Topical Collection "Open Questions in Black Hole Physics"

Editors

Collection Editor
Dr. Gonzalo J. Olmo

Depto. Física Teórica & IFIC,Centro Mixto Universidad de Valencia & CSIC, 46100 Burjassot, Valencia, Spain
Website | E-Mail
Interests: quantum gravity; black holes; Hawking radiation; Cosmology; inflation; modified theories of gravity; Palatini formalism; field theory; wormholes
Collection Editor
Dr. Diego Rubiera-Garcia

Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Edifício C8, Campo Grande, P-1749-016 Lisbon, Portugal
Website | E-Mail
Interests: Black holes; Modified Gravity; Cosmology; Bouncing solutions;Spacetime singularities; Nonlinear Electrodynamics; Field Theory; Topological Defects; Metric-affine geometry

Topical Collection Information

Dear Colleagues,

Black holes are fascinating objects that hold the key to uncover the intimate relation between gravitation and the quantum, as well as a number of astrophysical phenomena. The strength of their gravitational field is responsible for extreme deformations of the causal structure of space-time, leading to intriguing thermodynamic properties and serious tensions with our understanding of the quantum world. The current picture of the Universe could hardly be explained without recurring to them, even though they are very elusive and defy direct observational detection.

The coming years are likely to witness important breakthroughs in our view of black holes and their astrophysical and quantum properties through the direct detection of gravitational waves and Hawking radiation in analogue models. A number of fundamental open questions might find an answer and new ones may emerge. Are black holes, seen as regions of space-time hidden behind an event horizon, astrophysically realizable? Are there viable compact-object alternatives? Will black hole mergers allow us to experimentally probe the strong-field gravitational regime? What happens when two singularities meet in a black hole merger? Do singularities have a physical role? Can wormholes alleviate the issue of singularities? How much can we learn about black holes from analogue models? What is the end state of black hole evaporation? Why primordial black hole explosions have not been observed yet? What aspects of black holes are modified in a quantum theory of gravity? How does a supermassive black hole interact with its host galaxy? These are just a few examples of the Open Questions in Black Hole Physics we expect to address in this Special Issue of Universe.

Dr. Gonzalo J. Olmo
Collection Editor

Manuscript Submission Information

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Published Papers (22 papers)

