Alternative Gravities and Fundamental Cosmology

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

Deadline for manuscript submissions: closed (28 February 2022) | Viewed by 34972

Special Issue Editors


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Guest Editor
Szczecin Cosmology Group, Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland
Interests: general relativity and cosmology; alternative gravities; superstring and brane cosmology; quantum cosmology; standard and non-standard cosmological singularities; varying constants (theory, observations, experiment); superstring landscape and the multiverse; quantum gravity phenomenology; cosmological and black hole horizon entropy
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E-Mail Website
Guest Editor
Szczecin Cosmology Group, Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland
Interests: inhomogeneous universes; dark energy; varying fundamental constants; modified gravity; braneworld cosmology; quantum cosmology

E-Mail Website
Guest Editor
Szczecin Cosmology Group, Institute of Physics, University of Szczecin, Wielkopolska 15, 70-451 Szczecin, Poland
Interests: observational cosmology; variability of the physical constants (theory and observations); alternative gravities; dark energy

Special Issue Information

Dear Colleagues,

This Special Issue will gather together works presented at the conference “Alternative Gravities and Fundamental Cosmology” (Altecosmofun'21), which will take place in Szczecin, Poland, 6–10 September, 2021. This is the fifth of the fundamental cosmology conferences organized by the Cosmology Group at the University of Szczecin (after Cosmofun’2005, Grasscosmofun’09, Multicosmofun’12, Varcosmofun'16). The conference was originally planned for September 2020, but because of the pandemic, it was shifted and rescheduled to be held online one year later.

The task of the conference is to bring together specialists dealing with many modifications of Einstein’s general relativity—something we call “alternative gravities”. The number of theories which go beyond Einstein is growing, and this growth is motivated by the mystery of dark energy, the phenomenology of quantum gravity, and fundamental theories such as superstring, Brane, and M-theory. This conference is aimed towards all them, and participants who want to exchange current ideas on these topics are strongly encouraged to participate. Renowned specialists in the field have confirmed their attendance. Among them is Paul C.W. Davies, Astrid Eichhorn, Lavinia Heisenberg, Alexei Starobinsky, and many others have confirmed their attendance. The program of the conference will contain four invited plenary morning sessions and five contributed paper afternoon sessions. In addition, a special Doctoral Students Session during which students will be able to present and discuss actively their research will be held one morning.

This Special Issue of Universe will contain longer review articles by invited speakers, as well as selected by the Scientific Committee, short papers from contributed and student sessions speakers.

For more information about the conference, see: https://indico.cern.ch/event/873762

Prof. Dr. Mariusz P. Dąbrowski
Dr. Adam Balcerzak
Dr. Vincenzo Salzano
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

  • alternative gravities
  • dark energy
  • dark matter
  • gravitational waves
  • compact objects
  • quantum gravity as alternative gravity
  • quantum gravity phenomenology: GUP
  • Lorentz violation
  • varying constants
  • fundamental theories: strings, branes

Published Papers (19 papers)

