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Universe, Volume 2, Issue 2 (June 2016) – 7 articles

Cover Story (view full-size image): White holes have been long thought of as mathematical curiosities: solutions of the Einstein field equations that cannot be nevertheless dynamically produced in physical processes involving realistic matter. However, quantum gravity may supply the necessary spark to produce them in events that are ubiquitous in the universe, namely in the gravitational collapse of massive stars. In this scenario, the first corrections to general relativity at very short distances lead to repulsive forces that prevent the formation of singularities. These repulsive forces restore the symmetry under time reversal, resulting in quasi-periodic oscillations of the collapsing star that involve transient phases resembling black holes and white holes. This proposal would solve a number of open issues, such as the singularity problem or the information loss conundrum, providing moreover a falsifiable alternative. View this [...] Read more.
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Article
Charged and Electromagnetic Fields from Relativistic Quantum Geometry
Universe 2016, 2(2), 13; https://doi.org/10.3390/universe2020013 - 21 Jun 2016
Cited by 6 | Viewed by 1872
Abstract
In the recently introduced Relativistic Quantum Geometry (RQG) formalism, the possibility was explored that the variation of the tensor metric can be done in a Weylian integrable manifold using a geometric displacement, from a Riemannian to a Weylian integrable manifold, described by the [...] Read more.
In the recently introduced Relativistic Quantum Geometry (RQG) formalism, the possibility was explored that the variation of the tensor metric can be done in a Weylian integrable manifold using a geometric displacement, from a Riemannian to a Weylian integrable manifold, described by the dynamics of an auxiliary geometrical scalar field θ, in order that the Einstein tensor (and the Einstein equations) can be represented on a Weyl-like manifold. In this framework we study jointly the dynamics of electromagnetic fields produced by quantum complex vector fields, which describes charges without charges. We demonstrate that complex fields act as a source of tetra-vector fields which describe an extended Maxwell dynamics. Full article
Review
Brief Review on Black Hole Loop Quantization
Universe 2016, 2(2), 12; https://doi.org/10.3390/universe2020012 - 15 Jun 2016
Cited by 14 | Viewed by 2172
Abstract
Here, we present a review about the quantization of spherically-symmetric spacetimes adopting loop quantum gravity techniques. Several models that have been studied so far share similar properties: the resolution of the classical singularity and some of them an intrinsic discretization of the geometry. [...] Read more.
Here, we present a review about the quantization of spherically-symmetric spacetimes adopting loop quantum gravity techniques. Several models that have been studied so far share similar properties: the resolution of the classical singularity and some of them an intrinsic discretization of the geometry. We also explain the extension to Reissner–Nordström black holes. Besides, we review how quantum test fields on these quantum geometries allow us to study phenomena, like the Casimir effect or Hawking radiation. Finally, we briefly describe a recent proposal that incorporates spherically-symmetric matter, discussing its relevance for the understanding of black hole evolution. Full article
(This article belongs to the Special Issue Loop Quantum Cosmology and Quantum Black Holes)
Article
Virial Theorem in Nonlocal Newtonian Gravity
Universe 2016, 2(2), 9; https://doi.org/10.3390/universe2020009 - 30 May 2016
Cited by 3 | Viewed by 1995
Abstract
Nonlocal gravity is the recent classical nonlocal generalization of Einstein’s theory of gravitation in which the past history of the gravitational field is taken into account. In this theory, nonlocality appears to simulate dark matter. The virial theorem for the Newtonian regime of [...] Read more.
Nonlocal gravity is the recent classical nonlocal generalization of Einstein’s theory of gravitation in which the past history of the gravitational field is taken into account. In this theory, nonlocality appears to simulate dark matter. The virial theorem for the Newtonian regime of nonlocal gravity theory is derived and its consequences for “isolated” astronomical systems in virial equilibrium at the present epoch are investigated. In particular, for a sufficiently isolated nearby galaxy in virial equilibrium, the galaxy’s baryonic diameter D 0 —namely, the diameter of the smallest sphere that completely surrounds the baryonic system at the present time—is predicted to be larger than the effective dark matter fraction f D M times a universal length that is the basic nonlocality length scale λ 0 3 ± 2 kpc. Full article
Review
Einstein and Beyond: A Critical Perspective on General Relativity
Universe 2016, 2(2), 11; https://doi.org/10.3390/universe2020011 - 30 May 2016
Cited by 40 | Viewed by 2429
Abstract
An alternative approach to Einstein’s theory of General Relativity (GR) is reviewed, which is motivated by a range of serious theoretical issues inflicting the theory, such as the cosmological constant problem, presence of non-Machian solutions, problems related with the energy-stress tensor [...] Read more.
