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Universe: Feature Papers–Cosmology and Gravitation

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Published Papers

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

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 20115

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Special Issue Editors

Prof. Dr. Antonino Del Popolo

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Guest Editor

1. Department of Physics and Astronomy, University of Catania, 95100 Catania, Italy
2. INFN, Sezione di Catania, 95100 Catania, Italy
Interests: cosmological physics; CDM models; extra-planetary systems; gravitational systems
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Yi-Fu Cai

E-Mail Website
Guest Editor

Department of Astronomy, School of Physical Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, China
Interests: bounce cosmology; inflation; dark energy; curvaton; curvature perturbation; primordial gravitational waves; cosmic microwave background; modified gravity; black holes; cosmic strings; cosmological perturbations
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Jean-Michel Alimi

E-Mail Website
Guest Editor

Laboratory Universe and Theories, UMR 8102 CNRS, Observatoire de Paris, PSL Research University, 5 Place Jules Janssen, 92195 Meudon, France
Interests: cosmological physics; numerical cosmology; large scale structure formation; scalar-tensor and modified gravity theory; backreaction and inhomogeneous universes
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

In the last few decades, the standard model of cosmology, based on general relativity, has been shown to be able to explain a large number of observations describing the universe’s large scale structure formation and evolution, the state of the early universe, and the abundance of the different forms of matter and energy. However, such success cannot hide the tensions at both large and small scales that precision data are revealing. At the large scale, there are tensions of unknown origin between the value of the Hubble parameter, H0, and SNe Ia data, the 2013 Planck parameters, and σ8 obtained from cluster number counts and weak lensing. Also the Planck 2015 data are in tension with σ8 growth rate, and with CFHTLenS weak lensing data. Moreover, a quadrupole–octupole alignment, a power hemispherical asymmetry, and a cold spot are presented in the large angle fluctuations in the CMB. To these problems, we should add the absence of detection particles constituting dark matter, the so-called ”small scale problems” of the LCDM, the “cosmological constant fine tuning problem,” and the “cosmic coincidence problem.”

The quoted issues motivated the investigations of other explanations to the dark matter and dark energy problems. These alternative models generate the dark matter and dark energy effects through additional matter fields (e.g., quintessence), or modified gravity (MG) models. In some cases, the quoted theories tried to explain dark matter and the accelerated expansion as the manifestation of extra dimensions, or higher-order corrections effects, as in the Dvali-Gabadadze-Porrati (DGP) model and in scalar-tensor and f(R) gravity. Disentangling between the plethora of models is not an easy task. An interpretation of cosmic acceleration as resulting from the backreaction effects of matter inhomogeneities in inhomogeneous universe models has also been proposed.

However, some recent results, such as the discovery of gravitational waves, may be used to reject some modified gravity theories thanks to the fact that they predict gravitational waves to move at speeds different to that of light, in disagreement with observations. A discussion on comparison of the predictions of modified theories of gravitation and those of general relativity are of paramount importance.

By converse, the previous discussion also urges us to try to improve our understanding of gravitational interaction, both at the classical and quantum levels. It is well known that to date no theory has been able to have gravity and quantum mechanics speak the one to the other. A better understanding of gravity also leads us to look for solutions of the gravitational field equations describing compact object that in contradiction with the “cosmic censorship” idea have no horizons, and can reproduce the behaviour of some known solutions, as for example gravitational waves.

It aims to set itself at the cutting edge of the most recent advances in the following areas:

1) our theoretical, phenomenological and experimental understanding of the gravitational interaction and its even more numerous and intertwined ties with other fields at all relevant distance and energy scales.

2) the disagreement between local measurements of Hubble's constant, and the value obtained from the ΛCDM model in conjunction with the data extracted from the Planck satellite mission.

3) the status of modified gravity theories after the discovery, with a high degree of accuracy, that the speeds of propagation of gravitational and light waves are equal.

5) Interpretation of the dark components of the universe in the framework of inhomogeneous universes.

6) The cosmological probes of the nature of the dark component of the universe.

7) the search for alternative solutions of gravitational field equations representing horizonless compact objects able to mimic the gravitational wave signals attributed to black holes.

You are welcome to send short proposals for submissions of feature papers to our Editorial Office ([email protected]). They will be evaluated by Editors first, and the selected papers will be thoroughly and rigorously peer reviewed.