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Research

Jump to: Review

15 pages, 542 KiB  
Article
Avoiding Bias in Measurements of Fundamental Constants from High Resolution Quasar Spectra
by John K. Webb, Chung-Chi Lee and Dinko Milaković
Universe 2022, 8(5), 266; https://doi.org/10.3390/universe8050266 - 27 Apr 2022
Cited by 8 | Viewed by 1390
Abstract
Recent advances in spectroscopic instrumentation and calibration methods dramatically improve the quality of quasar spectra. Supercomputer calculations show that, at high spectral resolution, procedures used in some previous analyses of spacetime variations of fundamental constants are likely to generate spurious measurements, biased systematically [...] Read more.
Recent advances in spectroscopic instrumentation and calibration methods dramatically improve the quality of quasar spectra. Supercomputer calculations show that, at high spectral resolution, procedures used in some previous analyses of spacetime variations of fundamental constants are likely to generate spurious measurements, biased systematically towards a null result. Developments in analysis methods are also summarised and a prescription given for the analysis of new and forthcoming data. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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30 pages, 405 KiB  
Article
Ultraviolet Finiteness or Asymptotic Safety in Higher Derivative Gravitational Theories
by Lesław Rachwał
Universe 2022, 8(4), 229; https://doi.org/10.3390/universe8040229 - 08 Apr 2022
Viewed by 1352
Abstract
We present and discuss well known conditions for ultraviolet finiteness and asymptotic safety. The requirements for complete absence of ultraviolet divergences in quantum field theories and existence of a non-trivial fixed point for renormalization group flow in the ultraviolet regime are compared based [...] Read more.
We present and discuss well known conditions for ultraviolet finiteness and asymptotic safety. The requirements for complete absence of ultraviolet divergences in quantum field theories and existence of a non-trivial fixed point for renormalization group flow in the ultraviolet regime are compared based on the example of a six-derivative quantum gravitational theory in d=4 spacetime dimensions. In this model, it is possible for the first time to have fully UV-finite quantum theory without adding matter or special symmetry, but by inclusion of additional terms cubic in curvatures. We comment on similarities and some apparent differences between the two approaches, but we show that they are both compatible to each other. Finally, we motivate the claim that actually asymptotic safety needs UV-finite models for providing explicit form of the ultraviolet limit of Wilsonian effective actions describing special situations at fixed points. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
13 pages, 1018 KiB  
Article
Noncommutativity, Sáez–Ballester Theory and Kinetic Inflation
by S. M. M. Rasouli
Universe 2022, 8(3), 165; https://doi.org/10.3390/universe8030165 - 07 Mar 2022
Cited by 8 | Viewed by 1454
Abstract
This paper presents a noncommutative (NC) version of an extended Sáez–Ballester (SB) theory. Concretely, considering the spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) metric, we propose an appropriate dynamical deformation between the conjugate momenta and, applying the Hamiltonian formalism, obtain deformed equations of motion. In our [...] Read more.
This paper presents a noncommutative (NC) version of an extended Sáez–Ballester (SB) theory. Concretely, considering the spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) metric, we propose an appropriate dynamical deformation between the conjugate momenta and, applying the Hamiltonian formalism, obtain deformed equations of motion. In our model, the NC parameter appears linearly in the deformed Poisson bracket and the equations of the NC SB cosmology. When it goes to zero, we get the corresponding commutative counterparts. Even by restricting our attention to a particular case, where there is neither an ordinary matter nor a scalar potential, we show that the effects of the noncommutativity provide interesting results: applying numerical endeavors for very small values of the NC parameter, we show that (i) at the early times of the universe, there is an inflationary phase with a graceful exit, for which the relevant nominal condition is satisfied; (ii) for the late times, there is a zero acceleration epoch. By establishing an appropriate dynamical framework, we show that the results (i) and (ii) can be obtained for many sets of the initial conditions and the parameters of the model. Finally, we indicate that, at the level of the field equations, one may find a close resemblance between our NC model and the Starobinsky inflationary model. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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8 pages, 365 KiB  
Communication
Late Time Acceleration of the Universe from Quantum Gravity
by Xiankai Pang
Universe 2022, 8(3), 163; https://doi.org/10.3390/universe8030163 - 05 Mar 2022
