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Universe, Volume 4, Issue 5 (May 2018) – 5 articles

Cover Story (view full-size image): In the present article, black holes free of the central singularity, dubbed regular (or non-singular) black holes are investigated. Regular black holes have two horizons: an outer event horizon and an inner Cauchy horizon, a space-time structure similar to that resulting from studies of the gravitational collapse based on Loop Quantum Gravity. The extreme case, when the two horizons coincide, has a zero surface temperature and, therefore, they can be imagined to be in a “ground-state”, since no Hawking radiation is possible. The basic idea developed in this article assumes that the mass distribution scale characterizing the space-time is fixed by the Riemann invariant, computed in the context of Loop Quantum Gravity. In this case, extreme black holes would have masses comparable to the Planck mass (tens of micrograms), and would have been formed just after the inflation era. View Paper here.
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Article
On a Model of Magnetically Charged Black Hole with Nonlinear Electrodynamics
Universe 2018, 4(5), 66; https://doi.org/10.3390/universe4050066 - 19 May 2018
Cited by 14 | Viewed by 1561
Abstract
The Bronnikov model of nonlinear electrodynamics is investigated in general relativity. The magnetic black hole is considered and we obtain a solution giving corrections to the Reissner-Nordström solution. In this model spacetime at r becomes Minkowski’s spacetime. We calculate the magnetic [...] Read more.
The Bronnikov model of nonlinear electrodynamics is investigated in general relativity. The magnetic black hole is considered and we obtain a solution giving corrections to the Reissner-Nordström solution. In this model spacetime at r becomes Minkowski’s spacetime. We calculate the magnetic mass of the black hole and the metric function. At some parameters of the model there can be one, two or no horizons. The Hawking temperature and the heat capacity of black holes are calculated. We show that a second-order phase transition takes place and black holes are thermodynamically stable at some range of parameters. Full article
(This article belongs to the Collection Open Questions in Black Hole Physics)
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Article
Several Effects Unexplained by QCD
Universe 2018, 4(5), 65; https://doi.org/10.3390/universe4050065 - 16 May 2018
Cited by 9 | Viewed by 1325
Abstract
Several new experimental discoveries in high energy proton interactions, yet unexplained by QCD, are discussed in the paper. The increase of the cross sections with increasing energy from ISR to LHC, the correlation between it and the behavior of the slope of the [...] Read more.
Several new experimental discoveries in high energy proton interactions, yet unexplained by QCD, are discussed in the paper. The increase of the cross sections with increasing energy from ISR to LHC, the correlation between it and the behavior of the slope of the elastic diffraction cone, the unexpected increase of the survival probability of protons in the same energy range, the new structure of the elastic differential cross section at rather large transferred momenta (small distances) and the peculiar ridge effect in high multiplicity inelastic processes are still waiting for QCD interpretation and deeper insight in vacuum. Full article
(This article belongs to the Special Issue Interplay of QCD, Cosmology and Astroparticle Physics)
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Article
Neutrino Emissivity in the Quark-Hadron Mixed Phase
Universe 2018, 4(5), 64; https://doi.org/10.3390/universe4050064 - 16 May 2018
Cited by 9 | Viewed by 1172
Abstract
In this work we investigate the effect a crystalline quark–hadron mixed phase can have on the neutrino emissivity from the cores of neutron stars. To this end we use relativistic mean-field equations of state to model hadronic matter and a nonlocal extension of [...] Read more.
