Universe

Research

Jump to: Review

18 pages, 349 KiB

Open AccessArticle

Primordial Black Holes from Spatially Varying Cosmological Constant Induced by Field Fluctuations in Extra Dimensions

by Arkady A. Popov, Sergey G. Rubin and Alexander S. Sakharov

Universe 2024, 10(4), 166; https://doi.org/10.3390/universe10040166 - 31 Mar 2024

Viewed by 680

Abstract

The origin and evolution of supermassive black holes (SMBHs) in our universe have sparked controversy. In this study, we explore the hypothesis that some of these black holes may have seeded from the direct collapse of dark energy domains with density significantly higher [...] Read more.

The origin and evolution of supermassive black holes (SMBHs) in our universe have sparked controversy. In this study, we explore the hypothesis that some of these black holes may have seeded from the direct collapse of dark energy domains with density significantly higher than the surrounding regions. The mechanism of the origin of such domains relies on the inflationary evolution of a scalar field acting in D dimensions, which is associated with the cosmological constant in our four-dimensional spacetime manifold. Inner space quantum fluctuations of the field during inflation are responsible for the spatial variations of the dark energy density in our space. This finding holds particular significance, especially considering recent evidence from pulsar timing array observations, which supports the existence of a stochastic gravitational wave background consisting of SMBH mergers. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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12 pages, 269 KiB

Open AccessArticle

Conformally Invariant Gravity and Gravitating Mirages

by Victor Berezin and Inna Ivanova

Universe 2024, 10(3), 147; https://doi.org/10.3390/universe10030147 - 17 Mar 2024

Viewed by 745

Abstract

The action of an ideal fluid in Euler variables with a variable number of particles is used for the phenomenological description of the processes of particle creation in strong external fields. It has been demonstrated that the conformal invariance of the creation law [...] Read more.

The action of an ideal fluid in Euler variables with a variable number of particles is used for the phenomenological description of the processes of particle creation in strong external fields. It has been demonstrated that the conformal invariance of the creation law imposes quite strict restrictions on the possible types of sources. It is shown that combinations with the particle number density in the creation law can be interpreted as dark matter within the framework of this model. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

31 pages, 1805 KiB

Open AccessArticle

A Critical Discussion on the H₀ Tension

by Salvatore Capozziello, Giuseppe Sarracino and Giulia De Somma

Universe 2024, 10(3), 140; https://doi.org/10.3390/universe10030140 - 13 Mar 2024

Cited by 1 | Viewed by 985

Abstract

A critical discussion on the

H_{0}

Hubble constant tension is presented by considering both early and late-type observations. From recent precise measurements, discrepancies emerge when comparing results for some cosmological quantities obtained at different redshifts. We highlight the most relevant measurements of [...] Read more.

A critical discussion on the

H_{0}

Hubble constant tension is presented by considering both early and late-type observations. From recent precise measurements, discrepancies emerge when comparing results for some cosmological quantities obtained at different redshifts. We highlight the most relevant measurements of

H_{0}

and propose potential ideas to solve its tension. These solutions concern the exploration of new physics beyond the

Λ

CDM model or the evaluation of

H_{0}

by other methods. In particular, we focus on the role of the look-back time. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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13 pages, 285 KiB

Open AccessArticle

Regular Friedmann Universes and Matter Transformations

by Alexander Kamenshchik and Polina Petriakova

Universe 2024, 10(3), 137; https://doi.org/10.3390/universe10030137 - 13 Mar 2024

Cited by 1 | Viewed by 800

Abstract

We apply a very simple procedure to construct non-singular cosmological models for flat Friedmann universes filled with minimally coupled scalar fields or by tachyon Born–Infeld-type fields. Remarkably, for the minimally coupled scalar field and the tachyon field, the regularity of the cosmological evolution, [...] Read more.

