Schwarzschild–de Sitter Metric from Supernovae and Gamma-Ray Burst Observational Data
by Vladimir N. Yershov
Universe 2024, 10(8), 325; https://doi.org/10.3390/universe10080325 - 11 Aug 2024
Viewed by 918
Abstract
In developing his cosmological model of 1917, de Sitter theoretically predicted the phenomenon of cosmological redshift (the de Sitter effect), which he did long before the discovery of this phenomenon in observations. The de Sitter effect is gravitational by its nature, as it [...] Read more.
In developing his cosmological model of 1917, de Sitter theoretically predicted the phenomenon of cosmological redshift (the de Sitter effect), which he did long before the discovery of this phenomenon in observations. The de Sitter effect is gravitational by its nature, as it is due to differences between the coordinate systems of the observer and the distant source. However, the relationship between the redshift and distance derived from the de Sitter metric is at odds with observations, since this relationship is nonlinear (quadratic) for small redshifts, while the observed relationship between the same quantities is strictly linear. This paper discusses the possibility that cosmological redshift is gravitational by its nature, as in de Sitter’s 1917 model. At the same time, here, as in de Sitter’s model, an elliptical space is used, the main characteristic of which is the identification of its antipodal points. But, unlike de Sitter’s model, here, in order to ensure strict linear dependence of the redshift on distance, the origin of the reference system is transferred to the observer’s antipodal point. The Schwarzschild–de Sitter metric used in this model allows you to estimate the curvature of space from observational data. To achieve this, a theoretical Hubble diagram is built within the framework of the model with the Schwarzschild–de Sitter metric, which is compared with observations from the Pantheon+ catalogue of type Ia supernovae and the Amati catalogue of gamma-ray bursts in the redshift range of 0<z<8. As a result of this comparison, we found that the lower estimate of the radius of curvature of space was quite large: 2.4×1015 Mpc. This means that the observational data indicate a negligible curvature of space. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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13 pages, 485 KiB  
Article
Coupled Quintessence Inspired by Warm Inflation
by Paulo M. Sá
Universe 2024, 10(8), 324; https://doi.org/10.3390/universe10080324 - 10 Aug 2024
Cited by 2 | Viewed by 453
Abstract
We investigate a coupled quintessence cosmological model in which a dark-energy scalar field with an exponential potential interacts directly with a dark-matter fluid through a dissipative term inspired by warm inflation. The evolution equations of this model give rise to a three-dimensional dynamical [...] Read more.
We investigate a coupled quintessence cosmological model in which a dark-energy scalar field with an exponential potential interacts directly with a dark-matter fluid through a dissipative term inspired by warm inflation. The evolution equations of this model give rise to a three-dimensional dynamical system for which a thorough qualitative analysis is performed for all values of the relevant parameters. We find that the model is able to replicate the observed sequence of late-time cosmological eras, namely, a long enough matter-dominated era followed by a present era of accelerated expansion. In situations where there is a significant transfer of energy from dark energy to dark matter, temporary scaling-type solutions may arise, but, asymptotically, all solutions are dominated by dark energy. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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34 pages, 1106 KiB  
Article
Stochastic Tunneling in de Sitter Spacetime
by Taiga Miyachi, Jiro Soda and Junsei Tokuda
Universe 2024, 10(7), 292; https://doi.org/10.3390/universe10070292 - 11 Jul 2024
Cited by 3 | Viewed by 698
Abstract
Tunneling processes in de Sitter spacetime are studied by using the stochastic approach. We evaluate the Martin–Siggia–Rose–Janssen–de Dominicis (MSRJD) functional integral by using the saddle-point approximation to obtain the tunneling rate. The applicability conditions of this method are clarified using the Schwinger–Keldysh formalism. [...] Read more.
Tunneling processes in de Sitter spacetime are studied by using the stochastic approach. We evaluate the Martin–Siggia–Rose–Janssen–de Dominicis (MSRJD) functional integral by using the saddle-point approximation to obtain the tunneling rate. The applicability conditions of this method are clarified using the Schwinger–Keldysh formalism. In the case of a shallow potential barrier, we reproduce the Hawking–Moss (HM) tunneling rate. Remarkably, in contrast to the HM picture, the configuration derived from the MSRJD functional integral satisfies physically natural boundary conditions. We also discuss the case of a steep potential barrier and find an interesting Coleman–de Luccia (CDL) bubblelike configuration. Since the starting point of our analysis is the Schwinger–Keldysh path integral, which can be formulated in a more generic setup and incorporates quantum effects, our formalism sheds light on further studies of tunneling phenomena from a real-time perspective. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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12 pages, 681 KiB  
Communication
Radial Oscillations of Dark Matter Stars Admixed with Dark Energy
by Camila Sepúlveda and Grigoris Panotopoulos
Universe 2024, 10(1), 41; https://doi.org/10.3390/universe10010041 - 16 Jan 2024
Cited by 1 | Viewed by 1253
Abstract
We imagine spherically symmetric configurations made of both dark matter and dark energy in the halo of spiral galaxies. Adopting a polytropic equation of state for dark matter and the Extended Chaplygin gas equation of state for dark energy, we model the same [...] Read more.
We imagine spherically symmetric configurations made of both dark matter and dark energy in the halo of spiral galaxies. Adopting a polytropic equation of state for dark matter and the Extended Chaplygin gas equation of state for dark energy, we model the same object with three different dark matter–dark energy compositions. We compute the frequencies and the corresponding eigenfunctions of the ten lowest modes, integrating the equations for the radial perturbations by imposing the appropriate boundary conditions at the center and the surface of the object. Also, a comparison between the different models is made. Full article
(This article belongs to the Special Issue Cosmological Models of the Universe)
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