Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (29 February 2020) | Viewed by 19234

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Special Issue Information

Dear Colleagues,

Cosmology and particle physics are complementary frameworks based on two standard paradigms with several intertwined assumptions. Unfortunately, all attempts to unify the aforementioned scenarios have so far been unsuccessful and a wide number of challenges remain open. Examples include unifying gravity with quantum mechanics, relating quantum field theory to the LCDM model, searching for dark matter candidates, laboratory measurement of particles of dark matter, etc.

In lieu of considering separate approaches to addressing these issues, a possible landscape could provide an explanation for current universe dynamics at different epochs in terms of quantum physics, including the presence of dark matter as proof of a possible new physics. Thus, extensions of the models, among them extended theories of gravity, supersymmetry, string theories, dark fluids, etc., would represent possible alternatives to formulating a self-consistent and definitive theory of everything.

The main subject of this issue: "Theories of gravity: alternatives to the cosmological and particle standard models", is thus widely formulated to include several topics. The issue deals with alternative paradigms to cosmology, quantum field theory, extensions of the standard model of particle physics, dark energy and dark matter from the point of view of particle physics, etc. The target lies beyond the usual LCDM model, passing through quantum field theory applied to gravity.

As a consequence, the issue represents the interplay between particle physics and cosmology, with particular emphasis on the role played by particle quantum field theory in the early and the late universe.

Papers based on such topics are therefore warmly welcome for submission.

Prof. Dr. Stefano Bellucci
Prof. Dr. Orlando Luongo
Guest Editors

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Keywords

  • Particle dark matter
  • Quantum cosmology
  • Extended theories of gravity
  • Phenomenology of dark energy
  • Dark fluids and extensions of the ΛCDM paradigm
  • Supersymmetry
  • String theories and cosmology of strings
  • Inflation and effective field representation
  • Measurable quantum corrections
  • Entanglement in the early universe

Published Papers (5 papers)

