Universe

11 pages, 307 KiB

Open AccessArticle

Entropy and Energy of Static Spherically Symmetric Black Hole in f(R) Theory

by Yaoguang Zheng and Rong-Jia Yang

Universe 2020, 6(3), 47; https://doi.org/10.3390/universe6030047 - 20 Mar 2020

Cited by 5 | Viewed by 2142

We consider the new horizon first law in

f (R)

theory. We derive the general formulas to computed the entropy and energy for static spherically symmetric black hole. For applications, some nontrivial solutions in some popular

f (R)

theories [...] Read more.

We consider the new horizon first law in

f (R)

theory. We derive the general formulas to computed the entropy and energy for static spherically symmetric black hole. For applications, some nontrivial solutions in some popular

f (R)

theories are investigated, the entropies and the energies of static spherically symmetric black holes in these models are first calculated. Full article

(This article belongs to the Special Issue Black Hole Physics and Astrophysics)

33 pages, 3142 KiB

Open AccessArticle

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 14 | Viewed by 4260

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

(This article belongs to the Special Issue Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models)

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12 pages, 621 KiB

Open AccessArticle

Revisiting the Fraction of Radio-Loud Narrow Line Seyfert 1 Galaxies with LoTSS DR1

by Xu-Liang Fan

Universe 2020, 6(3), 45; https://doi.org/10.3390/universe6030045 - 18 Mar 2020

Cited by 3 | Viewed by 2347

Abstract

Radio-Loud narrow-line Seyfert 1 galaxies (NLS1s), especially the extremely radio-loud ones, are widely accepted as the jetted versions of NLS1s. We explore the radio-loud fraction for NLS1s with recently released LoTSS DR1 at 150 MHz. The radio detection rate is about 28% for [...] Read more.

Radio-Loud narrow-line Seyfert 1 galaxies (NLS1s), especially the extremely radio-loud ones, are widely accepted as the jetted versions of NLS1s. We explore the radio-loud fraction for NLS1s with recently released LoTSS DR1 at 150 MHz. The radio detection rate is about 28% for LoTSS DR1. The radio detected NLS1s have lower redshift than the non-detected ones. Moreover, the 150 MHz radio luminosities of NLS1s detected by LoTSS are about two orders of magnitude weaker than those of the previous samples. By defining the radio loudness with the ratio between 150 MHz radio flux and SDSS r band flux, the radio-loud fraction is about 1% with the critical radio loudness equalling 100. Radio loudness shows no dependence on central black hole mass, while weak correlations are found between radio loudness and disk luminosity, as well as Eddington ratio. Full article

(This article belongs to the Special Issue Seyfert Galaxies Astrophysics)

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20 pages, 1677 KiB

Open AccessArticle

Magnetized Particle Motion around Black Holes in Conformal Gravity: Can Magnetic Interaction Mimic Spin of Black Holes?

by Kamoliddin Haydarov, Ahmadjon Abdujabbarov, Javlon Rayimbaev and Bobomurat Ahmedov

Universe 2020, 6(3), 44; https://doi.org/10.3390/universe6030044 - 17 Mar 2020

Cited by 28 | Viewed by 2721

Abstract

Magnetized particle motion around black holes in conformal gravity immersed in asymptotically uniform magnetic field has been studied. We have also analyzed the behavior of magnetic fields near the horizon of the black hole in conformal gravity and shown that with the increase [...] Read more.

Magnetized particle motion around black holes in conformal gravity immersed in asymptotically uniform magnetic field has been studied. We have also analyzed the behavior of magnetic fields near the horizon of the black hole in conformal gravity and shown that with the increase of conformal parameters L and N the value of angular component of magnetic field at the stellar surface decreases. The maximum value of the effective potential corresponding to circular motion of the magnetized particle increases with the increase of conformal parameters. It is shown that in all cases of neutral, charged and magnetized particle collisions in the black hole environment the center-of-mass energy decreases with the increase of conformal parameters L and N. In the case of the magnetized and negatively charged particle collisions, the innermost collision point with the maximum center-of-mass energy comes closer to the central object due to the effects of the parameters of the conformal gravity. We have applied the results to the real astrophysical scenario when a pulsar treated as a magnetized particle is orbiting the super massive black hole (SMBH) Sgr A* in the center of our galaxy in order to obtain the estimation of magnetized compact object’s orbital parameter. The possible detection of pulsar in Sgr A* close environment can provide constraints on black hole parameters. Here we have shown that there is degeneracy between spin of SMBH and ambient magnetic field and consequently the interaction of magnetic field