2019

Jump to: 2018, 2017, 2016, 2015

Open AccessArticle
Identification of a Regular Black Hole by Its Shadow
Universe 2019, 5(7), 163; https://doi.org/10.3390/universe5070163
Received: 28 May 2019 / Revised: 28 June 2019 / Accepted: 1 July 2019 / Published: 3 July 2019
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Abstract
We study shadows of regular rotating black holes described by the axially symmetric solutions asymptotically Kerr for a distant observer, obtained from regular spherical solutions of the Kerr–Schild class specified by Ttt=Trr(pr= [...] Read more.
We study shadows of regular rotating black holes described by the axially symmetric solutions asymptotically Kerr for a distant observer, obtained from regular spherical solutions of the Kerr–Schild class specified by T t t = T r r ( p r = ε ) . All regular solutions obtained with the Newman–Janis algorithm belong to this class. Their basic generic feature is the de Sitter vacuum interior. Information about the interior content of a regular rotating de Sitter-Kerr black hole can be in principle extracted from observation of its shadow. We present the general formulae for description of shadows for this class of regular black holes, and numerical analysis for two particular regular black hole solutions. We show that the shadow of a de Sitter-Kerr black hole is typically smaller than that for the Kerr black hole, and the difference depends essentially on the interior density and on the pace of its decreasing. Full article
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Open AccessReview
Explaining Defects of the Universal Vacua with Black Holes-Hedgehogs and Strings
Received: 27 November 2018 / Revised: 6 March 2019 / Accepted: 6 March 2019 / Published: 12 March 2019
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Abstract
Assuming the Multiple Point Principle (MPP) as a new law of Nature, we considered the existence of the two degenerate vacua of the Universe: (a) the first Electroweak (EW) vacuum at v1246 GeV—“true vacuum”, and (b) the second Planck scale [...] Read more.
Assuming the Multiple Point Principle (MPP) as a new law of Nature, we considered the existence of the two degenerate vacua of the Universe: (a) the first Electroweak (EW) vacuum at v 1 246 GeV—“true vacuum”, and (b) the second Planck scale “false vacuum” at v 2 10 18 GeV. In these vacua, we investigated different topological defects. The main aim of the paper is an investigation of the black-hole-hedgehogs configurations as defects of the false vacuum. In the framework of the f ( R ) gravity, described by the Gravi-Weak unification model, we considered a black-hole solution, which corresponds to a “hedgehog”—global monopole, that has been “swallowed” by the black-hole with mass core M B H 10 18 GeV and radius δ 10 21 GeV 1 . Considering the results of the hedgehog lattice theory in the framework of the S U ( 2 ) Yang-Mills gauge-invariant theory with hedgehogs in the Wilson loops, we have used the critical value of temperature for the hedgehogs’ confinement phase ( T c 10 18 GeV). This result gave us the possibility to conclude that the SM shows a new physics (with contributions of the S U ( 2 ) -triplet Higgs bosons) at the scale ∼10 TeV. This theory predicts the stability of the EW-vacuum and the accuracy of the MPP. Full article
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Open AccessArticle
Horizon Areas and Logarithmic Correction to the Charged Accelerating Black Hole Entropy
Received: 15 November 2018 / Revised: 5 January 2019 / Accepted: 1 February 2019 / Published: 11 February 2019
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Abstract
It has been shown by explicit and exact calculation that the geometric product formula i.e., area (or entropy) product formula of outer horizon (H+) and inner horizon (H) for charged accelerating black hole (BH) should neither be [...] Read more.
It has been shown by explicit and exact calculation that the geometric product formula i.e., area (or entropy) product formula of outer horizon ( H + ) and inner horizon ( H ) for charged accelerating black hole (BH) should neither be mass-independent nor quantized. This implies that the area (or entropy ) product is mass-independent conjecture has been broken down for charged accelerating BH. This also further implies that the mass-independent feature of the area product of H ± is not a generic feature at all. We also compute the Cosmic-Censorship-Inequality for this BH. Moreover, we compute the specific heat for this BH to determine the local thermodynamic stability. Under certain criterion, the BH shows the second order phase transition. Furthermore, we compute logarithmic corrections to the entropy for the said BH due to small statistical fluctuations around the thermal equilibrium. Full article
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Open AccessArticle
On Quantization of a Slowly Rotating Kerr Black Hole in Teleparallel Gravity
Received: 3 December 2018 / Revised: 7 January 2019 / Accepted: 11 January 2019 / Published: 16 January 2019
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Abstract
In this article we calculate the total angular momentum for Kerr space-time for slow rotations in the context of teleparallel gravity. In order to analyze the role of such a quantity, we apply Weyl quantization method to obtain a quantum equation for the [...] Read more.
In this article we calculate the total angular momentum for Kerr space-time for slow rotations in the context of teleparallel gravity. In order to analyze the role of such a quantity, we apply Weyl quantization method to obtain a quantum equation for the z-component of the angular momentum density, and for the squared angular momentum density as well. We present an approximate solution using the Adomian decomposition method (AM), which reveals a discrete characteristic for angular momentum. Full article

