Special Issue "Universe: Feature Papers–Cosmology and Gravitation"

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

Deadline for manuscript submissions: closed (31 December 2021).

Special Issue Editors

Prof. Dr. Antonino Del Popolo
E-Mail Website
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 papers will be 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 (3 papers)

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Research

Article
Painlevé–Gullstrand form of the Lense–Thirring Spacetime
Universe 2021, 7(4), 105; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7040105 - 19 Apr 2021
Cited by 13 | Viewed by 562
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, [...] Read more.
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)
Article
The Trans-Planckian Censorship Conjecture in Different Frameworks of Viable Inflation
Universe 2021, 7(4), 95; https://0-doi-org.brum.beds.ac.uk/10.3390/universe7040095 - 09 Apr 2021
Cited by 1 | Viewed by 449
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.
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 and we investigate the cases of slow-roll 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)
Article
Critical Tidal Currents in General Relativity
Universe 2020, 6(8), 104; https://0-doi-org.brum.beds.ac.uk/10.3390/universe6080104 - 30 Jul 2020
Cited by 3 | Viewed by 778 | 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 [...] Read more.
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)
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