scholarly journals A Conceptual Model for the Origin of the Cutoff Parameter in Exotic Compact Objects

Symmetry ◽  
2020 ◽  
Vol 12 (12) ◽  
pp. 2072
Author(s):  
Wilson Alexander Rojas Castillo ◽  
Jose Robel Arenas Salazar
Keyword(s):  
Black Hole ◽  
Conceptual Model ◽  
Initial Position ◽  
Compact Objects ◽  
Field Equations ◽  
Gravitational Radius ◽  
Cutoff Parameter ◽  
Gravitational Field Equations ◽  
Dynamic Quantity ◽  
The Difference

A Black Hole (BH) is a spacetime region with a horizon and where geodesics converge to a singularity. At such a point, the gravitational field equations fail. As an alternative to the problem of the singularity arises the existence of Exotic Compact Objects (ECOs) that prevent the problem of the singularity through a transition phase of matter once it has crossed the horizon. ECOs are characterized by a closeness parameter or cutoff, ϵ, which measures the degree of compactness of the object. This parameter is established as the difference between the radius of the ECO’s surface and the gravitational radius. Thus, different values of ϵ correspond to different types of ECOs. If ϵ is very big, the ECO behaves more like a star than a black hole. On the contrary, if ϵ tends to a very small value, the ECO behaves like a black hole. It is considered a conceptual model of the origin of the cutoff for ECOs, when a dust shell contracts gravitationally from an initial position to near the Schwarzschild radius. This allowed us to find that the cutoff makes two types of contributions: a classical one governed by General Relativity and one of a quantum nature, if the ECO is very close to the horizon, when estimating that the maximum entropy is contained within the material that composes the shell. Such entropy coincides with the Bekenstein–Hawking entropy. The established cutoff corresponds to a dynamic quantity dependent on coordinate time that is measured by a Fiducial Observer (FIDO). Without knowing the details about quantum gravity, parameter ϵ is calculated, which, in general, allows distinguishing the ECOs from BHs. Specifically, a black shell (ECO) is undistinguishable from a BH.

PHYSICAL PROPERTIES OF THE THIN ACCRETION DISK OF THE TAUB-NUT BLACK HOLE

2021 ◽  
Vol 0 (1) ◽  
pp. 87-91
Author(s):  
R.M. YUSUPOVA ◽  
◽  
R.N. ZMAILOV ◽  
Keyword(s):  
Black Hole ◽  
Angular Momentum ◽  
Gravitational Field ◽  
Physical Properties ◽  
Accretion Disk ◽  
Energy Flow ◽  
Field Equations ◽  
Effective Potential Energy ◽  
Gravitational Field Equations ◽  
Vacuum Solutions

The Taub-NUT space-time metric is one of the vacuum solutions to Einstein's gravitational field equations. In this metric, the Newman-Unti-Tamburino parameter (NUT) and its effect on the physical properties of a thin accretion disk are of particular interest. In this paper, calculations are performed to determine the physical properties of a thin accretion disk around the Taub-NUT black hole based on the Page-Thorne model. The influence of the NUT parameter on the angular velocity, binding energy, angular momentum of particles, effective potential, energy flow, and temperature of the accretion disk is revealed. According to the data obtained, the temperature of the accretion disk of the Taub-NUT black hole decreases as the value of the NUT parameter increases.

scholarly journals An X-ray reverberation mass measurement of Cygnus X-1

2019 ◽  
Vol 488 (1) ◽  
pp. 348-361 ◽  
Author(s):  
Guglielmo Mastroserio ◽  
Adam Ingram ◽  
Michiel van der Klis
Keyword(s):  
Black Hole ◽  
Time Scales ◽  
Mass Measurement ◽  
Stellar Mass ◽  
Accretion Disc ◽  
X Rays ◽  
Gravitational Radius ◽  
X Ray ◽  
Wide Range ◽  
The Difference

ABSTRACT We present the first X-ray reverberation mass measurement of a stellar-mass black hole. Accreting stellar-mass and supermassive black holes display characteristic spectral features resulting from reprocessing of hard X-rays by the accretion disc, such as an Fe Kα line and a Compton hump. This emission probes the innermost region of the accretion disc through general relativistic distortions to the line profile. However, these spectral distortions are insensitive to black hole mass, since they depend on disc geometry in units of gravitational radii. Measuring the reverberation lag resulting from the difference in path-length between direct and reflected emission calibrates the absolute length of the gravitational radius. We use a relativistic model able to reproduce the behaviour of the lags as a function of energy for a wide range of variability time-scales, addressing both the reverberation lags on short time-scales and the intrinsic hard lags on longer time-scales. We jointly fit the time-averaged spectrum and the real and imaginary parts of the cross-spectrum as a function of energy for a range of Fourier frequencies to Rossi X-ray Timing Exporer data from the X-ray binary Cygnus X-1. We also show that introducing a self-consistently calculated radial ionisation profile in the disc improves the fit, but requires us to impose an upper limit on ionization profile peak to allow a plausible value of the accretion disc density. This limit leads to a mass value more consistent with the existing dynamical measurement.