We apply a very simple procedure to construct non-singular cosmological models for flat Friedmann universes filled with minimally coupled scalar fields or by tachyon Born–Infeld-type fields. Remarkably, for the minimally coupled scalar field and the tachyon field, the regularity of the cosmological evolution, or in other words, the existence of bounce, implies the necessity of the transition between scalar fields with standard kinetic terms to those with phantom ones. In both cases, the potentials in the vicinity of the point of the transition have a non-analyticity of the cusp form that is characterized by the same exponent and is equal to

\frac{2}{3}

. If, in the tachyon model’s evolution, the pressure changes its sign, then another transformation of the Born–Infeld-type field occurs: the tachyon transforms into a pseudotachyon, and vice versa. We also undertake an analysis of the stability of the cosmological evolution in our models; we rely on the study of the speed of sound squared. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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9 pages, 245 KiB

Open AccessArticle

On Phase Transitions during Collisions near the Horizon of Black Holes

by Andrey A. Grib and Yuri V. Pavlov

Universe 2024, 10(3), 131; https://doi.org/10.3390/universe10030131 - 07 Mar 2024

Viewed by 817

Abstract

During particle collisions in the vicinity of the horizon of black holes, it is possible to achieve energies and temperatures corresponding to phase transitions in particle physics. It is shown that the sizes of the regions of the new phase are of the [...] Read more.

During particle collisions in the vicinity of the horizon of black holes, it is possible to achieve energies and temperatures corresponding to phase transitions in particle physics. It is shown that the sizes of the regions of the new phase are of the order of the Compton length for the corresponding mass scale. The lifetime is also on the order of the Compton time. It is shown that the inverse influence of the energy density in the electro-weak phase transition in collisions on the space–time metric can be neglected. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

11 pages, 829 KiB

Open AccessArticle

Search for Wormhole Candidates: Accreting Wormholes with Monopole Magnetic Fields

by Mikhail Piotrovich, Serguei Krasnikov, Stanislava Buliga and Tinatin Natsvlishvili

Universe 2024, 10(3), 108; https://doi.org/10.3390/universe10030108 - 27 Feb 2024

Viewed by 903

Abstract

The existence of even the simplest magnetized wormholes may lead to observable consequences. In the case where both the wormhole and the magnetic field around its mouths are static and spherically symmetric, and gas in the region near the wormhole falls radially into [...] Read more.

The existence of even the simplest magnetized wormholes may lead to observable consequences. In the case where both the wormhole and the magnetic field around its mouths are static and spherically symmetric, and gas in the region near the wormhole falls radially into it, the former’s spectrum contains bright cyclotron or synchrotron lines due to the interaction of charged plasma particles with the magnetic field. At the same time, due to spherical symmetry, the radiation is non-polarized. The emission of this just-described exotic type (non-thermal, but non-polarized) may be a wormhole signature. Also, in this scenario, the formation of an accretion disk is still quite possible at some distance from the wormhole, but a monopole magnetic field could complicate this process and lead to the emergence of asymmetrical and one-sided relativistic jets. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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11 pages, 292 KiB

Open AccessArticle

No-Boundary Wave Functional and Own Mass of the Universe

by Natalia Gorobey, Alexander Lukyanenko and Alexander V. Goltsev

Universe 2024, 10(2), 101; https://doi.org/10.3390/universe10020101 - 19 Feb 2024

Viewed by 1026

Abstract

An alternative formulation of the no-boundary initial state of the universe in the Euclidean quantum theory of gravity is proposed. Unlike the no-boundary Hartle–Hawking wave function, in which time appears together with macroscopic space–time in the semiclassical approximation, in the proposed formalism, time [...] Read more.

An alternative formulation of the no-boundary initial state of the universe in the Euclidean quantum theory of gravity is proposed. Unlike the no-boundary Hartle–Hawking wave function, in which time appears together with macroscopic space–time in the semiclassical approximation, in the proposed formalism, time is present from the very beginning on an equal footing with spatial coordinates. The main element of the formalism is the wave functional, which is defined based on the world histories of the universe. This ensures formal 4D covariance of the theory. The wave functional is defined independently of the wave function as an eigenvector of the action operator. The shape of the Origin region, together with the boundary conditions, is determined by the structure of the total energy of the universe, which includes a 3D-invariant contribution of the expansion energy. The own mass of the universe arises as a non-zero value of the expansion energy in the Origin. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

11 pages, 299 KiB

Open AccessArticle

On the Possibility of a Static Universe

by Júlio C. Fabris, Felipe T. Falciano, Luiz F. Guimarães and Nelson Pinto-Neto

Universe 2024, 10(2), 92; https://doi.org/10.3390/universe10020092 - 16 Feb 2024

Viewed by 1105

Abstract

After a century of cosmological observations, we have a solid standard model of cosmology. However, from a theoretical viewpoint, it is a compelling question if the cosmological data inevitably require an expanding universe independently of the theoretical framework. The possibility of obtaining a [...] Read more.