Cited by 1 | Viewed by 1380
Abstract
We show that the accelerating expansion phase of the universe can emerge from the group field theory formalism, a candidate theory of quantum gravity. The cosmological evolution can be extracted from condensate states using the mean field approximation, in a form of modified [...] Read more.
We show that the accelerating expansion phase of the universe can emerge from the group field theory formalism, a candidate theory of quantum gravity. The cosmological evolution can be extracted from condensate states using the mean field approximation, in a form of modified FLRW equations. By introducing an effective equation of state w, we can reveal the relevant features of the evolution and show that, with the proper choice of the parameters, w will approach 1, leading to an accelerating phase dominated by the cosmological constant effectively. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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9 pages, 281 KiB  
Article
Bouncing Cosmology in Fourth-Order Gravity
by Marcello Miranda, Daniele Vernieri, Salvatore Capozziello and Francisco S. N. Lobo
Universe 2022, 8(3), 161; https://doi.org/10.3390/universe8030161 - 04 Mar 2022
Cited by 3 | Viewed by 1391
Abstract
The Big Bang initial singularity problem can be solved by means of bouncing solutions. In the context of extended theories of gravity, we will look for covariant effective actions whose field equations contain up to fourth-order derivatives of the metric tensor. In finding [...] Read more.
The Big Bang initial singularity problem can be solved by means of bouncing solutions. In the context of extended theories of gravity, we will look for covariant effective actions whose field equations contain up to fourth-order derivatives of the metric tensor. In finding such bouncing solutions, we will make use of an order reduction technique based on a perturbative approach. Reducing the order of the field equations to second-order, we are able to find solutions which are perturbatively close to General Relativity. We will build the covariant effective actions of the resulting order reduced theories. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
11 pages, 471 KiB  
Article
Testing Screening Mechanisms with Mass Profiles of Galaxy Clusters
by Lorenzo Pizzuti
Universe 2022, 8(3), 157; https://doi.org/10.3390/universe8030157 - 01 Mar 2022
Cited by 1 | Viewed by 1451
Abstract
We present MG-MAMPOSSt, a license-free code to constrain modified gravity models by reconstructing the mass profile of galaxy clusters with the kinematics of the cluster’s member galaxies. We describe the main features of the code and we show the capability of the [...] Read more.
We present MG-MAMPOSSt, a license-free code to constrain modified gravity models by reconstructing the mass profile of galaxy clusters with the kinematics of the cluster’s member galaxies. We describe the main features of the code and we show the capability of the method when the kinematic information is combined with lensing data. We discuss recent results and forecasts on two classes of models currently implemented in the code, characterized by different screening mechanisms, namely, chameleon and Vainshtein screening. We further explore the impact of possible systematics in view of application to the data from upcoming surveys. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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6 pages, 281 KiB  
Communication
Can Dark Energy Emerge from a Varying G and Spacetime Geometry?
by Ekim Taylan Hanımeli, Isaac Tutusaus, Brahim Lamine and Alain Blanchard
Universe 2022, 8(3), 148; https://doi.org/10.3390/universe8030148 - 25 Feb 2022
Cited by 2 | Viewed by 1624
Abstract
The accelerated expansion of the universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. We will discuss an alternative gravity [...] Read more.
The accelerated expansion of the universe implies the existence of an energy contribution known as dark energy. Associated with the cosmological constant in the standard model of cosmology, the nature of this dark energy is still unknown. We will discuss an alternative gravity model in which this dark energy contribution emerges naturally, as a result of allowing for a time-dependence on the gravitational constant, G, in Einstein’s field equations. With this modification, Bianchi’s identities require an additional tensor field to be introduced so that the usual conservation equation for matter and radiation is satisfied. The equation of state of this tensor field is obtained using additional constraints, coming from the assumption that this tensor field represents the space-time response to the variation of G. We will also present the predictions of this model for the late-universe data, and show that the energy contribution of this new tensor is able to explain the accelerated expansion of the universe without the addition of a cosmological constant. Unlike many other alternative gravities with varying gravitational strength, the predicted G evolution is also consistent with local observations and therefore this model does not require screening. We will finish by discussing possible other implications this approach might have for cosmology and some future prospects. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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12 pages, 391 KiB  
