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10 pages, 289 KiB

Open AccessArticle

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 599

Abstract

The equation of state of quantum fluctuations of the gravitational field of the universe depends on

H^{4}

, 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

H^{4}

, 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

z \leq - 0.68

, 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

Open AccessArticle

Ellipsoidal Universe and Cosmic Shear

by Luigi Tedesco

Universe 2024, 10(9), 363; https://doi.org/10.3390/universe10090363 - 10 Sep 2024

Cited by 1 | Viewed by 668

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

Open AccessArticle

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 629

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

Open AccessArticle

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 1048

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 \times 10^{15}

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

Open AccessArticle

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 533

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

Open AccessArticle

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 793

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

Open AccessCommunication

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 1347

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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Review

Jump to: Research

28 pages, 2411 KiB

Open AccessReview

Cosmological Models in Lovelock Gravity: An Overview of Recent Progress

by Sergey Pavluchenko

Universe 2024, 10(11), 429; https://doi.org/10.3390/universe10110429 - 18 Nov 2024

Viewed by 364

Abstract

In the current review, we provide a summary of the recent progress made in the cosmological aspect of extra-dimensional Lovelock gravity. Our review covers a wide variety of particular model/matter source combinations: Einstein–Gauss–Bonnet as well as cubic Lovelock gravities with vacuum, cosmological constant, [...] Read more.

In the current review, we provide a summary of the recent progress made in the cosmological aspect of extra-dimensional Lovelock gravity. Our review covers a wide variety of particular model/matter source combinations: Einstein–Gauss–Bonnet as well as cubic Lovelock gravities with vacuum, cosmological constant, perfect fluid, spatial curvature, and some of their combinations. Our analysis suggests that it is possible to set constraints on the parameters of the above-mentioned models from the simple requirement of the existence of a smooth transition from the initial singularity to a realistic low-energy regime. Initially, anisotropic space naturally evolves into a configuration with two isotropic subspaces, and if one of these subspaces is three-dimensional and is expanding while another is contracting, we call it realistic compactification. Of course, the process is not devoid of obstacles, and in our paper, we review the results of the compactification occurrence investigation for the above-mentioned models. In particular, for vacuum and

Λ

-term EGB models, compactification is not suppressed (but is not the only possible outcome either) if the number of extra dimensions is

D ⩾ 2

; for vacuum cubic Lovelock gravities it is always present (however, cubic Lovelock gravity is defined only for

D ⩾ 3

number of extra dimensions); for the EGB model with perfect fluid it is present for

D = 2

(we have not considered this model in higher dimensions yet), and in the presence of spatial curvature, the realistic stabilization of extra dimensions is always present (however, such a model is well-defined only in

D ⩾ 4

number of extra dimensions). Full article

(This article belongs to the Special Issue Cosmological Models of the Universe)

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