In this work we investigate the effect a crystalline quark–hadron mixed phase can have on the neutrino emissivity from the cores of neutron stars. To this end we use relativistic mean-field equations of state to model hadronic matter and a nonlocal extension of the three-flavor Nambu–Jona–Lasinio model for quark matter. Next we determine the extent of the quark–hadron mixed phase and its crystalline structure using the Glendenning construction, allowing for the formation of spherical blob, rod, and slab rare phase geometries. Finally, we calculate the neutrino emissivity due to electron–lattice interactions utilizing the formalism developed for the analogous process in neutron star crusts. We find that the contribution to the neutrino emissivity due to the presence of a crystalline quark–hadron mixed phase is substantial compared to other mechanisms at fairly low temperatures (≲10 9 K) and quark fractions (≲30%), and that contributions due to lattice vibrations are insignificant compared to static-lattice contributions. There are a number of open issues that need to be addressed in a future study on the neutrino emission rates caused by electron–quark blob bremsstrahlung. Chiefly among them are the role of collective oscillations of matter, electron band structures, and of gaps at the boundaries of the Brillouin zones on bremsstrahlung, as discussed in the summary section of this paper. We hope this paper will stimulate studies addressing these issues. Full article
(This article belongs to the Special Issue Compact Stars in the QCD Phase Diagram)
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Review
Generic Features of Thermodynamics of Horizons in Regular Spherical Space-Times of the Kerr-Schild Class
Universe 2018, 4(5), 63; https://doi.org/10.3390/universe4050063 - 11 May 2018
Cited by 9 | Viewed by 1387
Abstract
We present a systematic review of thermodynamics of horizons in regular spherically symmetric spacetimes of the Kerr-Schild class, [...] Read more.
We present a systematic review of thermodynamics of horizons in regular spherically symmetric spacetimes of the Kerr-Schild class, d s 2 = g ( r ) d t 2 g 1 ( r ) d r 2 r 2 d Ω 2 , both asymptotically flat and with a positive background cosmological constant λ . Regular solutions of this class have obligatory de Sitter center. A source term in the Einstein equations satisfies T t t = T r r and represents an anisotropic vacuum dark fluid ( p r = ρ ), defined by the algebraic structure of its stress-energy tensor, which describes a time-evolving and spatially inhomogeneous, distributed or clustering, vacuum dark energy intrinsically related to space-time symmetry. In the case of two vacuum scales it connects smoothly two de Sitter vacua, 8 π G T ν μ = Λ δ ν μ as r 0 , 8 π G T ν μ = λ δ ν μ as r with λ < Λ . In the range of the mass parameter M c r 1 M M c r 2 it describes a regular cosmological black hole directly related to a vacuum dark energy. Space-time has at most three horizons: a cosmological horizon r c , a black hole horizon r b < r c , and an internal horizon r a < r b , which is the cosmological horizon for an observer in the internal R-region asymptotically de Sitter as r 0 . Asymptotically flat regular black holes ( λ = 0 ) can have at most two horizons, r b and r a . We present the basic generic features of thermodynamics of horizons revealed with using the Padmanabhan approach relevant for a multi-horizon space-time with a non-zero pressure. Quantum evaporation of a regular black hole involves a phase transition in which the specific heat capacity is broken and changes sign while a temperature achieves its maximal value, and leaves behind the thermodynamically stable double-horizon ( r a = r b ) remnant with zero temperature and positive specific heat. The mass of objects with the de Sitter center is generically related to vacuum dark energy and to breaking of space-time symmetry. In the cosmological context space-time symmetry provides a mechanism for relaxing cosmological constant to a certain non-zero value. We discuss also observational applications of the presented results. Full article
(This article belongs to the Special Issue Black Hole Thermodynamics)
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Article
Primordial Regular Black Holes: Thermodynamics and Dark Matter
Universe 2018, 4(5), 62; https://doi.org/10.3390/universe4050062 - 03 May 2018
Cited by 5 | Viewed by 1747
Abstract
The possibility that dark matter particles could be constituted by extreme regular primordial black holes is discussed. Extreme black holes have zero surface temperature, and are not subjected to the Hawking evaporation process. Assuming that the common horizon radius of these black holes [...] Read more.
The possibility that dark matter particles could be constituted by extreme regular primordial black holes is discussed. Extreme black holes have zero surface temperature, and are not subjected to the Hawking evaporation process. Assuming that the common horizon radius of these black holes is fixed by the minimum distance that is derived from the Riemann invariant computed from loop quantum gravity, the masses of these non-singular stable black holes are of the order of the Planck mass. However, if they are formed just after inflation, during reheating, their initial masses are about six orders of magnitude higher. After a short period of growth by the accretion of relativistic matter, they evaporate until reaching the extreme solution. Only a fraction of 3.8 × 10−22 of relativistic matter is required to be converted into primordial black holes (PBHs) in order to explain the present abundance of dark matter particles. Full article
(This article belongs to the Special Issue Black Hole Thermodynamics)
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