Article
Gravastars with Kuchowicz Metric in Energy-Momentum Squared Gravity
by Saba Naz and Muhammad Sharif
Universe 2022, 8(3), 142; https://doi.org/10.3390/universe8030142 - 22 Feb 2022
Cited by 11 | Viewed by 1531
Abstract
This paper investigates the geometry of a gravitational vacuum star (also known as a gravastar) from the perspective of f(R,T2) gravity. The gravastar can be treated as a black hole substitute with three domains: (i) the inner [...] Read more.
This paper investigates the geometry of a gravitational vacuum star (also known as a gravastar) from the perspective of f(R,T2) gravity. The gravastar can be treated as a black hole substitute with three domains: (i) the inner domain, (ii) the intrinsic shell, and (iii) the outer domain. We examine these geometries using Kuchowicz ansatz for temporal metric function corresponding to a specific f(R,T2) model. We compute a nonsingular radial metric potential for both the interior and intermediate domains. The matching of these domains with exterior Schwarzschild vacuum results in boundary conditions that assist in the evaluation of unknown constants. Finally, we examine various attributes of gravastar domains, such as the equation of state parameter, proper length, energy, and surface redshift. We conclude that the gravastar model is a viable alternative to the black hole in the background of this gravity. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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7 pages, 365 KiB  
Communication
Constraining MOdified Gravity with the S2 Star
by Riccardo Della Monica, Ivan de Martino and Mariafelicia de Laurentis
Universe 2022, 8(2), 137; https://doi.org/10.3390/universe8020137 - 21 Feb 2022
Cited by 9 | Viewed by 1419
Abstract
We have used publicly available kinematic data for the S2 star to constrain the parameter space of MOdified Gravity. Integrating geodesics and using a Markov Chain Monte Carlo algorithm, we have provided the first constraint on the scales of the Galactic Centre for [...] Read more.
We have used publicly available kinematic data for the S2 star to constrain the parameter space of MOdified Gravity. Integrating geodesics and using a Markov Chain Monte Carlo algorithm, we have provided the first constraint on the scales of the Galactic Centre for the parameter α of the theory, which represents the fractional increment of the gravitational constant G with respect to its Newtonian value. Namely, α0.662 at 99.7% confidence level (where α=0 reduces the theory to General Relativity). Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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20 pages, 2876 KiB  
Article
Study of Decoupled Anisotropic Solutions in f(R, T, RρηTρη) Theory
by Tayyab Naseer and Muhammad Sharif
Universe 2022, 8(2), 62; https://doi.org/10.3390/universe8020062 - 19 Jan 2022
Cited by 36 | Viewed by 1654
Abstract
In this paper, we consider isotropic solution and extend it to two different exact well-behaved spherical anisotropic solutions through minimal geometric deformation method in f(R,T,RρηTρη) gravity. We only deform the radial [...] Read more.
In this paper, we consider isotropic solution and extend it to two different exact well-behaved spherical anisotropic solutions through minimal geometric deformation method in f(R,T,RρηTρη) gravity. We only deform the radial metric component that separates the field equations into two sets corresponding to their original sources. The first set corresponds to perfect matter distribution while the other set exhibits the effects of additional source, i.e., anisotropy. The isotropic system is resolved by assuming the metric potentials proposed by Krori-Barua while the second set needs one constraint to be solved. The physical acceptability and consistency of the obtained solutions are analyzed through graphical analysis of effective matter components and energy bounds. We also examine mass, surface redshift and compactness of the resulting solutions. For particular values of the decoupling parameter, our both solutions turn out to be viable and stable. We conclude that this curvature-matter coupling gravity provides more stable solutions corresponding to a self-gravitating geometry. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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16 pages, 800 KiB  
Article
Vacuum Energy in Saez-Ballester Theory and Stabilization of Extra Dimensions
by Pheiroijam Suranjoy Singh and Kangujam Priyokumar Singh
Universe 2022, 8(2), 60; https://doi.org/10.3390/universe8020060 - 18 Jan 2022
Cited by 5 | Viewed by 1865
Abstract
In this work, we study a spherically symmetric metric in 5D within the framework of Saez-Ballester Theory, where minimal dark energy-matter interaction occurs. We predict that the expanding isotropic universe will be progressively DE dominated. We estimate few values of the deceleration parameter, [...] Read more.