After a century of cosmological observations, we have a solid standard model of cosmology. However, from a theoretical viewpoint, it is a compelling question if the cosmological data inevitably require an expanding universe independently of the theoretical framework. The possibility of obtaining a viable cosmological model with a constant scale-factor is discussed in the context of the Brans–Dicke class of scalar–tensor theories. It is shown that a flat spatial section requires the presence of a stiff matter fluid. However, some kinematical properties of the standard cosmological model can be reproduced. A realistic scenario may require a more complex class of scalar–tensor theories. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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17 pages, 602 KiB

Open AccessArticle

Kantowski–Sachs Model with a Running Cosmological Constant and Radiation

by Vinícius Guilherme Oliveira, Gil de Oliveira Neto and Ilya L. Shapiro

Universe 2024, 10(2), 83; https://doi.org/10.3390/universe10020083 - 08 Feb 2024

Viewed by 894

Abstract

The simplest anisotropic model of the early universe is the one with two conformal factors, which can be identified as the Kantowski–Sachs metric, or the reduced version of the Bianchi-I metric. To fit the existing observational data, it is important that the anisotropy [...] Read more.

The simplest anisotropic model of the early universe is the one with two conformal factors, which can be identified as the Kantowski–Sachs metric, or the reduced version of the Bianchi-I metric. To fit the existing observational data, it is important that the anisotropy is washed out in the early stage of the evolution. We explore the possible effects of the running cosmological constant on the dynamics of isotropy in the case of space filled by radiation. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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9 pages, 428 KiB

Open AccessArticle

Nonrelativistic Quantum Mechanical Problem for the Cornell Potential in Lobachevsky Space

by Laszlo Jenkovszky, Yurii Andreevich Kurochkin, N. D. Shaikovskaya and Vladimir Olegovich Soloviev

Universe 2024, 10(2), 76; https://doi.org/10.3390/universe10020076 - 05 Feb 2024

Viewed by 952

Abstract

In Friedmann–Lobachevsky space-time with a radius of curvature slowly varying over time, we study numerically the problem of motion of a particle moving in the Cornell potential. The mass of the particle is taken to be a reduced mass of the charmonium system. [...] Read more.

In Friedmann–Lobachevsky space-time with a radius of curvature slowly varying over time, we study numerically the problem of motion of a particle moving in the Cornell potential. The mass of the particle is taken to be a reduced mass of the charmonium system. In contrast to the similar problem in flat space, in Lobachevsky space the Cornell potential has a finite depth and, as a consequence, the number of bound states of the system is finite and motion with a continuum energy spectrum is also possible. In this paper, we study the bound states as well as the scattering states of the system. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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14 pages, 579 KiB

Open AccessArticle

An Isotropic Cosmological Model with Aetherically Active Axionic Dark Matter

by Alexander Balakin and Amir Shakirzyanov

Universe 2024, 10(2), 74; https://doi.org/10.3390/universe10020074 - 04 Feb 2024

Viewed by 956

Abstract

Within the framework of the extended Einstein–aether–axion theory, we studied the model of a two-level aetheric control over the evolution of a spatially isotropic homogeneous Universe filled with axionic dark matter. Two guiding functions are introduced, which depend on the expansion scalar of [...] Read more.