2018

Jump to: 2019, 2017, 2016, 2015

Open AccessFeature PaperArticle
Greybody Factors for Schwarzschild Black Holes: Path-Ordered Exponentials and Product Integrals
Received: 14 August 2018 / Revised: 28 August 2018 / Accepted: 28 August 2018 / Published: 3 September 2018
Cited by 4 | PDF Full-text (997 KB) | HTML Full-text | XML Full-text
Abstract
In earlier work concerning the sparsity of the Hawking flux, we found it necessary to re-examine what is known regarding the greybody factors of black holes, with a view to extending and expanding on some old results from the 1970s. Focusing specifically on [...] Read more.
In earlier work concerning the sparsity of the Hawking flux, we found it necessary to re-examine what is known regarding the greybody factors of black holes, with a view to extending and expanding on some old results from the 1970s. Focusing specifically on Schwarzschild black holes, we have re-calculated and re-assessed the greybody factors using a path-ordered-exponential approach, a technique which has the virtue of providing a pedagogically useful semi-explicit formula for the relevant Bogoliubov coefficients. These path-ordered-exponentials, being based on a variant of the “transfer matrix” formalism, are closely related to so-called “product integrals”, leading to quite straightforward and direct numerical evaluation, while side-stepping any need for numerically solving the relevant ordinary differential equations. Furthermore, while considerable analytic information is already available regarding both the high-frequency and low-frequency asymptotics of these greybody factors, numerical approaches seem better adapted to finding suitable “global models” for these greybody factors in the intermediate frequency regime, where most of the Hawking flux is actually concentrated. Working in a more general context, these path-ordered-exponential techniques are also likely to be of interest for generic barrier-penetration problems. Full article
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Open AccessArticle
High Speed Cylindrical Gravitational Collapse with Anisotropic Pressure
Received: 28 April 2018 / Revised: 19 May 2018 / Accepted: 30 May 2018 / Published: 5 June 2018
Cited by 1 | PDF Full-text (258 KB) | HTML Full-text | XML Full-text
Abstract
This paper focuses on the cylindrical symmetric gravitational collapse in the presence of anisotropic fluid. The high speed approximation scheme was used. In this perspective, the effect of anisotropy of pressure in fluid distribution on the collapsing process with the Equation of State [...] Read more.
This paper focuses on the cylindrical symmetric gravitational collapse in the presence of anisotropic fluid. The high speed approximation scheme was used. In this perspective, the effect of anisotropy of pressure in fluid distribution on the collapsing process with the Equation of State (EoS) p t = λ ρ and p r = l ρ , ( l + 2 λ < - 1 ) . The effect of pressure on collapse in radial and tangential direction was observed for all values of λ and l. It is determined that, for some values of constants, i.e., λ and l, collapse results in a Naked Singularity (NS) while, for some values of constants, it does not form NS or Black Hole (BH). This study presents the effect on the collapsing process for all values of λ and l. Full article
Open AccessArticle
On a Model of Magnetically Charged Black Hole with Nonlinear Electrodynamics
Received: 13 April 2018 / Revised: 10 May 2018 / Accepted: 17 May 2018 / Published: 19 May 2018
Cited by 4 | PDF Full-text (892 KB) | HTML Full-text | XML Full-text
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
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Open AccessReview
Scalar-Tensor Black Holes Embedded in an Expanding Universe
Received: 22 November 2017 / Revised: 20 December 2017 / Accepted: 27 December 2017 / Published: 6 February 2018
Cited by 5 | PDF Full-text (320 KB) | HTML Full-text | XML Full-text
Abstract
In this review, we focus our attention on scalar-tensor gravity models and their empirical verification in terms of black hole and wormhole physics. We focus on black holes, embedded in an expanding universe, describing both cosmological and astrophysical scales. We show that in [...] Read more.
In this review, we focus our attention on scalar-tensor gravity models and their empirical verification in terms of black hole and wormhole physics. We focus on black holes, embedded in an expanding universe, describing both cosmological and astrophysical scales. We show that in scalar-tensor gravity it is quite common that the local geometry is isolated from the cosmological expansion, so that it does not backreact on the black hole metric. We try to extract common features of scalar-tensor black holes in an expanding universe and point out the issues that are not fully investigated. Full article