In this work, we study a spherically symmetric metric in 5D within the framework of Saez-Ballester Theory, where minimal dark energy-matter interaction occurs. We predict that the expanding isotropic universe will be progressively DE dominated. We estimate few values of the deceleration parameter, very close to the recently predicted values. We obtain the value of the DE EoS parameter as ω=1. Additionally, we measure the value of the overall density parameter as Ω=0.97(1), in line with the notion of a close to or nearly (not exactly) flat universe. We predict that the model universe starts with the Big-Bang and ends at the Big Freeze singularity. In general, we cannot find conditions for stabilization of extra dimensions in general relativity, and all dimensions want to be dynamical. Here, we present two possible conditions to solve this stabilization problem in general relativity. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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10 pages, 1177 KiB  
Communication
One-Point Statistics Matter in Extended Cosmologies
by Alex Gough and Cora Uhlemann
Universe 2022, 8(1), 55; https://doi.org/10.3390/universe8010055 - 17 Jan 2022
Cited by 4 | Viewed by 1864
Abstract
The late universe contains a wealth of information about fundamental physics and gravity, wrapped up in non-Gaussian fields. To make use of as much information as possible, it is necessary to go beyond two-point statistics. Rather than going to higher-order N-point correlation [...] Read more.
The late universe contains a wealth of information about fundamental physics and gravity, wrapped up in non-Gaussian fields. To make use of as much information as possible, it is necessary to go beyond two-point statistics. Rather than going to higher-order N-point correlation functions, we demonstrate that the probability distribution function (PDF) of spheres in the matter field (a one-point function) already contains a significant amount of this non-Gaussian information. The matter PDF dissects different density environments which are lumped together in two-point statistics, making it particularly useful for probing modifications of gravity or expansion history. Our approach in Cataneo et al. 2021 extends the success of Large Deviation Theory for predicting the matter PDF in ΛCDM in these “extended” cosmologies. A Fisher forecast demonstrates the information content in the matter PDF via constraints for a Euclid-like survey volume combining the 3D matter PDF with the 3D matter power spectrum. Adding the matter PDF halves the uncertainties on parameters in an evolving dark energy model, relative to the power spectrum alone. Additionally, the matter PDF contains enough non-linear information to substantially increase the detection significance of departures from General Relativity, with improvements up to six times the power spectrum alone. This analysis demonstrates that the matter PDF is a promising non-Gaussian statistic for extracting cosmological information, particularly for beyond ΛCDM models. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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17 pages, 2366 KiB  
Article
Estimating the Cosmological Constant from Shadows of Kerr–de Sitter Black Holes
by Misba Afrin and Sushant G. Ghosh
Universe 2022, 8(1), 52; https://doi.org/10.3390/universe8010052 - 16 Jan 2022
Cited by 15 | Viewed by 2180
Abstract
The Event Horizon Telescope collaboration has revealed the first direct image of a black hole, as per the shadow of a Kerr black hole of general relativity. However, other Kerr-like rotating black holes of modified gravity theories cannot be ignored, and they are [...] Read more.
The Event Horizon Telescope collaboration has revealed the first direct image of a black hole, as per the shadow of a Kerr black hole of general relativity. However, other Kerr-like rotating black holes of modified gravity theories cannot be ignored, and they are essential as they offer an arena in which these theories can be tested through astrophysical observation. This motivates us to investigate asymptotically de Sitter rotating black holes wherein interpreting the cosmological constant Λ as the vacuum energy leads to a deformation in the vicinity of a black hole—new Kerr–de Sitter solution, which has a richer geometric structure than the original one. We derive an analytical formula necessary for the shadow of the new Kerr–de Sitter black holes and then visualize the shadow of black holes for various parameters for an observer at given coordinates (r0,θ0) in the domain (r0,rc) and estimate the cosmological constant Λ from its shadow observables. The shadow observables of the new Kerr–de Sitter black holes significantly deviate from the corresponding observables of the Kerr–de Sitter black hole over an appreciable range of the parameter space. Interestingly, we find a finite parameter space for (Λ, a) where the observables of the two black holes are indistinguishable. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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Review