\sim 10^{2}

Gauss with magnetic moment of magnetized neutron star can in principle mimic spin of Kerr black holes up to

0.6

. Full article

(This article belongs to the Special Issue Relativistic Astrophysics)

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73 pages, 6133 KiB

Open AccessEditor’s ChoiceReview

Quantum Black Holes in the Sky

by Jahed Abedi, Niayesh Afshordi, Naritaka Oshita and Qingwen Wang

Universe 2020, 6(3), 43; https://doi.org/10.3390/universe6030043 - 10 Mar 2020

Cited by 52 | Viewed by 5430

Abstract

Black Holes are possibly the most enigmatic objects in our universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past four years. Nevertheless, [...] Read more.

Black Holes are possibly the most enigmatic objects in our universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past four years. Nevertheless, they remain active playgrounds for strong gravity and quantum effects, where novel aspects of the elusive theory of quantum gravity may be hard at work. In this review article, we provide an overview of the strong motivations for why “Quantum Black Holes” may be radically different from their classical counterparts in Einstein’s General Relativity. We then discuss the observational signatures of quantum black holes, focusing on gravitational wave echoes as smoking guns for quantum horizons (or exotic compact objects), which have led to significant recent excitement and activity. We review the theoretical underpinning of gravitational wave echoes and critically examine the seemingly contradictory observational claims regarding their (non-)existence. Finally, we discuss the future theoretical and observational landscape for unraveling the “Quantum Black Holes in the Sky”. Full article

(This article belongs to the Special Issue Probing New Physics with Black Holes)

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15 pages, 419 KiB

Open AccessArticle

Evolution of Quasiperiodic Structures in a Non-Ideal Hydrodynamic Description of Phase Transitions

by D. N. Voskresensky

Universe 2020, 6(3), 42; https://doi.org/10.3390/universe6030042 - 7 Mar 2020

Cited by 3 | Viewed by 2323

Abstract

Various phase transitions could have taken place in the early universe, and may occur in the course of heavy-ion collisions and supernova explosions, in proto-neutron stars, in cold compact stars, and in the condensed matter at terrestrial conditions. Most generally, the dynamics of [...] Read more.

Various phase transitions could have taken place in the early universe, and may occur in the course of heavy-ion collisions and supernova explosions, in proto-neutron stars, in cold compact stars, and in the condensed matter at terrestrial conditions. Most generally, the dynamics of the density and temperature at first- and second-order phase transitions can be described with the help of the equations of non-ideal hydrodynamics. In the given work, some novel solutions are found describing the evolution of quasiperiodic structures that are formed in the course of the phase transitions. Although this consideration is very general, particular examples of quark-hadron and nuclear liquid-gas first-order phase transitions to the uniform

k_{0} = 0

state and of a pion-condensate second-order phase transition to a non-uniform

k_{0} \neq 0

state in dense baryon matter are considered. Full article

(This article belongs to the Special Issue Shedding Light to the Dark Sides of the Universe: Cosmology from Strong Interactions)

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23 pages, 2201 KiB

Open AccessReview

Exploring Light Sterile Neutrinos at Long Baseline Experiments: A Review

by Antonio Palazzo

Universe 2020, 6(3), 41; https://doi.org/10.3390/universe6030041 - 7 Mar 2020

Cited by 9 | Viewed by 3436

Abstract

Several anomalies observed in short-baseline neutrino experiments suggest the existence of new light sterile neutrino species. In this review, we describe the potential role of long-baseline experiments in the searches of sterile neutrino properties and, in particular, the new CP-violation phases that appear [...] Read more.