Prof. Dr. Antonino Del Popolo
Prof. Dr. Yi-Fu Cai
Prof. Dr. Jean-Michel Alimi
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Problems of the LCDM model
Large-scale structure formation
Modified gravity theories
Gravity at classical and quantum level

Published Papers (10 papers)

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Research

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11 pages, 307 KiB

Open AccessArticle

The Higgs Mechanism and Cosmological Constant Today

by Irina Dymnikova

Universe 2022, 8(6), 305; https://doi.org/10.3390/universe8060305 - 28 May 2022

Cited by 2 | Viewed by 1477

Abstract

The Higgs mechanism, as responsible for the first inflation, powers the initial accelerated expansion and further preheating via the symmetry breaking from its false vacuum state corresponding to the Sitter vacuum of the GUT scale with

Λ = 8 π G ρ_{Λ}

[...] Read more.

Λ = 8 π G ρ_{Λ}

, whose decay provides necessary energetic support. Here we address the question of the possibility of symmetry restoration of the Higgs field at the presently observed vacuum scale which would make it responsible for the today value of the cosmological constant

λ = 8 π G ρ_{λ}

. We find the existence of the possibility of symmetry restoration in the minisuperspace model of quantum cosmology and show that

λ

today must have a non-zero value. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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Figure 1

16 pages, 661 KiB

Open AccessArticle

Polymer Dynamics of Isotropic Universe in Ashtekar and in Volume Variables

by Eleonora Giovannetti, Gabriele Barca, Federico Mandini and Giovanni Montani

Universe 2022, 8(6), 302; https://doi.org/10.3390/universe8060302 - 26 May 2022

Cited by 4 | Viewed by 1284

Abstract

We analyze the semiclassical and quantum polymer dynamics of the isotropic Universe in terms of both the standard Ashtekar-Barbero-Immirzi connection and its conjugate momentum and also of the new generalized coordinate conjugate to the Universe volume. We study the properties of the resulting bouncing cosmology that emerges in both the representations and we show that the Big Bounce is an intrinsic cut-off on the cosmological dynamics only when the volume variable is implemented, while in terms of the standard connection the Universe Bounce energy density is fixed by the initial conditions on the prepared wavepacket. As a phenomenological implication, we introduce particle creation as a dissipative term and study the production of entropy in the two formulations. Then, we compare the obtained dynamics with what emerges in Loop Quantum Cosmology, where the same difference in the nature of the Big Bounce is associated to fixing a minimum area eigenvalue in a comoving or in a physical representation. We conclude that the privileged character of the Ashtekar-Barbero-Immirzi connection suggests that the natural scenario in the polymer framework is a Big Bounce that is not a Universal cut-off. However, by a parallelism between the polymer and Loop Quantum Cosmology properties of the basic operators, we also develop some considerations in favour of the viability of the

\bar{μ}

scheme of Loop Quantum Cosmology on a semiclassical level. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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16 pages, 1206 KiB

Open AccessArticle

Cosmological Parameter Estimation Using Current and Future Observations of Strong Gravitational Lensing

by Jing-Zhao Qi, Wei-Hong Hu, Yu Cui, Jing-Fei Zhang and Xin Zhang

Universe 2022, 8(5), 254; https://doi.org/10.3390/universe8050254 - 20 Apr 2022

Cited by 5 | Viewed by 1729

Abstract

The remarkable development of cosmology benefits from the increasingly improved measurements of cosmic distances, including absolute distances and relative distances. In recent years, however, the emerged cosmological tensions have motivated us to explore independent and precise late-universe probes. The two observational effects of strong gravitational lensing (SGL), the velocity dispersions of lens galaxies and the time delays between multiple images can provide measurements of relative and absolute distances, respectively, and their combination makes it possible to break the degeneracies between cosmological parameters and enable tight constraints on them. In this paper, we combine the observed 130 SGL systems with velocity-dispersion measurements and 7 SGL systems with time-delay measurements to constrain dark-energy cosmological models. It is found that the combination of the two effects does not significantly break the degeneracies between cosmological parameters as expected. However, with the simulations of 8000 SGL systems with well-measured velocity dispersions and 55 SGL systems with well-measured time delays based on the forthcoming LSST survey, we find that the combination of two effects can significantly break the parameter degeneracies, and make the constraint precision of cosmological parameters meet the standard of precision cosmology. We conclude that the observations of SGL will become a useful late-universe probe for precisely measuring cosmological parameters. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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8 pages, 266 KiB