Jump to: Research

11 pages, 315 KiB  
Review
Moffat MOdified Gravity (MOG)
by Sreekanth Harikumar
Universe 2022, 8(5), 259; https://doi.org/10.3390/universe8050259 - 24 Apr 2022
Viewed by 1754
Abstract
Scalar Tensor Vector Gravity (STVG) or MOdified Gravity (MOG) is a metric theory of gravity with dynamical scalar fields and a massive vector field introduced in addition to the metric tensor. In the weak field approximation, MOG modifies the Newtonian acceleration with a [...] Read more.
Scalar Tensor Vector Gravity (STVG) or MOdified Gravity (MOG) is a metric theory of gravity with dynamical scalar fields and a massive vector field introduced in addition to the metric tensor. In the weak field approximation, MOG modifies the Newtonian acceleration with a Yukawa-like repulsive term due to a Maxwell–Proca type Lagrangian. This associates matter with a fifth force and a modified equation of motion. MOG has been successful in explaining galaxy rotation curves, cosmological observations and all other solar system observations without the need for dark matter. In this article, we discuss the key concepts of MOG theory. Then, we discuss existing observational bounds on MOG weak field parameters. In particular, we will present our original results obtained from the X-COP sample of galaxy clusters. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
22 pages, 5361 KiB  
Review
How the Big Bang Ends Up Inside a Black Hole
by Enrique Gaztanaga
Universe 2022, 8(5), 257; https://doi.org/10.3390/universe8050257 - 21 Apr 2022
Cited by 12 | Viewed by 3452
Abstract
The standard model of cosmology assumes that our Universe began 14 Gyrs (billion years) ago from a singular Big Bang creation. This can explain a vast range of different astrophysical data from a handful of free cosmological parameters. However, we have no direct [...] Read more.
The standard model of cosmology assumes that our Universe began 14 Gyrs (billion years) ago from a singular Big Bang creation. This can explain a vast range of different astrophysical data from a handful of free cosmological parameters. However, we have no direct evidence or fundamental understanding of some key assumptions: Inflation, Dark Matter and Dark Energy. Here we review the idea that cosmic expansion originates instead from gravitational collapse and bounce. The collapse generates a Black Hole (BH) of mass M5×1022M that formed 25 Gyrs ago. As there is no pressure support, the cold collapse can continue inside in free fall until it reaches atomic nuclear saturation (GeV), when is halted by Quantum Mechanics, as two particles cannot occupy the same quantum state. The collapse then bounces like a core-collapse supernovae, producing the Big Bang expansion. Cosmic acceleration results from the BH event horizon. During collapse, perturbations exit the horizon to re-enter during expansion, giving rise to the observed universe without the need for Inflation or Dark Energy. Using Ockham’s razor, this makes the BH Universe (BHU) model more compelling than the standard singular Big Bang creation. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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12 pages, 1079 KiB  
Review
The Scale Invariant Vacuum Paradigm: Main Results and Current Progress
by Vesselin G. Gueorguiev and Andre Maeder
Universe 2022, 8(4), 213; https://doi.org/10.3390/universe8040213 - 27 Mar 2022
Cited by 2 | Viewed by 1583
Abstract
We present a summary of the main results within the Scale Invariant Vacuum (SIV) paradigm as related to the Weyl Integrable Geometry (WIG) as an extension to the standard Einstein General Relativity (EGR). After a brief review of the mathematical framework, we will [...] Read more.
We present a summary of the main results within the Scale Invariant Vacuum (SIV) paradigm as related to the Weyl Integrable Geometry (WIG) as an extension to the standard Einstein General Relativity (EGR). After a brief review of the mathematical framework, we will highlight the main results related to inflation within the SIV, the growth of the density fluctuations, and the application of the SIV to scale-invariant dynamics of galaxies, MOND, dark matter, and the dwarf spheroidals. The possible connection between the weak-field SIV equations and the notion of un-proper time parametrization within the reparametrization paradigm is also discussed. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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17 pages, 841 KiB  
Review
Compact Objects in Alternative Gravities
by Jose Luis Blázquez-Salcedo, Burkhard Kleihaus and Jutta Kunz
Universe 2022, 8(3), 153; https://doi.org/10.3390/universe8030153 - 28 Feb 2022