scholarly journals Thermodynamic approach to field equations in Lovelock gravity and f(R) gravity revisited

2014 ◽  
Vol 23 (11) ◽  
pp. 1450093 ◽  
Author(s):  
Yan-Gang Miao ◽  
Fang-Fang Yuan ◽  
Zheng-Zheng Zhang
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Gravitational Field ◽  
Black Hole Thermodynamics ◽  
Thermodynamic Approach ◽  
Field Equations ◽  
Lovelock Gravity ◽  
First Law Of Thermodynamics ◽  
Charged Black Holes ◽  
Gravitational Field Equations

The first law of thermodynamics at black hole horizons is known to be obtainable from the gravitational field equations. A recent study claims that the contributions at inner horizons should be considered in order to give the conventional first law of black hole thermodynamics. Following this method, we revisit the thermodynamic aspects of field equations in the Lovelock gravity and f(R) gravity by focusing on two typical classes of charged black holes in the two theories.

BRANEWORLD BLACK HOLES IN COSMOLOGY AND ASTROPHYSICS

2005 ◽  
Vol 14 (07) ◽  
pp. 1095-1129 ◽  
Author(s):  
A. S. MAJUMDAR ◽  
N. MUKHERJEE
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Gravitational Lensing ◽  
Strong Field ◽  
Planck Scale ◽  
High Energy ◽  
Field Equations ◽  
Large Extra Dimension ◽  
Gravitational Field Equations ◽  
Fundamental Scale

The braneworld description of our universe entails a large extra dimension and a fundamental scale of gravity that might be lower by several orders of magnitude compared to the Planck scale. An interesting consequence of the braneworld scenario is in the nature of spherically symmetric vacuum solutions to the brane gravitational field equations which could represent black holes with properties quite distinct compared to ordinary black holes in 4-dimensions. We discuss certain key features of some braneworld black hole geometries. Such black holes are likely to have diverse cosmological and astrophysical ramifications. The cosmological evolution of primordial braneworld black holes is described highlighting their longevity due to modified evaporation and effective accretion of radiation during the early braneworld high energy era. Observational abundance of various evaporation products of the black holes at different eras impose constraints on their initial mass fraction. Surviving primordial black holes could be candidates of dark matter present in galactic haloes. We discuss gravitational lensing by braneworld black holes. Observables related to the relativistic images of strong field gravitational lensing could in principle be used to distinguish between different braneworld black hole metrics in future observations.

scholarly journals The thermodynamics of collapsars

2016 ◽  
Author(s):  
Trevor W. Marshall
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Neutron Stars ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Field Equations ◽  
Gravitational Radius ◽  
Final Density ◽  
Shell Models ◽  
Independent Case

This article argues that there is a consistent description of gravitationally collapsed bodies, including neutron stars above the Tolman-Oppenheimer-Volkoff mass and also supermassive galactic centres, according to which collapse stops before the object reaches its gravitational radius, the density reaching a maximum close to the surface and then decreasing towards the centre. Models for such shell-like objects have been constructed using classic formulations found in the 1939 articles of Oppenheimer-Volkoff and Oppenheimer-Snyder. It was possible to modify the conclusions of the first article by changing the authors’ boundary conditions at r = 0. In the second case we find that the authors’ solution of the field equations needs no changes, but that the choice of their article’s title led many of their successors to believe that it supports the black-hole hypothesis. However, it is easily demonstrated that their final density distribution accords with the shell models found in our articles. Because black holes, according to many formulations, "have no hair", their thermodynamics is rather simple. The kind of collapsar which our models describe are more like main-sequence stars; they have spatiotemporal distributions of pressure, density and temperature, that is they have hair. In this article we shall concentrate on the dynamics of the Oppenheimer-Snyder collapsar; both pressure and temperature are everywhere zero, so there can be no thermodynamics. Only in the time independent case of Oppenheimer-Volkoff type models is it currently feasible to consider some thermodynamic implications; here some valuable new insights are obtained through the incorporation of the Oppenheimer-Snyder dynamics.

scholarly journals Conceptual Model for Cutoff Origin in Exotic Compact Objects

2020 ◽  
Author(s):  
Wilson Alexander Rojas Castillo ◽  
Jose Robel Arenas Salazar
Keyword(s):  
Conceptual Model ◽  
Observation Time ◽  
Compact Objects ◽  
Second Phase ◽  
Planck Length ◽  
Dust Shell ◽  
External Observer ◽  
Gravitational Radius ◽  
Specific Position ◽  
Rapid Contraction