Open AccessCommunication

Qubit Clock in Quantum Cosmology

by Yasusada Nambu

Universe 2022, 8(2), 99; https://doi.org/10.3390/universe8020099 - 04 Feb 2022

Viewed by 972

Abstract

We investigate the emergent time scenario in quantum cosmology based on the Page–Wotters approach. Using a quantum cosmological model with a qubit clock, it is demonstrated how the entanglement between the qubit clock and the geometry derives emergence of a time parameter, which defines evolution of the timeless quantum state of the universe. We show that the universe wave function conditioned by a qubit clock obeys the standard Schrödinger equation and the Fisher information for the clock state, which quantifies entanglement between the universe and the clock, and contributes as a negative energy density. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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54 pages, 706 KiB

Open AccessArticle

A Real Scalar Field Unifying the Early Inflation and the Late Accelerating Expansion of the Universe through a Quadratic Equation of State: The Vacuumon

by Pierre-Henri Chavanis

Universe 2022, 8(2), 92; https://doi.org/10.3390/universe8020092 - 31 Jan 2022

Cited by 2 | Viewed by 2032

Abstract

In a previous paper we introduced a cosmological model describing the early inflation, the intermediate decelerated expansion, and the late accelerating expansion of the universe in terms of a single barotropic fluid characterized by a quadratic equation of state. We obtained a scalar [...] Read more.

V (ϕ)

connecting the inflaton potential in the early universe to the quintessence potential in the late universe. This scalar field has later been called the ‘vacuumon’ by other authors, in the context of the Running Vacuum model. In this paper, we study how the scalar field potential is modified by the presence of other cosmic components such as stiff matter, black-body radiation, baryonic matter, and dark matter. We also determine the mass m and the self-interaction constant

λ

of the scalar field given by the second and fourth derivatives of the potential at its extrema. We find that its mass is imaginary in the early universe with a modulus of the order of the Planck mass

M_{P} = {(ℏ c / G)}^{1 / 2} = 1.22 \times 10^{19} GeV / c^{2}

and real in the late universe with a value of the order of the cosmon mass

m_{Λ} = {(Λ ℏ^{2} / c^{4})}^{1 / 2} = 2.08 \times 10^{- 33} eV / c^{2}

predicted by string theory. Although our model is able to describe the evolution of the homogeneous background for all times, it cannot account for the spectrum of fluctuations in the early universe. Indeed, by applying the Hamilton–Jacobi formalism to our model of early inflation, we find that the Hubble hierarchy parameters and the spectral indices lead to severe discrepancies with the observations. This suggests that the vacuumon potential is just an effective classical potential that cannot be directly used to compute the fluctuations in the early universe. A fully quantum field theory may be required to achieve that goal. Finally, we discuss the connection between our model based on a quadratic equation of state and the Running Vacuum model which assumes a variation of the cosmological constant with the Hubble parameter. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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23 pages, 369 KiB

Open AccessArticle

Painlevé–Gullstrand form of the Lense–Thirring Spacetime

by Joshua Baines, Thomas Berry, Alex Simpson and Matt Visser

Universe 2021, 7(4), 105; https://doi.org/10.3390/universe7040105 - 19 Apr 2021

Cited by 33 | Viewed by 1807

Abstract

The standard Lense–Thirring metric is a century-old slow-rotation large-distance approximation to the gravitational field outside a rotating massive body, depending only on the total mass and angular momentum of the source. Although it is not an exact solution to the vacuum Einstein equations, asymptotically the Lense–Thirring metric approaches the Kerr metric at large distances. Herein we shall discuss a specific variant of the standard Lense–Thirring metric, carefully chosen for simplicity, clarity, and various forms of improved mathematical and physical behaviour, (to be more carefully defined in the body of the article). We shall see that this Lense–Thirring variant can be viewed as arising from the linearization of a suitably chosen tetrad representing the Kerr spacetime. In particular, we shall construct an explicit unit-lapse Painlevé–Gullstrand variant of the Lense–Thirring spacetime, one that has flat spatial slices, a very simple and physically intuitive tetrad, and extremely simple curvature tensors. We shall verify that this variant of the Lense–Thirring spacetime is Petrov type I, (so it is not algebraically special), but nevertheless possesses some very straightforward timelike geodesics, (the “rain” geodesics). We shall also discuss on-axis and equatorial geodesics, ISCOs (innermost stable circular orbits) and circular photon orbits. Finally, we wrap up by discussing some astrophysically relevant estimates, and analyze what happens if we extrapolate down to small values of r; verifying that for sufficiently slow rotation we explicitly recover slowly rotating Schwarzschild geometry. This Lense–Thirring variant can be viewed, in its own right, as a “black hole mimic”, of direct interest to the observational astronomy community. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