Within the framework of the extended Einstein–aether–axion theory, we studied the model of a two-level aetheric control over the evolution of a spatially isotropic homogeneous Universe filled with axionic dark matter. Two guiding functions are introduced, which depend on the expansion scalar of the aether flow being equal to the tripled Hubble function. The guiding function of the first type enters the aetheric effective metric, which modifies the kinetic term of the axionic system; the guiding function of the second type predetermines the structure of the potential axion field. We obtained new exact solutions to the total set of master equations in the model (with and without cosmological constant), and studied four analytically solvable submodels in detail, for which both guiding functions are reconstructed and illustrations of their behavior are presented. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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30 pages, 703 KiB

Open AccessArticle

Graviton to Photon Conversion in Curved Space-Time and External Magnetic Field

by Alexander D. Dolgov, Lyubov A. Panasenko and Vladimir A. Bochko

Universe 2024, 10(1), 7; https://doi.org/10.3390/universe10010007 - 25 Dec 2023

Cited by 1 | Viewed by 979

Abstract

The suppression of relic gravitational waves due to their conversion into electromagnetic radiation in a cosmological magnetic field is studied. The coupled system of equations describing gravitational and electromagnetic wave propagation in an arbitrary curved space-time and in external magnetic field is derived. [...] Read more.

The suppression of relic gravitational waves due to their conversion into electromagnetic radiation in a cosmological magnetic field is studied. The coupled system of equations describing gravitational and electromagnetic wave propagation in an arbitrary curved space-time and in external magnetic field is derived. The subsequent elimination of photons from the beam due to their interaction with the primary plasma is taken into account. The resulting system of equations is solved numerically in the Friedman–LeMaitre–Robertson–Walker metric for the upper limit of the intergalactic magnetic field strength of 1 nGs. We conclude that the gravitational wave conversion into photons in the intergalactic magnetic field cannot significantly change the amplitude of the relic gravitational wave and their frequency spectrum. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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18 pages, 520 KiB

Open AccessArticle

Entropic Inflation in Presence of Scalar Field

by Sergei D. Odintsov, Simone D’Onofrio and Tanmoy Paul

Universe 2024, 10(1), 4; https://doi.org/10.3390/universe10010004 - 21 Dec 2023

Cited by 1 | Viewed by 1007

Abstract

In spirit of the recently proposed four-parameter generalized entropy of apparent horizon, we investigate inflationary cosmology where the matter field inside of the horizon is dominated by a scalar field with a power law potential (i.e., the form of

ϕ^{n}

where

ϕ

[...] Read more.

In spirit of the recently proposed four-parameter generalized entropy of apparent horizon, we investigate inflationary cosmology where the matter field inside of the horizon is dominated by a scalar field with a power law potential (i.e., the form of

ϕ^{n}

where

ϕ

is the scalar field under consideration). Actually without any matter inside of the horizon, the entropic cosmology leads to a de-Sitter spacetime, or equivalently, an eternal inflation with no exit. Thus in order to achieve a viable inflation, we consider a minimally coupled scalar field inside the horizon, and moreover, with the simplest quadratic potential. It is well known that the

ϕ^{2}

potential in standard scalar field cosmology is ruled out from inflationary perspective as it is not consistent with the recent Planck 2018 data; (here it may be mentioned that in the realm of “apparent horizon thermodynamics”, the standard scalar field cosmology is analogous to the case where the entropy of the apparent horizon is given by the Bekenstein–Hawking entropy). However, the story becomes different if the horizon entropy is of generalized entropic form, in which case, the effective energy density coming from the horizon entropy plays a significant role during the evolution of the universe. In particular, it turns out that in the context of generalized entropic cosmology, the

ϕ^{2}

potential indeed leads to a viable inflation (according to the Planck data) with a graceful exit, and thus the potential can be made back in the scene. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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15 pages, 295 KiB

Open AccessArticle

Conventionalism, Cosmology and Teleparallel Gravity

by Laur Järv and Piret Kuusk

Universe 2024, 10(1), 1; https://doi.org/10.3390/universe10010001 - 19 Dec 2023

Cited by 1 | Viewed by 1026

Abstract

We consider homogeneous and isotropic cosmological models in the framework of three geometrical theories of gravitation. In Einstein’s general relativity, they are given in terms of the curvature of the Levi-Civita connection in torsion-free metric spacetimes; in the teleparallel equivalent of general relativity, [...] Read more.