Several anomalies observed in short-baseline neutrino experiments suggest the existence of new light sterile neutrino species. In this review, we describe the potential role of long-baseline experiments in the searches of sterile neutrino properties and, in particular, the new CP-violation phases that appear in the enlarged 3 + 1 scheme. We also assess the impact of light sterile states on the discovery potential of long-baseline experiments of important targets such as the standard 3-flavor CP violation, the neutrino mass hierarchy, and the octant of

θ_{23}

. Full article

(This article belongs to the Special Issue Neutrino Oscillations)

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18 pages, 352 KiB

Open AccessArticle

Curvature Spinors in Locally Inertial Frame and the Relations with Sedenion

by In Ki Hong, Choong Sun Kim and Gyung Hyun Min

Universe 2020, 6(3), 40; https://doi.org/10.3390/universe6030040 - 6 Mar 2020

Viewed by 1980

Abstract

In the 2-spinor formalism, the gravity can be dealt with curvature spinors with four spinor indices. Here we show a new effective method to express the components of curvature spinors in the rank-2

4 \times 4

tensor representation for the gravity in a [...] Read more.

In the 2-spinor formalism, the gravity can be dealt with curvature spinors with four spinor indices. Here we show a new effective method to express the components of curvature spinors in the rank-2

4 \times 4

tensor representation for the gravity in a locally inertial frame. In the process we have developed a few manipulating techniques, through which the roles of each component of Riemann curvature tensor are revealed. We define a new algebra ‘sedon’, the structure of which is almost the same as sedenion except for the basis multiplication rule. Finally we also show that curvature spinors can be represented in the sedon form and observe the chiral structure in curvature spinors. A few applications of the sedon representation, which includes the quaternion form of differential Binanchi identity and hand-in-hand couplings of curvature spinors, are also presented. Full article

(This article belongs to the Section Gravitation)

26 pages, 387 KiB

Open AccessArticle

Deformed General Relativity and Quantum Black Holes Interior

by Denis Arruga, Jibril Ben Achour and Karim Noui

Universe 2020, 6(3), 39; https://doi.org/10.3390/universe6030039 - 4 Mar 2020

Cited by 37 | Viewed by 2721

Abstract

Effective models of black holes interior have led to several proposals for regular black holes. In the so-called polymer models, based on effective deformations of the phase space of spherically symmetric general relativity in vacuum, one considers a deformed Hamiltonian constraint while keeping [...] Read more.

Effective models of black holes interior have led to several proposals for regular black holes. In the so-called polymer models, based on effective deformations of the phase space of spherically symmetric general relativity in vacuum, one considers a deformed Hamiltonian constraint while keeping a non-deformed vectorial constraint, leading under some conditions to a notion of deformed covariance. In this article, we revisit and study further the question of covariance in these deformed gravity models. In particular, we propose a Lagrangian formulation for these deformed gravity models where polymer-like deformations are introduced at the level of the full theory prior to the symmetry reduction and prior to the Legendre transformation. This enables us to test whether the concept of deformed covariance found in spherically symmetric vacuum gravity can be extended to the full theory, and we show that, in the large class of models we are considering, the deformed covariance cannot be realized beyond spherical symmetry in the sense that the only deformed theory which leads to a closed constraints algebra is general relativity. Hence, we focus on the spherically symmetric sector, where there exist non-trivial deformed but closed constraints algebras. We investigate the possibility to deform the vectorial constraint as well and we prove that non-trivial deformations of the vectorial constraint with the condition that the constraints algebra remains closed do not exist. Then, we compute the most general deformed Hamiltonian constraint which admits a closed constraints algebra and thus leads to a well-defined effective theory associated with a notion of deformed covariance. Finally, we study static solutions of these effective theories and, remarkably, we solve explicitly and in full generality the corresponding modified Einstein equations, even for the effective theories which do not satisfy the closeness condition. In particular, we give the expressions of the components of the effective metric (for spherically symmetric black holes interior) in terms of the functions that govern the deformations of the theory. Full article