2017

Jump to: 2019, 2018, 2016, 2015

Open AccessArticle
On the Near-Horizon Canonical Quantum Microstates from AdS2/CFT1 and Conformal Weyl Gravity
Received: 22 March 2017 / Revised: 18 June 2017 / Accepted: 13 July 2017 / Published: 17 July 2017
Cited by 1 | PDF Full-text (293 KB) | HTML Full-text | XML Full-text
Abstract
We compute the full asymptotic symmetry group of black holes belonging to the same equivalence class of solutions within the conformal Weyl gravity formalism. We do this within an AdS2/CFT1 correspondence and by performing a [...] Read more.
We compute the full asymptotic symmetry group of black holes belonging to the same equivalence class of solutions within the conformal Weyl gravity formalism. We do this within an A d S 2 / C F T 1 correspondence and by performing a Robinson–Wilczek two-dimensional reduction, thus enabling the construction of effective quantum theory of the remaining field content. The resulting energy momentum tensors generate asymptotic Virasoro algebras to s-waves, with calculable central extensions. These centers, in conjunction with their proper regularized lowest Virasoro eigenmodes, yield the Bekenstein–Hawking black hole entropy via the statistical Cardy formula. We also analyze quantum holomorphic fluxes of the dual conformal field theories (CFTs) in the near horizon, giving rise to finite Hawking temperatures weighted by the central charges of the respective black hole spacetimes. We conclude with a discussion and outlook for future work. Full article
Open AccessReview
Black Holes: Eliminating Information or Illuminating New Physics?
Received: 6 March 2017 / Revised: 3 July 2017 / Accepted: 7 July 2017 / Published: 13 July 2017
Cited by 23 | PDF Full-text (700 KB) | HTML Full-text | XML Full-text
Abstract
Black holes, initially thought of as very interesting mathematical and geometric solutions of general relativity, over time, have come up with surprises and challenges for modern physics. In modern times, they have started to test our confidence in the fundamental understanding of nature. [...] Read more.
Black holes, initially thought of as very interesting mathematical and geometric solutions of general relativity, over time, have come up with surprises and challenges for modern physics. In modern times, they have started to test our confidence in the fundamental understanding of nature. The most serious charge on the black holes is that they eat up information, never to release and subsequently erase it. This goes absolutely against the sacred principles of all other branches of fundamental sciences. This realization has shaken the very base of foundational concepts, both in quantum theory and gravity, which we always took for granted. Attempts to get rid of of this charge, have led us to crossroads with concepts, hold dearly in quantum theory. The sphere of black hole’s tussle with quantum theory has readily and steadily grown, from the advent of the Hawking radiation some four decades back, into domain of quantum information theory in modern times, most aptly, recently put in the form of the firewall puzzle. Do black holes really indicate something sinister about their existence or do they really point towards the troubles of ignoring the fundamental issues, our modern theories are seemingly plagued with? In this review, we focus on issues pertaining to black hole evaporation, the development of the information loss paradox, its recent formulation, the leading debates and promising directions in the community. Full article
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Open AccessArticle
Non-Linear Stationary Solutions in Realistic Models for Analog Black-Hole Lasers
Received: 9 March 2017 / Revised: 25 June 2017 / Accepted: 4 July 2017 / Published: 10 July 2017
Cited by 1 | PDF Full-text (987 KB) | HTML Full-text | XML Full-text
Abstract
From both a theoretical and an experimental point of view, Bose–Einstein condensates are good candidates for studying gravitational analogues of black holes and black-hole lasers. In particular, a recent experiment has shown that a black-hole laser configuration can be created in the laboratory. [...] Read more.