An alternative approach to Einstein’s theory of General Relativity (GR) is reviewed, which is motivated by a range of serious theoretical issues inflicting the theory, such as the cosmological constant problem, presence of non-Machian solutions, problems related with the energy-stress tensor T i k and unphysical solutions. The new approach emanates from a critical analysis of these problems, providing a novel insight that the matter fields, together with the ensuing gravitational field, are already present inherently in the spacetime without taking recourse to T i k . Supported by lots of evidence, the new insight revolutionizes our views on the representation of the source of gravitation and establishes the spacetime itself as the source, which becomes crucial for understanding the unresolved issues in a unified manner. This leads to a new paradigm in GR by establishing equation R i k = 0 as the field equation of gravitation plus inertia in the very presence of matter. Full article
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Article
Reissner–Nordström Anti-de Sitter Black Holes in Mimetic F(R) Gravity
Universe 2016, 2(2), 10; https://doi.org/10.3390/universe2020010 - 30 May 2016
Cited by 33 | Viewed by 2355
Abstract
In this paper, we study under which conditions the Reissner–Nordström anti-de Sitter black hole can be a solution of the vacuum mimetic F ( R ) gravity with Lagrange multiplier and mimetic scalar potential. As the author demonstrates, the resulting picture in the [...] Read more.
In this paper, we study under which conditions the Reissner–Nordström anti-de Sitter black hole can be a solution of the vacuum mimetic F ( R ) gravity with Lagrange multiplier and mimetic scalar potential. As the author demonstrates, the resulting picture in the mimetic F ( R ) gravity case is a trivial extension of the standard F ( R ) approach, and in effect, the metric perturbations in the mimetic F ( R ) gravity case, for the Reissner–Nordström anti-de Sitter black hole metric, at the first order of the perturbed variables are the same at the leading order. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
Article
Black Holes and Exotic Spinors
Universe 2016, 2(2), 8; https://doi.org/10.3390/universe2020008 - 26 May 2016
Cited by 9 | Viewed by 2396
Abstract
Exotic spin structures are non-trivial liftings, of the orthogonal bundle to the spin bundle, on orientable manifolds that admit spin structures according to the celebrated Geroch theorem. Exotic spin structures play a role of paramount importance in different areas of physics, from quantum [...] Read more.
Exotic spin structures are non-trivial liftings, of the orthogonal bundle to the spin bundle, on orientable manifolds that admit spin structures according to the celebrated Geroch theorem. Exotic spin structures play a role of paramount importance in different areas of physics, from quantum field theory, in particular at Planck length scales, to gravity, and in cosmological scales. Here, we introduce an in-depth panorama in this field, providing black hole physics as the fount of spacetime exoticness. Black holes are then studied as the generators of a non-trivial topology that also can correspond to some inequivalent spin structure. Moreover, we investigate exotic spinor fields in this context and the way exotic spinor fields branch new physics. We also calculate the tunneling probability of exotic fermions across a Kerr-Sen black hole, showing that the exotic term does affect the tunneling probability, altering the black hole evaporation rate. Finally we show that it complies with the Hawking temperature universal law. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
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Review
Where Does the Physics of Extreme Gravitational Collapse Reside?
Universe 2016, 2(2), 7; https://doi.org/10.3390/universe2020007 - 13 May 2016
Cited by 38 | Viewed by 2742
Abstract
The gravitational collapse of massive stars serves to manifest the most severe deviations of general relativity with respect to Newtonian gravity: the formation of horizons and spacetime singularities. Both features have proven to be catalysts of deep physical developments, especially when combined with [...] Read more.
The gravitational collapse of massive stars serves to manifest the most severe deviations of general relativity with respect to Newtonian gravity: the formation of horizons and spacetime singularities. Both features have proven to be catalysts of deep physical developments, especially when combined with the principles of quantum mechanics. Nonetheless, it is seldom remarked that it is hardly possible to combine all these developments into a unified theoretical model, while maintaining reasonable prospects for the independent experimental corroboration of its different parts. In this paper we review the current theoretical understanding of the physics of gravitational collapse in order to highlight this tension, stating the position that the standard view on evaporating black holes stands for. This serves as the motivation for the discussion of a recent proposal that offers the opposite perspective, represented by a set of geometries that regularize the classical singular behavior and present modifications of the near-horizon Schwarzschild geometry as the result of the propagation of non-perturbative ultraviolet effects originated in regions of high curvature. We present an extensive exploration of the necessary steps on the explicit construction of these geometries, and discuss how this proposal could change our present understanding of astrophysical black holes and even offer the possibility of detecting genuine ultraviolet effects in gravitational-wave experiments. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
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