16 pages, 336 KiB

Open AccessArticle

The Trans-Planckian Censorship Conjecture in Different Frameworks of Viable Inflation

by Bruno Sanna and Lorenzo Sebastiani

Universe 2021, 7(4), 95; https://doi.org/10.3390/universe7040095 - 09 Apr 2021

Cited by 1 | Viewed by 1802

Abstract

We review the recently proposed Trans-Planckian Censorship Conjecture (TCC) that stems from the trans-Planckian problem of cosmological perturbations. We analyze the implications and constraints that the TCC introduces in different frameworks of viable inflation. We revisit the case of slow-roll scalar field inflation [...] Read more.

f (R)

and

f (R, ϕ)

-gravity. Finally, we consider the conjecture in the context of constant-roll scalar field inflation. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

18 pages, 327 KiB

Open AccessArticle

Critical Tidal Currents in General Relativity

by Bahram Mashhoon

Universe 2020, 6(8), 104; https://doi.org/10.3390/universe6080104 - 30 Jul 2020

Cited by 3 | Viewed by 1829 | Correction

Abstract

Relativistic tidal equations are formulated with respect to the rest frame of a central gravitational source and their solutions are studied. The existence of certain relativistic critical tidal currents are thereby elucidated. Specifically, observers that are spatially at rest in the exterior Kerr spacetime are considered in detail; in effect, these fiducial observers define the rest frame of the Kerr source. The general tidal equations for the free motion of test particles are worked out with respect to the Kerr background. The analytic solutions of these equations are investigated and the existence of a tidal acceleration mechanism is emphasized. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

Review

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17 pages, 883 KiB

Open AccessEditor’s ChoiceReview

Low Energy Supersymmetry Confronted with Current Experiments: An Overview

by Fei Wang, Wenyu Wang, Jinmin Yang, Yang Zhang and Bin Zhu

Universe 2022, 8(3), 178; https://doi.org/10.3390/universe8030178 - 12 Mar 2022

Cited by 18 | Viewed by 2506

Abstract

This study provides a brief overview of low energy supersymmetry (SUSY) in light of current experimental constraints, such as collider searches, dark matter searches, and muon

g - 2

measurements. In addition, we survey a variety of low energy supersymmetric models: the phenomenological [...] Read more.

This study provides a brief overview of low energy supersymmetry (SUSY) in light of current experimental constraints, such as collider searches, dark matter searches, and muon

g - 2

measurements. In addition, we survey a variety of low energy supersymmetric models: the phenomenological minimal supersymmetric model (MSSM); the supersymmetric models with cut-off-scale boundary conditions, i.e., the minimal supergravity (mSUGRA) or the constrained MSSM (CMSSM), the gauge mediation of SUSY breaking (GMSB), and the anomaly mediation of SUSY breaking (AMSB), as well as their extensions. The conclusion is that the low energy SUSY can survive all current experimental constraints and remains compelling, albeit suffering from a slight fine-tuning problem. The advanced models such as mSUGRA, GMSB, and AMSB need to be extended if the muon

g - 2

anomaly comes from new physics. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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23 pages, 914 KiB

Open AccessReview

Observational Cosmology with Artificial Neural Networks

by Juan de Dios Rojas Olvera, Isidro Gómez-Vargas and Jose Alberto Vázquez

Universe 2022, 8(2), 120; https://doi.org/10.3390/universe8020120 - 12 Feb 2022

Cited by 8 | Viewed by 3526

Abstract

In cosmology, the analysis of observational evidence is very important when testing theoretical models of the Universe. Artificial neural networks are powerful and versatile computational tools for data modelling and have recently been considered in the analysis of cosmological data. The main goal of this paper is to provide an introduction to artificial neural networks and to describe some of their applications to cosmology. We present an overview on the fundamentals of neural networks and their technical details. Through three examples, we show their capabilities in the modelling of cosmological data, numerical tasks (saving computational time), and the classification of stellar objects. Artificial neural networks offer interesting qualities that make them viable alternatives for data analysis in cosmological research. Full article

(This article belongs to the Special Issue Universe: Feature Papers–Cosmology and Gravitation)

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