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Research

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33 pages, 3142 KiB  
Article
The Scale-Invariant Vacuum (SIV) Theory: A Possible Origin of Dark Matter and Dark Energy
by Andre Maeder and Vesselin G. Gueorguiev
Universe 2020, 6(3), 46; https://doi.org/10.3390/universe6030046 - 18 Mar 2020
Cited by 9 | Viewed by 3796
Abstract
The Scale Invariant Vacuum (SIV) theory rests on the basic hypothesis that the macroscopic empty space is scale invariant. This hypothesis is applied in the context of the Integrable Weyl Geometry, where it leads to considerable simplifications in the scale covariant cosmological equations. [...] Read more.
The Scale Invariant Vacuum (SIV) theory rests on the basic hypothesis that the macroscopic empty space is scale invariant. This hypothesis is applied in the context of the Integrable Weyl Geometry, where it leads to considerable simplifications in the scale covariant cosmological equations. After an initial explosion and a phase of braking, the cosmological models show a continuous acceleration of the expansion. Several observational tests of the SIV cosmology are performed: on the relation between H 0 and the age of the Universe, on the m z diagram for SNIa data and its extension to z = 7 with quasars and GRBs, and on the H ( z ) vs. z relation. All comparisons show a very good agreement between SIV predictions and observations. Predictions for the future observations of the redshift drifts are also given. In the weak field approximation, the equation of motion contains, in addition to the classical Newtonian term, an acceleration term (usually very small) depending on the velocity. The two-body problem is studied, showing a slow expansion of the classical conics. The new equation has been applied to clusters of galaxies, to rotating galaxies (some proximities with Modifies Newtonian Dynamics, MOND, are noticed), to the velocity dispersion vs. the age of the stars in the Milky Way, and to the growth of the density fluctuations in the Universe. We point out the similarity of the mechanical effects of the SIV hypothesis in cosmology and in the Newtonian approximation. In both cases, it results in an additional acceleration in the direction of motions. In cosmology, these effects are currently interpreted in terms of the dark energy hypothesis, while in the Newtonian approximation they are accounted for in terms of the dark matter (DM) hypothesis. These hypotheses appear no longer necessary in the SIV context. Full article
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24 pages, 656 KiB  
Article
The Dynamical Origin of the Graviton Mass in the Non-Linear Theory of Massive Gravity
by Ivan Arraut
Universe 2019, 5(7), 166; https://doi.org/10.3390/universe5070166 - 6 Jul 2019
Cited by 3 | Viewed by 2370
Abstract
We compare the standard Higgs mechanism corresponding to the scalar field, with the dynamical origin of the graviton mass inside the scenario of the dRGT theory of massive gravity. We demonstrate that the effective mass perceived locally by different observers depends on how [...] Read more.
We compare the standard Higgs mechanism corresponding to the scalar field, with the dynamical origin of the graviton mass inside the scenario of the dRGT theory of massive gravity. We demonstrate that the effective mass perceived locally by different observers depends on how they define the local time with respect to the preferred notion of time defined by the Stückelberg function T 0 ( r , t ) . Full article
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21 pages, 541 KiB  
Article
Thermodynamic Geometry of Yang–Mills Vacua
by Stefano Bellucci and Bhupendra Nath Tiwari
Universe 2019, 5(4), 90; https://doi.org/10.3390/universe5040090 - 10 Apr 2019
Cited by 1 | Viewed by 2456
Abstract
We study vacuum fluctuation properties of an ensemble of S U ( N ) gauge theory configurations, in the limit of many colors, viz. N c , and explore the statistical nature of the topological susceptibility by analyzing its critical behavior [...] Read more.
We study vacuum fluctuation properties of an ensemble of S U ( N ) gauge theory configurations, in the limit of many colors, viz. N c , and explore the statistical nature of the topological susceptibility by analyzing its critical behavior at a non-zero-vacuum parameter θ and temperature T. We find that the system undergoes a vacuum phase transition at the chiral symmetry restoration temperature as well as at an absolute value of θ . On the other hand, the long-range correlation length solely depends on θ for the theories with critical exponent e = 2 or T = T d + 1 , where T d is the decoherence temperature. Furthermore, it is worth noticing that the unit-critical exponent vacuum configuration corresponds to a non-interacting statistical basis pertaining to a constant mass of η . Full article
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Review

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86 pages, 23085 KiB  
Review
Dark Matters on the Scale of Galaxies
by Ivan de Martino, Sankha S. Chakrabarty, Valentina Cesare, Arianna Gallo, Luisa Ostorero and Antonaldo Diaferio
Universe 2020, 6(8), 107; https://doi.org/10.3390/universe6080107 - 1 Aug 2020
Cited by 68 | Viewed by 6333
Abstract
The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales [...] Read more.
The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed within the cold dark-matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and f(R) gravity. Full article
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30 pages, 533 KiB  
Review
Embedding Black Holes and Other Inhomogeneities in the Universe in Various Theories of Gravity: A Short Review
by Valerio Faraoni
Universe 2018, 4(10), 109; https://doi.org/10.3390/universe4100109 - 16 Oct 2018
Cited by 23 | Viewed by 2869
Abstract
Classic black hole mechanics and thermodynamics are formulated for stationary black holes with event horizons. Alternative theories of gravity of interest for cosmology contain a built-in time-dependent cosmological “constant” and black holes are not stationary. Realistic black holes are anyway dynamical because they [...] Read more.
Classic black hole mechanics and thermodynamics are formulated for stationary black holes with event horizons. Alternative theories of gravity of interest for cosmology contain a built-in time-dependent cosmological “constant” and black holes are not stationary. Realistic black holes are anyway dynamical because they interact with astrophysical environments or, at a more fundamental level, because of backreaction by Hawking radiation. In these situations, the teleological concept of event horizon fails and apparent or trapping horizons are used instead. Even as toy models, black holes embedded in cosmological “backgrounds” and other inhomogeneous universes constitute an interesting class of solutions of various theories of gravity. We discuss the known phenomenology of apparent and trapping horizons in these geometries, focusing on spherically symmetric inhomogeneous universes. Full article
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