From both a theoretical and an experimental point of view, Bose–Einstein condensates are good candidates for studying gravitational analogues of black holes and black-hole lasers. In particular, a recent experiment has shown that a black-hole laser configuration can be created in the laboratory. However, the most considered theoretical models for analog black-hole lasers are quite difficult to implement experimentally. In order to fill this gap, we devote this work to present more realistic models for black-hole lasers. For that purpose, we first prove that, by symmetrically extending every black-hole configuration, one can obtain a black-hole laser configuration with an arbitrarily large supersonic region. Based on this result, we propose the use of an attractive square well and a double delta-barrier, which can be implemented using standard experimental tools, for studying black-hole lasers. We also compute the different stationary states of these setups, identifying the true ground state of the system and discussing the relation between the obtained solutions and the appearance of dynamical instabilities. Full article
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Open AccessArticle
A Model of Black Hole Evaporation and 4D Weyl Anomaly
Received: 19 May 2017 / Revised: 12 June 2017 / Accepted: 12 June 2017 / Published: 16 June 2017
Cited by 25 | PDF Full-text (1151 KB) | HTML Full-text | XML Full-text
Abstract
We analyze the time evolution of a spherically-symmetric collapsing matter from the point of view that black holes evaporate by nature. We consider conformal matters and solve the semi-classical Einstein equation Gμν=8πGTμν [...] Read more.
We analyze the time evolution of a spherically-symmetric collapsing matter from the point of view that black holes evaporate by nature. We consider conformal matters and solve the semi-classical Einstein equation G μ ν = 8 π G T μ ν by using the four-dimensional Weyl anomaly with a large c coefficient. Here, T μ ν contains the contribution from both the collapsing matter and Hawking radiation. The solution indicates that the collapsing matter forms a dense object and evaporates without horizon or singularity, and it has a surface, but looks like an ordinary black hole from the outside. Any object we recognize as a black hole should be such an object. Full article
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Open AccessReview
Classical Collapse to Black Holes and Quantum Bounces: A Review
Received: 12 March 2017 / Revised: 18 May 2017 / Accepted: 22 May 2017 / Published: 25 May 2017
Cited by 26 | PDF Full-text (1333 KB) | HTML Full-text | XML Full-text
Abstract
In the last four decades, different programs have been carried out aiming at understanding the final fate of gravitational collapse of massive bodies once some prescriptions for the behaviour of gravity in the strong field regime are provided. The general picture arising from [...] Read more.
In the last four decades, different programs have been carried out aiming at understanding the final fate of gravitational collapse of massive bodies once some prescriptions for the behaviour of gravity in the strong field regime are provided. The general picture arising from most of these scenarios is that the classical singularity at the end of collapse is replaced by a bounce. The most striking consequence of the bounce is that the black hole horizon may live for only a finite time. The possible implications for astrophysics are important since, if these models capture the essence of the collapse of a massive star, an observable signature of quantum gravity may be hiding in astrophysical phenomena. One intriguing idea that is implied by these models is the possible existence of exotic compact objects, of high density and finite size, that may not be covered by an horizon. The present article outlines the main features of these collapse models and some of the most relevant open problems. The aim is to provide a comprehensive (as much as possible) overview of the current status of the field from the point of view of astrophysics. As a little extra, a new toy model for collapse leading to the formation of a quasi static compact object is presented. Full article
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Open AccessReview
Janis–Newman Algorithm: Generating Rotating and NUT Charged Black Holes
Received: 27 December 2016 / Revised: 26 January 2017 / Accepted: 24 February 2017 / Published: 7 March 2017