Cited by 6 | Viewed by 2142
Abstract
We address neutron stars and black holes in alternative gravities, after recalling their basic properties in General Relativity. Among the plethora of interesting alternative gravities we here focus on an interesting set of scalar-tensor theories. We discuss the phenomenon of spontaneous scalarization, that [...] Read more.
We address neutron stars and black holes in alternative gravities, after recalling their basic properties in General Relativity. Among the plethora of interesting alternative gravities we here focus on an interesting set of scalar-tensor theories. We discuss the phenomenon of spontaneous scalarization, that is, matter induced for neutron stars and curvature induced for black holes. Along with other relevant physical properties, we address the quasi-normal modes of these compact objects. In particular, we consider universal relations of neutron stars to largely reduce the dependence on the equation of state, and we briefly address the shadow of black holes. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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13 pages, 323 KiB  
Review
Graviton Mass in the Era of Multi-Messenger Astronomy
by Aleksandra Piórkowska-Kurpas
Universe 2022, 8(2), 83; https://doi.org/10.3390/universe8020083 - 27 Jan 2022
Cited by 3 | Viewed by 1635
Abstract
The idea of massive graviton plays a fundamental role in modern physics as a landmark of most scenarios related to modified gravity theories. Limits on graviton mass can be obtained through different methods, using all the capabilities of multi-messenger astronomy available today. In [...] Read more.
The idea of massive graviton plays a fundamental role in modern physics as a landmark of most scenarios related to modified gravity theories. Limits on graviton mass can be obtained through different methods, using all the capabilities of multi-messenger astronomy available today. In this paper, we consider some emerging opportunities. In particular, modified relativistic dispersion relations of massive gravitons may lead to changes in the travel time of gravitational waves (GWs) emitted from distant astrophysical objects. Strong gravitational lensing of signals from a carefully selected class of extra-galactic sources such as compact object binaries (actually, binary neutron stars) is predicted to play an important role in this context. Comparing time delays between images of the lensed GW signal and its electromagnetic (EM) counterpart may be a new model-independent strategy (proposed by us in X.-L. Fan et al., 2017), which is especially promising in light of the fruitful observing runs of interferometric GW detectors, resulting in numerous GW signals. In addition to this direct, kinematic method, one can use an indirect, static method. In this approach, the non-zero graviton mass would modify estimates of the total cluster mass via a Yukawa term, influencing the Newtonian potential. In A. Piórkowska-Kurpas et al., 2022, using the X-COP galaxy cluster sample, we obtained mg<(4.996.79)×1029 eV (at 95% C.L.), which is one of the best available constraints. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
11 pages, 352 KiB  
Review
Polynomial Affine Model of Gravity in Three-Dimensions
by Oscar Castillo-Felisola, Bastian Grez, Oscar Orellana, Jose Perdiguero, Francisca Ramirez, Aureliano Skirzewski and Alfonso R. Zerwekh
Universe 2022, 8(2), 68; https://doi.org/10.3390/universe8020068 - 23 Jan 2022
Cited by 3 | Viewed by 1850
Abstract
In this work, we explore a three-dimensional formulation of the polynomial affine model of gravity, which is a model that extends general relativity by relaxing the equivalence principle through the exclusion of the metric from the set of fundamental fields. In particular, in [...] Read more.
In this work, we explore a three-dimensional formulation of the polynomial affine model of gravity, which is a model that extends general relativity by relaxing the equivalence principle through the exclusion of the metric from the set of fundamental fields. In particular, in an attempt to gain insight of the role of the torsion and nonmetricity in the gravitational models, we consider homogeneous and isotropic cosmological models, for which their solutions are classified in a decisions tree. We also show a few of these explicit solutions that allow the definition of (alternative/emergent) metrics derived from the connection. Full article
(This article belongs to the Special Issue Alternative Gravities and Fundamental Cosmology)
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