We consider homogeneous and isotropic cosmological models in the framework of three geometrical theories of gravitation. In Einstein’s general relativity, they are given in terms of the curvature of the Levi-Civita connection in torsion-free metric spacetimes; in the teleparallel equivalent of general relativity, they are given in terms of the torsion of flat metric spacetimes; and in the symmetric teleparallel equivalent of general relativity, they are given in terms of the nonmetricity of flat torsion-free spacetimes. We argue that although these three formulations seem to be different, the corresponding cosmological models are in fact equivalent and their choice is conventional. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

21 pages, 6897 KiB

Open AccessArticle

Possible Wormholes in a Friedmann Universe

by Kirill A. Bronnikov, Pavel E. Kashargin and Sergey V. Sushkov

Universe 2023, 9(11), 465; https://doi.org/10.3390/universe9110465 - 29 Oct 2023

Viewed by 3885

Abstract

We study the properties of evolving wormholes able to exist in a closed Friedmann dust-filled universe and described by a particular branch of the well-known Lemaître–Tolman–Bondi solution to the Einstein equations and its generalization with a nonzero cosmological constant and an electromagnetic field. [...] Read more.

We study the properties of evolving wormholes able to exist in a closed Friedmann dust-filled universe and described by a particular branch of the well-known Lemaître–Tolman–Bondi solution to the Einstein equations and its generalization with a nonzero cosmological constant and an electromagnetic field. Most of the results are obtained with pure dust solutions. It is shown, in particular, that the lifetime of wormhole throats is much shorter than that of the whole wormhole region in the universe (which coincides with the lifetime of the universe as a whole), and that the density of matter near the boundary of the wormhole region is a few times smaller than the mean density of matter in the universe. Explicit examples of wormhole solutions and the corresponding numerical estimates are presented. The traversability of the wormhole under study is shown by a numerical analysis of radial null geodesics. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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10 pages, 598 KiB

Open AccessCommunication

Improved Model of Primordial Black Hole Formation after Starobinsky Inflation

by Sultan Saburov and Sergei V. Ketov

Universe 2023, 9(7), 323; https://doi.org/10.3390/universe9070323 - 06 Jul 2023

Cited by 7 | Viewed by 690

Abstract

A new (improved) model of inflation and primordial black hole (PBH) formation is proposed by combining the Starobinsky model of inflation, Appleby–Battye–Starobinsky (ABS) model of dark energy, and a quantum correction in the modified

F (R)

gravity. The energy scale parameter [...] Read more.

A new (improved) model of inflation and primordial black hole (PBH) formation is proposed by combining the Starobinsky model of inflation, Appleby–Battye–Starobinsky (ABS) model of dark energy, and a quantum correction in the modified

F (R)

gravity. The energy scale parameter in the ABS model is taken to be close to the inflationary scale, in order to describe double inflation instead of dark energy. The quantum correction is given by the term quartic in the spacetime scalar curvature R with a negative coefficient

(- δ)

in the

F (R)

function. It is demonstrated that very good agreement (within

1 σ

) with current measurements of the cosmic microwave background (CMB) radiation can be achieved by choosing the proper value of

δ

, thus solving the problem of low values of the tilt of CMB scalar perturbations in the earlier proposed model in arXiv:2205.00603. A large (by a factor of

10^{7}

against CMB) enhancement in the power spectrum of scalar perturbations is achieved by fine tuning the parameters of the model. It is found by numerical analysis that it can lead to formation of asteroid-size PBHs with masses up to

10^{20}

g, which may form dark matter in the current universe. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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12 pages, 445 KiB

Open AccessArticle

Spinor Field in FLRW Cosmology

by Bijan Saha

Universe 2023, 9(5), 243; https://doi.org/10.3390/universe9050243 - 22 May 2023

Cited by 1 | Viewed by 898

Abstract

Within the scope of a Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model we study the role of a nonlinear spinor field in the evolution of the universe. In doing so, we exploit the FLRW models given in both Cartesian and spherical coordinates. It is found that [...] Read more.