We propose a conceptual model for the closeness parameter $\epsilon$, which characterizes exotic compact objects (ECOs). To estimate $\epsilon$, a thin spherical dust shell is considered, which gravitationally contracts from a specific position $r(t_{0})$ to near its gravitational radius $r(t_{2})=r_{s} + \epsilon$, in a finite time $t_{2}$, measured in the frame of a fiducial observer (FIDO). For an external observer, the shell’s kinematics is characterized by two clearly distinguishable phases: one of rapid contraction, where the shell is far away from the gravitational radius, $r(t_{0})\gg r_{s}$, and a second phase quasi-stationary, $r(t)\sim r_{s}$, where all of the shell’s mass is concentrated around the associated horizon, such that for a FIDO, a black hole (BH)is undistinguishable from a shell configured as a black shell (BS). \\ In the semi-classical approximation $E\ll \kappa_{0}l_{p}^{2}$ and tends to zero when the observation time of collapse $t_{2}$, measured by FIDO, tends to infinity; $\kappa_{0}$ and $l_{p}$ are surface gravity and Planck length, respectively. The quantum effects are significant when $\epsilon\ll r(t_{2})$ and $\epsilon$ tends to $\kappa_{0}l_{p}^{2}$. \\ Without knowing details on quantum gravity, parameter $\epsilon$ is calculated, which, in general, allows distinguishing the ECOs from BHs. Specifically, a BS (ECO) is undistinguishable from a BH.

scholarly journals Compact scalar field solutions in EiBI gravity

2020 ◽  
Vol 29 (11) ◽  
pp. 2041011
Author(s):  
Victor I. Afonso
Keyword(s):  
General Relativity ◽  
Black Hole ◽  
Scalar Field ◽  
Proper Time ◽  
Compact Object ◽  
Compact Objects ◽  
Antipodal Point ◽  
Field Equations ◽  
Structural Modifications ◽  
Schwarzschild Radius

We discuss exact scalar field solutions describing gravitating compact objects in the Eddington-inspired Born–Infeld (EiBI) gravity, a member of the class of (metric-affine formulated) Ricci-based gravity (RBG) theories. We include a detailed account of the RBGs/GR correspondence exploited to analytically solve the field equations. The single parameter [Formula: see text] of the EiBI model defines two branches for the solution. The [Formula: see text] branch may be described as a “shell with no interior”, and constitutes an ill-defined, geodesically incomplete spacetime. The more interesting [Formula: see text] branch admits the interpretation of a “wormhole membrane”, an exotic horizonless compact object with the ability to transfer particles and light from any point on its surface (located slightly below the would-be Schwarzschild radius) to its antipodal point, in a vanishing fraction of proper time. This is a single example illustrating how the structural modifications introduced by the metric-affine formulation may lead to significant departures from General relativity (GR) even at astrophysically relevant scales, giving rise to physically plausible objects radically different from those we are used to think of in the metric approach, and that could act as a black hole mimickers whose shadows might present distinguishable signals.

scholarly journals Black hole solutions and Euler equation in Rastall and generalized Rastall theories of gravity

2019 ◽  
Vol 34 (37) ◽  
pp. 1950304 ◽  
Author(s):  
H. Moradpour ◽  
Y. Heydarzade ◽  
C. Corda ◽  
A. H. Ziaie ◽  
S. Ghaffari
Keyword(s):  
Black Hole ◽  
Euler Equation ◽  
Mass Function ◽  
Conformally Flat ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Gravitational Field Equations ◽  
Theories Of Gravity ◽  
Minimal Curvature ◽  
Thermodynamic Pressure

Focusing on the special case of generalized Rastall theory, as a subclass of the non-minimal curvature-matter coupling theories in which the field equations are mathematically similar to the Einstein field equations in the presence of cosmological constant, we find two classes of black hole (BH) solutions including (i) conformally flat solutions and (ii) non-singular BHs. Accepting the mass function definition and by using the entropy contents of the solutions along with thermodynamic definitions of temperature and pressure, we study the validity of Euler equation on the corresponding horizons. Our results show that the thermodynamic pressure, meeting the Euler equation, is not always equal to the pressure components appeared in the gravitational field equations and satisfies the first law of thermodynamics, a result which in fact depends on the presumed energy definition. The requirements of having solutions with equal thermodynamic and Hawking temperatures are also studied. Additionally, we study the conformally flat BHs in the Rastall framework. The consequences of employing generalized Misner–Sharp mass in studying the validity of the Euler equation are also addressed.

scholarly journals Horizon radiation reaction forces

2020 ◽  
Vol 2020 (10) ◽  
Author(s):  
Walter D. Goldberger ◽  
Ira Z. Rothstein
Keyword(s):  
Black Hole ◽  
Equations Of Motion ◽  
Finite Size ◽  
Radiation Reaction ◽  
Effective Field ◽  
Compact Objects ◽  
Finite Size Effect ◽  
Binary Black Holes ◽  
Dissipative Effects ◽  
Reaction Forces

Abstract Using Effective Field Theory (EFT) methods, we compute the effects of horizon dissipation on the gravitational interactions of relativistic binary black hole systems. We assume that the dynamics is perturbative, i.e it admits an expansion in powers of Newton’s constant (post-Minkowskian, or PM, approximation). As applications, we compute corrections to the scattering angle in a black hole collision due to dissipative effects to leading PM order, as well as the post-Newtonian (PN) corrections to the equations of motion of binary black holes in non-relativistic orbits, which represents the leading order finite size effect in the equations of motion. The methods developed here are also applicable to the case of more general compact objects, eg. neutron stars, where the magnitude of the dissipative effects depends on non-gravitational physics (e.g, the equation of state for nuclear matter).

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