Stability of anisotropic compact objects in f(T) gravity

2017 ◽  
Vol 32 (07) ◽  
pp. 1750042 ◽  
Author(s):  
M. Zaeem-ul-Haq Bhatti ◽  
Z. Yousaf ◽  
Sonia Hanif
Keyword(s):  
Degrees Of Freedom ◽  
Evolutionary Process ◽  
Compact Object ◽  
Compact Objects ◽  
Dynamical Instability ◽  
Field Equations ◽  
Small Perturbations ◽  
Unstable Configuration ◽  
Teleparallel Theory ◽  
Fluid Configuration

We exhibit the dynamical instability of cylindrical compact object in the gravitational field of f(T) gravity, which is the simplest modification of teleparallel theory (TPT). We explore the field equations and conservation laws to provide the extra degrees of freedom governed by f(T) gravity. We investigate the behavior of small perturbations on geometric and material profile in the background of collapsing fluid configuration. The un/stable eras are studied under Newtonian (N) and post-Newtonian (pN) approximations. Our results show that the stiffness parameter has major role in determining the un/stable epochs of cylindrical object. The dark source terms of f(T) gravity lead to relatively more unstable configuration during its evolutionary process.

scholarly journals Scalar perturbation potentials in a homogeneous and isotropic Weitzenböck geometry

2016 ◽  
Vol 25 (07) ◽  
pp. 1650087
Author(s):  
A. Behboodi ◽  
S. Akhshabi ◽  
K. Nozari
Keyword(s):  
Degrees Of Freedom ◽  
Teleparallel Gravity ◽  
Scalar Perturbation ◽  
Coordinate Frame ◽  
Field Equations ◽  
Tetrad Field ◽  
Cosmological Perturbation ◽  
Gauge Invariant ◽  
Teleparallel Theory ◽  
Perturbation Parameters

We describe the fully gauge invariant cosmological perturbation equations in teleparallel gravity by using the gauge covariant version of the Stewart lemma for obtaining the variations in tetrad perturbations. In teleparallel theory, perturbations are the result of small fluctuations in the tetrad field. The tetrad transforms as a vector in both its holonomic and anholonomic indices. As a result, in the gauge invariant formalism, physical degrees of freedom are those combinations of perturbation parameters which remain invariant under a diffeomorphism in the coordinate frame, followed by an arbitrary rotation of the local inertial (Lorentz) frame. We derive these gauge invariant perturbation potentials for scalar perturbations and present the gauge invariant field equations governing their evolution.

DUAL NATURE OF RICCI SCALAR AND MODELS OF THE EARLY UNIVERSE

1999 ◽  
Vol 14 (16) ◽  
pp. 1021-1031 ◽  
Author(s):  
S. K. SRIVASTAVA
Keyword(s):  
Compact Object ◽  
Gravitational Effect ◽  
High Energy ◽  
Matter Field ◽  
Compact Objects ◽  
Ricci Scalar ◽  
Field Equations ◽  
Dual Nature ◽  
Gravitational Action ◽  
High Energy Level

In some of the earlier papers, it was noticed that at a high energy level Ricci scalar behaved in dual manner: (a) like a matter field and (b) like a geometrical field. In this letter, dual nature of the Ricci scalar is also obtained from a gravitational action where R2 and R3 terms dominate the Einstein–Hilbert Lagrangian in the gravitational action. Cosmological models are derived using dual role of the Ricci scalar. In an expanding model of the universe, local gravitational effect of a compact object is ignored. These models are interesting in the sense that these have capability of exhibiting gravitational effect of compact objects also in an expanding universe. Moreover, these models provide an inhomogeneous generalization of Robertson–Walker type models. Another important feature of the letter is the derivation of these models through physical theories like phase transition and spontaneous symmetry breaking, not through conventional approach of solving complicated Einstein's field equations.

scholarly journals Quasi-Periodic Oscillatory Motion of Particles Orbiting a Distorted, Deformed Compact Object

Universe ◽  
2021 ◽  
Vol 7 (11) ◽  
pp. 447
Author(s):  
Shokoufe Faraji ◽  
Audrey Trova
Keyword(s):  
Observational Data ◽  
High Frequency ◽  
Degrees Of Freedom ◽  
Dynamic Properties ◽  
Compact Object ◽  
Periodic Oscillation ◽  
Compact Objects ◽  
Periodic Oscillations ◽  
Quasi Periodic Oscillation ◽  
Test Particles

This work explores the dynamic properties of test particles surrounding a distorted, deformed compact object. The astrophysical motivation was to choose such a background as to constitute a more reasonable model of a real situation that arises in the vicinity of compact objects with the possibility of having parameters such as the extra physical degrees of freedom. This can facilitate associating observational data with astrophysical systems. This work’s main goal is to study the dynamic regime of motion and quasi-periodic oscillation in this background, depending on different parameters of the system. In addition, we exercise the resonant phenomena of the radial and vertical oscillations at their observed quasi-periodic oscillations frequency ratio 3:2 and show that the oscillatory frequencies of charged particles can be adequately related to the frequencies of the twin high-frequency quasi-periodic oscillations observed in some sources of the microquasar observational data.