Within the scope of a Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model we study the role of a nonlinear spinor field in the evolution of the universe. In doing so, we exploit the FLRW models given in both Cartesian and spherical coordinates. It is found that if the FLRW model is given in the spherical coordinates the energy-momentum tensor (EMT) of the spinor field possesses nontrivial non-diagonal components, which is not the case for Cartesian coordinates. These non-diagonal components do not depend on either the spinor field nonlinearity or the parameter k that defines the type of curvature of the FLRW model. The presence of such components imposes some restrictions on the spinor field. The problem is studied for open, flat and close geometries and the spinor field is used to simulate different types of sources including dark energies. Some qualitative numerical solutions are given. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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10 pages, 312 KiB

Open AccessArticle

Particles of Negative and Zero Energy in Black Holes and Cosmological Models

by Andrey A. Grib and Yuri V. Pavlov

Universe 2023, 9(5), 217; https://doi.org/10.3390/universe9050217 - 01 May 2023

Viewed by 1013

Abstract

Particles with negative energies are considered for three different cases: inside the horizon of a Schwarzschild black hole, Milne’s coordinates in flat Minkowski space–time (Milne’s universe using nonsynchronous coordinates) and in the cosmological Gödel model of the rotating universe. It is shown that, [...] Read more.

Particles with negative energies are considered for three different cases: inside the horizon of a Schwarzschild black hole, Milne’s coordinates in flat Minkowski space–time (Milne’s universe using nonsynchronous coordinates) and in the cosmological Gödel model of the rotating universe. It is shown that, differently from the Gödel model with a nondiagonal term, where it occurs that negative energies are impossible, they are present in all other cases considered in the paper. Particles with zero energy are also possible in the first two cases. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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Review

Jump to: Research

21 pages, 685 KiB

Open AccessReview

The Spectral Condition, Plane Waves, and Harmonic Analysis in de Sitter and Anti-de Sitter Quantum Field Theories

by Ugo Moschella

Universe 2024, 10(5), 199; https://doi.org/10.3390/universe10050199 - 28 Apr 2024

Viewed by 174

Abstract

We review the role of the spectral condition as a characteristic of Minkowski, de Sitter, and anti-de Sitter quantum field theories. We also discuss the role of plane waves that are compatible with the relevant analyticity domains linked to the spectral condition(s) and [...] Read more.

We review the role of the spectral condition as a characteristic of Minkowski, de Sitter, and anti-de Sitter quantum field theories. We also discuss the role of plane waves that are compatible with the relevant analyticity domains linked to the spectral condition(s) and discuss harmonic analysis in terms of them. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

19 pages, 2294 KiB

Open AccessReview

Neutrino at Different Epochs of the Friedmann Universe

by Alexandre V. Ivanchik, Oleg A. Kurichin and Vlad Yu. Yurchenko

Universe 2024, 10(4), 169; https://doi.org/10.3390/universe10040169 - 02 Apr 2024

Viewed by 733

Abstract

At least two relics of the Big Bang have survived: the cosmological microwave background (CMB) and the cosmological neutrino background (C

ν

B). Being the second most abundant particle in the universe, the neutrino has a significant impact on its evolution from the [...] Read more.

At least two relics of the Big Bang have survived: the cosmological microwave background (CMB) and the cosmological neutrino background (C

ν

B). Being the second most abundant particle in the universe, the neutrino has a significant impact on its evolution from the Big Bang to the present day. Neutrinos affect the following cosmological processes: the expansion rate of the universe, its chemical and isotopic composition, the CMB anisotropy and the formation of the large-scale structure of the universe. Another relic neutrino background is theoretically predicted, it consists of non-equilibrium antineutrinos of Primordial Nucleosynthesis arising as a result of the decay of neutrons and tritium nuclei. Such antineutrinos are an indicator of the baryon asymmetry of the universe. In addition to experimentally detectable active neutrinos, the existence of sterile neutrinos is theoretically predicted to generate neutrino masses and explain their oscillations. Sterile neutrinos can also solve such cosmological problems as the baryonic asymmetry of the universe and the nature of dark matter. The recent results of several independent experiments point to the possibility of the existence of a light sterile neutrino. However, the existence of such a neutrino is inconsistent with the predictions of the Standard Cosmological Model. The inclusion of a non-zero lepton asymmetry of the universe and/or increasing the energy density of active neutrinos can eliminate these contradictions and reconcile the possible existence of sterile neutrinos with Primordial Nucleosynthesis, the CMB anisotropy, and also reduce the H₀-tension. In this brief review, we discuss the influence of the physical properties of active and sterile neutrinos on the evolution of the universe from the Big Bang to the present day. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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17 pages, 455 KiB

Open AccessReview

The Nature of Dark Energy and Constraints on Its Hypothetical Constituents from Force Measurements

by Galina L. Klimchitskaya and Vladimir M. Mostepanenko

Universe 2024, 10(3), 119; https://doi.org/10.3390/universe10030119 - 04 Mar 2024

Cited by 1 | Viewed by 872

Abstract

This review considers the theoretical approaches to the understanding of dark energy, which comprises approximately 68% of the energy of our Universe and explains the acceleration in its expansion. Following a discussion of the main approach based on Einstein’s equations with the cosmological [...] Read more.

