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Universe
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New Directions in Gravitational Physics and Cosmology
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New Directions in Gravitational Physics and Cosmology

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

Deadline for manuscript submissions: closed (20 July 2022) | Viewed by 7679

Special Issue Editors

Prof. Dr. Abdel Nasser Tawfik

E-Mail Website
Guest Editor
1. Egyptian Center for Theoretical Physics (ECTP), Juhayna Square of 26th-July-Corridor, Giza 12588, Egypt
2. World Laboratory for Cosmology And Particle Physics (WLCAPP), Cairo 11571, Egypt
Interests: quantum gravity; early universe; cosmological equation of state; cosmological constant problem
Prof. Dr. Tiberiu Harko

E-Mail Website
Guest Editor
Department of Physics, Babes-Bolyai University, 400084 Cluj-Napoca, Romania
Interests: general relativity; modified gravity; cosmology; mathematical physics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Presently, we are witnessing a period of quick and profound change in our understanding of gravitational force, and of the cosmological dynamics, and evolution. Since the important observational discoveries of the late 1990s, our knowledge of the structure and properties of the Universe are quickly increasing. With the arrival of new observational and experimental techniques, new cosmological paradigms did emerge, with the $\Lambda$CDM providing the best fit for the cosmological observations. Hence, it seems that the present-day Universe is dominated by two mysterious components—dark energy and dark matter—the former determining the accelerated expansion of the Universe, while the latter could explain the strange behavior of the galactic rotation curves. However, the nature of the cosmological constant, or of dark energy, remains elusive. Moreover, no direct detection/observation of the dark matter particle has been reported, and the only evidence for its existence is the gravitational interaction with baryonic matter.

Hence, these astronomical observations suggest that, at large astrophysical and cosmological scales, the force of gravity may not behave according to standard general relativity, as derived from the Hilbert–Einstein action, and that a generalization of the Hilbert–Einstein action, either at the geometric level or at the matter level, may be required for a full understanding of the gravitational interaction.

This Special Issue is focused on the extensions of the standard theoretical as well as observational concepts in gravity and cosmology, and can hopefully offer a platform for the presentation and discussion of the latest trends and directions in gravitational physics, as well as in the understanding of the dynamical and structural properties of the Universe. Extensions of general relativity, be they geometrically or physically motivated, may open a new window on the nature of the gravitational interaction. We are looking forward to promoting state-of-the-art research contributions on modified gravity, dark energy, and the cosmological constant problem. Innovative theoretical, observational, or experimental studies about dark matter may shed some light on the properties, or the very existence, of the dark matter particle. The quantum nature of gravity, if any, is still unknown, and new approaches may be necessary to solve this problem. Microphysics and macrophysics may be related via the generalized uncertainty principles, and their study would certainly offer new insights into the quantum nature of the Universe. The experimental study of the gravitational waves could provide a deep view into the interior of stellar type objects or lead to a better understanding of the black hole properties.

We hope that this Special Issue, devoted to new directions in gravitation and cosmology, will serve as a reference for initiating and developing innovative ideas in the fundamental fields of gravitational physics and cosmology.

Prof. Dr. Abdel Nasser Tawfik
Dr. Tiberiu Harko
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

  • modified gravity theories
  • curvature-matter couplings
  • geometric extensions of general relativity
  • gravitational waves
  • quantum cosmology
  • the cosmological constant problem
  • the hubble tension
  • quantum theories of gravity
  • generalized uncertainty relations and their applications

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Published Papers (3 papers)