Cited by 8 | PDF Full-text (356 KB) | HTML Full-text | XML Full-text
Abstract
In this review we present the most general form of the Janis–Newman algorithm. This extension allows generating configurations which contain all bosonic fields with spin less than or equal to two (real and complex scalar fields, gauge fields, metric field) and with five [...] Read more.
In this review we present the most general form of the Janis–Newman algorithm. This extension allows generating configurations which contain all bosonic fields with spin less than or equal to two (real and complex scalar fields, gauge fields, metric field) and with five of the six parameters of the Plebański–Demiański metric (mass, electric charge, magnetic charge, NUT charge and angular momentum). Several examples are included to illustrate the algorithm. We also discuss the extension of the algorithm to other dimensions. Full article
Open AccessArticle
A Zeroth Law Compatible Model to Kerr Black Hole Thermodynamics
Received: 17 December 2016 / Revised: 6 February 2017 / Accepted: 14 February 2017 / Published: 16 February 2017
Cited by 7 | PDF Full-text (295 KB) | HTML Full-text | XML Full-text
Abstract
We consider the thermodynamic and stability problem of Kerr black holes arising from the nonextensive/nonadditive nature of the Bekenstein–Hawking entropy formula. Nonadditive thermodynamics is often criticized by asserting that the zeroth law cannot be compatible with nonadditive composition rules, so in this work [...] Read more.
We consider the thermodynamic and stability problem of Kerr black holes arising from the nonextensive/nonadditive nature of the Bekenstein–Hawking entropy formula. Nonadditive thermodynamics is often criticized by asserting that the zeroth law cannot be compatible with nonadditive composition rules, so in this work we follow the so-called formal logarithm method to derive an additive entropy function for Kerr black holes also satisfying the zeroth law’s requirement. Starting from the most general, equilibrium compatible, nonadditive entropy composition rule of Abe, we consider the simplest non-parametric approach that is generated by the explicit nonadditive form of the Bekenstein–Hawking formula. This analysis extends our previous results on the Schwarzschild case, and shows that the zeroth law-compatible temperature function in the model is independent of the mass–energy parameter of the black hole. By applying the Poincaré turning point method, we also study the thermodynamic stability problem in the system. Full article
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Open AccessArticle
Peccei–Quinn Transformations and Black Holes: Orbit Transmutations and Entanglement Generation
Received: 1 December 2016 / Accepted: 4 February 2017 / Published: 14 February 2017
Cited by 2 | PDF Full-text (432 KB) | HTML Full-text | XML Full-text
Abstract
In a recent paper (Mod. Phys. Lett. A 2015, 30, 1550104), the black-hole/qubit correspondence (BHQC) was exploited to define “black hole quantum circuits” allowing for a change of the supersymmetry-preserving features of electromagnetic charge configurations supporting the black hole solution. This resulted in [...] Read more.
In a recent paper (Mod. Phys. Lett. A 2015, 30, 1550104), the black-hole/qubit correspondence (BHQC) was exploited to define “black hole quantum circuits” allowing for a change of the supersymmetry-preserving features of electromagnetic charge configurations supporting the black hole solution. This resulted in switching from one U-duality orbit to another, or equivalently, from an element of the corresponding Freudenthal triple system with a definite rank to another one. On the supergravity side of BHQC, such quantum gates are related to particular symplectic transformations acting on the black hole charges; namely, such transformations cannot belong to the U-duality group, otherwise switching among orbits would be impossible. In this paper, we consider a particular class of such symplectic transformations, namely the ones belonging to the so-called Peccei–Quinn symplectic group, introduced some time ago within the study of very special Kähler geometries of the vector multiplets’ scalar manifolds in N = 2 supergravity in D =4 spacetime dimensions. Full article