This review considers the theoretical approaches to the understanding of dark energy, which comprises approximately 68% of the energy of our Universe and explains the acceleration in its expansion. Following a discussion of the main approach based on Einstein’s equations with the cosmological term, the explanations of dark energy using the concept of some kind of scalar field are elucidated. These include the concept of a quintessence and modifications of the general theory of relativity by means of the scalar–tensor gravity exploiting the chameleon, symmetron and environment-dependent dilaton fields and corresponding particles. After mentioning several laboratory experiments allowing us to constrain the hypothetical scalar fields modeling the dark energy, special attention is devoted to the possibility of constraining the parameters of chameleon, symmetron and environment-dependent dilaton fields from measuring the Casimir force. It is concluded that the parameters of each of these fields can be significantly strengthened in near future by using the next-generation setups in preparation suitable for measuring the Casimir force at larger separations. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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25 pages, 3041 KiB

Open AccessReview

Prediction of the Expansion of the Universe Made by Alexander Friedmann and the Effect of Particle Creation in Cosmology

by Vladimir M. Mostepanenko

Universe 2024, 10(2), 84; https://doi.org/10.3390/universe10020084 - 09 Feb 2024

Viewed by 1086

Abstract

This review devoted to the centenary of Alexander Friedmann’s prediction of the Universe expansion presents the results obtained by him in 1922 and 1924 and an overview of their further developments. Special attention is paid to the role of mathematics, which enabled Friedmann [...] Read more.

This review devoted to the centenary of Alexander Friedmann’s prediction of the Universe expansion presents the results obtained by him in 1922 and 1924 and an overview of their further developments. Special attention is paid to the role of mathematics, which enabled Friedmann to perform a radical departure from the conventional practice of considering our universe as a static system. The effect of particle creation in the expanding universe is discussed concurrently with the earlier investigated phenomenon of pair creation from a vacuum by an external electric field. The numbers of scalar and spinor particles created at different stages of the Universe’s evolution are presented, and the possible role of the effect of the creation of particles in the formation of relativistic plasma and cold dark matter after the inflationary period is noted. It is stressed that by introducing the concept of the expanding universe, Friedmann made a contribution towards the understanding of the world around us that is compatible with those made by Ptolemy, Copernicus, and Newton in previous epochs. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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14 pages, 320 KiB

Open AccessReview

DeWitt Boundary Condition in One-Loop Quantum Cosmology

by Giampiero Esposito

Universe 2023, 9(4), 187; https://doi.org/10.3390/universe9040187 - 14 Apr 2023

Cited by 1 | Viewed by 819

Abstract

DeWitt’s suggestion that the wave function of the universe should vanish at the classical Big Bang singularity is considered here within the framework of one-loop quantum cosmology. For pure gravity at one loop about a flat four-dimensional background bounded by a 3-sphere, three [...] Read more.

DeWitt’s suggestion that the wave function of the universe should vanish at the classical Big Bang singularity is considered here within the framework of one-loop quantum cosmology. For pure gravity at one loop about a flat four-dimensional background bounded by a 3-sphere, three choices of boundary conditions are considered: vanishing of the linearized magnetic curvature when only transverse-traceless gravitational modes are quantized; a one-parameter family of mixed boundary conditions for gravitational and ghost modes; and diffeomorphism-invariant boundary conditions for metric perturbations and ghost modes. A positive

ζ (0)

value in these cases ensures that, when the three-sphere boundary approaches zero, the resulting one-loop wave function approaches zero. This property may be interpreted by saying that, in the limit of small three-geometry, the resulting one-loop wave function describes a singularity-free universe. This property holds for one-loop functional integrals, which are not necessarily equivalent to solutions of the quantum constraint equations. Full article

(This article belongs to the Special Issue The Friedmann Cosmology: A Century Later)

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