(This article belongs to the Special Issue Probing New Physics with Black Holes)

9 pages, 255 KiB

Open AccessArticle

On the Solution of the Schrödinger Equation with Position-Dependent Mass

by Mehmet Sezgin

Universe 2020, 6(3), 38; https://doi.org/10.3390/universe6030038 - 4 Mar 2020

Cited by 1 | Viewed by 2173

Abstract

We have considered the Iwasawa and Gauss decompositions for the Lie group SL(2,R). According to these decompositions, the Casimir operators of the group and the Hamiltonians with position-dependent mass were expressed. Then, the unbound state solutions of the Schrödinger equations with position-dependent mass [...] Read more.

We have considered the Iwasawa and Gauss decompositions for the Lie group SL(2,R). According to these decompositions, the Casimir operators of the group and the Hamiltonians with position-dependent mass were expressed. Then, the unbound state solutions of the Schrödinger equations with position-dependent mass were given. Full article

30 pages, 2985 KiB

Open AccessReview

Neutrino Oscillations and Lorentz Invariance Violation

by Marco Danilo Claudio Torri

Universe 2020, 6(3), 37; https://doi.org/10.3390/universe6030037 - 27 Feb 2020

Cited by 16 | Viewed by 3718

Abstract

This work explores the possibility of resorting to neutrino phenomenology to detect evidence of new physics, caused by the residual signals of the supposed quantum structure of spacetime. In particular, this work investigates the effects on neutrino oscillations and mass hierarchy detection, predicted [...] Read more.

This work explores the possibility of resorting to neutrino phenomenology to detect evidence of new physics, caused by the residual signals of the supposed quantum structure of spacetime. In particular, this work investigates the effects on neutrino oscillations and mass hierarchy detection, predicted by models that violate Lorentz invariance, preserving the spacetime isotropy and homogeneity. Neutrino physics is the ideal environment where conducting the search for new “exotic” physics, since the oscillation phenomenon is not included in the original formulation of the minimal Standard Model (SM) of particles. The confirmed observation of the neutrino oscillation phenomenon is, therefore, the first example of physics beyond the SM and can indicate the necessity to resort to new theoretical models. In this work, the hypothesis that the supposed Lorentz Invariance Violation (LIV) perturbations can influence the oscillation pattern is investigated. LIV theories are indeed constructed assuming modified kinematics, caused by the interaction of massive particles with the spacetime background. This means that the dispersion relations are modified, so it appears natural to search for effects caused by LIV in physical phenomena governed by masses, as in the case of neutrino oscillations. In addition, the neutrino oscillation phenomenon is interesting since there are three different mass eigenstates and in a LIV scenario, which preserves isotropy, at least two different species of particle must interact. Full article

(This article belongs to the Special Issue Neutrino Oscillations)

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23 pages, 377 KiB

Open AccessArticle

Critical Evaluation of Common Claims in Loop Quantum Cosmology

by Martin Bojowald

Universe 2020, 6(3), 36; https://doi.org/10.3390/universe6030036 - 26 Feb 2020

Cited by 44 | Viewed by 3300

Abstract

A large number of models have been analyzed in loop quantum cosmology, using mainly minisuperspace constructions and perturbations. At the same time, general physics principles from effective field theory and covariance have often been ignored. A consistent introduction of these ingredients requires substantial [...] Read more.

A large number of models have been analyzed in loop quantum cosmology, using mainly minisuperspace constructions and perturbations. At the same time, general physics principles from effective field theory and covariance have often been ignored. A consistent introduction of these ingredients requires substantial modifications of existing scenarios. As a consequence, none of the broader claims made mainly by the Ashtekar school—such as the genericness of bounces with astonishingly semiclassical dynamics, robustness with respect to quantization ambiguities, the realization of covariance, and the relevance of certain technical results for potential observations—hold up to scrutiny. Several useful lessons for a sustainable version of quantum cosmology can be drawn from this evaluation. Full article

(This article belongs to the Special Issue Quantum Models for Cosmology)

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Universe, Volume 6, Issue 3 (March 2020) – 12 articles

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