2016

Jump to: 2019, 2018, 2017, 2015

Open AccessArticle
The Problem of Embedded Eigenvalues for the Dirac Equation in the Schwarzschild Black Hole Metric
Received: 18 September 2016 / Revised: 20 November 2016 / Accepted: 28 November 2016 / Published: 2 December 2016
Cited by 10 | PDF Full-text (304 KB) | HTML Full-text | XML Full-text
Abstract
We use the Dirac equation in a fixed black hole background and different independent techniques to demonstrate the absence of fermionic bound states around a Schwarzschild black hole. In particular, we show that no embedded eigenvalues exist which has been claimed for the [...] Read more.
We use the Dirac equation in a fixed black hole background and different independent techniques to demonstrate the absence of fermionic bound states around a Schwarzschild black hole. In particular, we show that no embedded eigenvalues exist which has been claimed for the case when the energy is less than the particle’s mass. We explicitly prove that the claims regarding the embedded eigenvalues can be traced back to an oversimplified approximation in the calculation. We conclude that no bound states exist regardless of the value of the mass. Full article
Open AccessArticle
Reissner–Nordström Anti-de Sitter Black Holes in Mimetic F(R) Gravity
Received: 18 April 2016 / Revised: 11 May 2016 / Accepted: 18 May 2016 / Published: 30 May 2016
Cited by 19 | PDF Full-text (270 KB) | HTML Full-text | XML Full-text
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
Open AccessArticle
Black Holes and Exotic Spinors
Received: 24 March 2016 / Revised: 11 May 2016 / Accepted: 17 May 2016 / Published: 26 May 2016
Cited by 3 | PDF Full-text (408 KB) | HTML Full-text | XML Full-text
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
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Open AccessReview
Where Does the Physics of Extreme Gravitational Collapse Reside?
Received: 20 October 2015 / Revised: 28 April 2016 / Accepted: 3 May 2016 / Published: 13 May 2016
Cited by 29 | PDF Full-text (828 KB) | HTML Full-text | XML Full-text
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
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2015

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Open AccessArticle
Thermodynamic Analysis of Non-Linear Reissner-Nordström Black Holes
Universe 2015, 1(3), 412-421; https://doi.org/10.3390/universe1030412
Received: 27 October 2015 / Revised: 6 November 2015 / Accepted: 9 November 2015 / Published: 16 November 2015
Cited by 3 | PDF Full-text (1491 KB) | HTML Full-text | XML Full-text
Abstract
In the present article we study the Inverse Electrodynamics Model. This model is a gauge and parity invariant non-linear Electrodynamics theory, which respects the conformal invariance of standard Electrodynamics. This modified Electrodynamics model, when minimally coupled to General Relativity, is compatible with static [...] Read more.
In the present article we study the Inverse Electrodynamics Model. This model is a gauge and parity invariant non-linear Electrodynamics theory, which respects the conformal invariance of standard Electrodynamics. This modified Electrodynamics model, when minimally coupled to General Relativity, is compatible with static and spherically symmetric Reissner-Nordström-like black-hole solutions. However, these black-hole solutions present more complex thermodynamic properties than their Reissner-Nordström black-hole solutions counterparts in standard Electrodynamics. In particular, in the Inverse Model a new stability region, with both the heat capacity and the free energy negative, arises. Moreover, unlike the scenario in standard Electrodynamics, a sole transition phase is possible for a suitable choice in the set of parameters of these solutions. Full article
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Open AccessArticle
Nonsingular Black Holes in ƒ (R) Theories
Universe 2015, 1(2), 173-185; https://doi.org/10.3390/universe1020173
Received: 21 May 2015 / Revised: 28 July 2015 / Accepted: 29 July 2015 / Published: 4 August 2015
Cited by 42 | PDF Full-text (289 KB) | HTML Full-text | XML Full-text
Abstract
We study the structure of a family of static, spherically symmetric space-times generated by an anisotropic fluid and governed by a particular type of f(R) theory. We find that for a range of parameters with physical interest, such solutions represent black holes with [...] Read more.
We study the structure of a family of static, spherically symmetric space-times generated by an anisotropic fluid and governed by a particular type of f(R) theory. We find that for a range of parameters with physical interest, such solutions represent black holes with the central singularity replaced by a finite size wormhole. We show that time-like geodesics and null geodesics with nonzero angular momentum never reach the wormhole throat due to an infinite potential barrier. For null radial geodesics, it takes an infinite affine time to reach the wormhole. This means that the resulting space-time is geodesically complete and, therefore, nonsingular despite the generic existence of curvature divergences at the wormhole throat. Full article
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