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22 pages, 361 KiB  
Article
Dynamical Signature: Complex Manifolds, Gauge Fields and Non-Flat Tangent Space
by Sergey Bondarenko
Universe 2022, 8(10), 497; https://doi.org/10.3390/universe8100497 - 22 Sep 2022
Cited by 12 | Viewed by 1677
Abstract
Theoretical possibilities of models of gravity with dynamical signature are discussed. The different scenarios of the signature change are proposed in the framework of Einstein-Cartan gravity. We consider, subsequently, the dynamical signature in the model of the complex manifold with complex coordinates and [...] Read more.
Theoretical possibilities of models of gravity with dynamical signature are discussed. The different scenarios of the signature change are proposed in the framework of Einstein-Cartan gravity. We consider, subsequently, the dynamical signature in the model of the complex manifold with complex coordinates and complex metrics are introduced, a complexification of the manifold and coordinates through new gauge fields, an additional gauge symmetry for the Einstein-Cartan vierbein fields, and non-flat tangent space for the metric in the Einstein-Cartan gravity. A new small parameter, which characterizes a degree of the deviation of the signature from the background one, is introduced in all models. The zero value of this parameter corresponds to the signature of an initial background metric. In turn, in the models with gauge fields present, this parameter represents a coupling constant of the gauge symmetry group. The mechanism of metric determination through induced gauge fields with defined signatures in the corresponding models is considered. The ways of the signature change through the gauge field dynamics are reviewed, and the consequences and applications of the proposed ideas are discussed as well. Full article
(This article belongs to the Special Issue New Directions in Gravitational Physics and Cosmology)
12 pages, 388 KiB  
Article
A New Analytic Approximation of Luminosity Distance in Cosmology Using the Parker–Sochacki Method
by Joseph Sultana
Universe 2022, 8(6), 300; https://doi.org/10.3390/universe8060300 - 26 May 2022
Viewed by 1787
Abstract
The luminosity distance dL is possibly the most important distance scale in cosmology and therefore accurate and efficient methods for its computation is paramount in modern precision cosmology. Yet in most cosmological models the luminosity distance cannot be expressed by a simple [...] Read more.
The luminosity distance dL is possibly the most important distance scale in cosmology and therefore accurate and efficient methods for its computation is paramount in modern precision cosmology. Yet in most cosmological models the luminosity distance cannot be expressed by a simple analytic function in terms of the redshift z and the cosmological parameters, and is instead represented in terms of an integral. Although one can revert to numerical integration techniques utilizing quadrature algorithms to evaluate such an integral, the high accuracy required in modern cosmology makes this a computationally demanding process. In this paper, we use the Parker–Sochacki method (PSM) to generate a series approximate solution for the luminosity distance in spatially flat ΛCDM cosmology by solving a polynomial system of nonlinear differential equations. When compared with other techniques proposed recently, which are mainly based on the Padé approximant, the expression for the luminosity distance obtained via the PSM leads to a significant improvement in the accuracy in the redshift range 0≤z≤2.5. Moreover, we show that this technique can be easily applied to other more complicated cosmological models, and its multistage approach can be used to generate analytic approximations that are valid on a wider redshift range. Full article
(This article belongs to the Special Issue New Directions in Gravitational Physics and Cosmology)
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16 pages, 1258 KiB  
Article
Is the Hubble Crisis Connected with the Extinction of Dinosaurs?
by Leandros Perivolaropoulos
Universe 2022, 8(5), 263; https://doi.org/10.3390/universe8050263 - 26 Apr 2022
Cited by 26 | Viewed by 3198
Abstract
It has recently been suggested that a gravitational transition of the effective Newton’s constant Geff by about 10%, 50–150 Myrs ago could lead to the resolution of both the Hubble crisis and the growth tension of the standard ΛCDM model. Hints [...] Read more.
It has recently been suggested that a gravitational transition of the effective Newton’s constant Geff by about 10%, 50–150 Myrs ago could lead to the resolution of both the Hubble crisis and the growth tension of the standard ΛCDM model. Hints for such an abrupt transition with weaker gravity at times before the transition, have recently been identified in Tully–Fisher galactic mass-velocity data, and also in Cepheid SnIa calibrator data. Here we use Monte-Carlo simulations to show that such a transition could significantly increase (by a factor of 3 or more) the number of long period comets (LPCs) impacting the solar system from the Oort cloud (semi-major axis of orbits ≳104AU). This increase is consistent with observational evidence from the terrestrial and lunar cratering rates, indicating that the impact flux of kilometer sized objects increased by at least a factor of 2 over that last 100 Myrs compared to the long term average. This increase may also be connected with the Chicxulub impactor event that produced the Cretaceous–Tertiary (K-T) extinction of 75% of life on Earth (including dinosaurs) about 66 Myrs ago. We use Monte-Carlo simulations to show that for isotropic Oort cloud comet distribution with initially circular orbits, random velocity perturbations (induced e.g., by passing stars and/or galactic tidal effects), lead to a deformation of the orbits that increases significantly when Geff increases. A 10% increase in Geff leads to an increase in the probability of the comets to enter the loss cone and reach the planetary region (pericenter of less than 10 AU) by a factor that ranges from 5% (for velocity perturbation much smaller than the comet initial velocity) to more than 300% (for total velocity perturbations comparable with the initial comet velocity). Full article
(This article belongs to the Special Issue New Directions in Gravitational Physics and Cosmology)
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