The second rise of general relativity in astrophysics

2019 ◽  
Vol 34 (30) ◽  
pp. 1950244
Author(s):  
L. Neslušan
Keyword(s):  
General Relativity ◽  
Outer Surface ◽  
Compact Object ◽  
Large Mass ◽  
Compact Objects ◽  
Spherically Symmetric ◽  
Field Equations ◽  
Mathematical Basis ◽  
Symmetric Configuration ◽  
Newtonian Case

The field equations, which are the mathematical basis of the theory of general relativity, provide us with a much larger variety of solutions to model the neutron stars and other compact objects than are used in the current astrophysics. We point out some important consequences of the new kind of solutions of the field equations, which can be obtained if the astrophysical usage of general relativity is not constrained, and outline an impact of these solutions on the models of internal structure of compact objects. If general relativity is not constrained, it enables to construct the stable object, with the outer surface above the event horizon, of whatever large mass. A new concept of relativistic compact object is a consequence of newly discovered property of gravity, yielded by the field equations in a spherically symmetric configuration of matter: in comparison with the Newtonian case, a particle is more effectively attracted by a nearer than a more distant matter.

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.

Fixing redundant degrees of teleparallel equivalent of general relativity

2017 ◽  
Vol 14 (11) ◽  
pp. 1750154
Author(s):  
Gamal G. L. Nashed ◽  
B. Elkhatib
Keyword(s):  
General Relativity ◽  
Lorentz Transformation ◽  
Arbitrary Function ◽  
Degrees Of Freedom ◽  
Symmetric Tensor ◽  
Cylindrical Symmetry ◽  
Radial Coordinate ◽  
Field Equations ◽  
Local Lorentz Transformation ◽  
Teleparallel Theory

It is well known that the field equation of teleparallel theory which is equivalent to general relativity completely agrees with the field equations of general relativity. However, teleparallel equivalent of general relativity has six redundant degrees of freedom which spoil the true physics. These extra degrees are related to the local Lorentz transformation. In this study, we give three different tetrad fields having cylindrical symmetry and depend only on the radial coordinate. One of these tetrads contains an arbitrary function, which is responsible to reproduce the other solutions, which come from local Lorentz transformation. We show by explicate calculations that this arbitrary function spoils the calculations of the conserved charges. We formulate a skew-symmetric tensor whose vanishing value puts a constraint on this arbitrary function. This constraint fixed the redundant degrees of freedom which characterize the teleparallel equivalent of general relativity.

scholarly journals Mass-radius relation of compact objects

2019 ◽  
Vol 15 (S356) ◽  
pp. 383-384
Author(s):  
Seman Abaraya ◽  
Tolu Biressa
Keyword(s):  
Numerical Analysis ◽  
Equation Of State ◽  
Compact Objects ◽  
Mass Limit ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Formation And Evolution ◽  
Active Research ◽  
Compact Sources ◽  
Astrophysical Research

AbstractCompact objects are of great interest in astrophysical research. There are active research interests in understanding better various aspects of formation and evolution of these objects. In this paper we addressed some problems related to the compact objects mass limit. We employed Einstein field equations (EFEs) to derive the equation of state (EoS). With the assumption of high densities and low temperature of compact sources, the derived equation of state is reduced to polytropic kind. Studying the polytropic equations we obtained similar physical implications, in agreement to previous works. Using the latest version of Mathematica-11 in our numerical analysis, we also obtained similar results except slight differences in accuracy.

scholarly journals Compact stars in f(ℛ,𝒢,𝒯 ) gravity

2021 ◽  
Vol 36 (24) ◽  
pp. 2150165
Author(s):  
M. Ilyas
Keyword(s):  
Test Particle ◽  
Gravitational Theory ◽  
Momentum Tensor ◽  
Conservation Equation ◽  
Compact Objects ◽  
Compact Stars ◽  
Energy Conditions ◽  
Field Equations ◽  
Physical Features ◽  
The Impact

This work is to introduce a new kind of modified gravitational theory, named as [Formula: see text] (also [Formula: see text]) gravity, where [Formula: see text] is the Ricci scalar, [Formula: see text] is Gauss–Bonnet invariant and [Formula: see text] is the trace of the energy–momentum tensor. With the help of different models in this gravity, we investigate some physical features of different relativistic compact stars. For this purpose, we develop the effectively modified field equations, conservation equation, and the equation of motion for test particle. Then, we check the impact of additional force (massive test particle followed by a nongeodesic line of geometry) on compact objects. Furthermore, we took three notable stars named as [Formula: see text], [Formula: see text] and [Formula: see text]. The physical behavior of the energy density, anisotropic pressures, different energy conditions, stability, anisotropy, and the equilibrium scenario of these strange compact stars are analyzed through various plots. Finally, we conclude that the energy conditions hold, and the core of these stars is so dense.

scholarly journals Cosmological black holes are not described by the Thakurta metric: LIGO-Virgo bounds on PBHs remain unchanged

2021 ◽  
Vol 81 (11) ◽  
Author(s):  
Gert Hütsi ◽  
Tomi Koivisto ◽  
Martti Raidal ◽  
Ville Vaskonen ◽  
Hardi Veermäe
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Dark Matter ◽  
Compact Object ◽  
Compact Objects ◽  
Primordial Black Hole ◽  
Mass Growth ◽  
Black Hole Binaries ◽  
Physical Conditions ◽  
Accretion Physics

AbstractWe show that the physical conditions which induce the Thakurta metric, recently studied by Bœhm et al. in the context of time-dependent black hole masses, correspond to a single accreting compact object in the entire Universe filled with isotropic non-interacting dust. In such a case, accretion physics is not local but tied to the properties of the whole Universe. We show that radiation, primordial black holes or particle dark matter cannot produce the specific energy flux required for supporting the mass growth of the compact objects described by the Thakurta metric. In particular, this solution does not apply to black hole binaries. We conclude that compact dark matter candidates and their mass growth cannot be described by the Thakurta metric, and thus existing constraints on the primordial black hole abundance from the LIGO-Virgo and the CMB measurements remain valid.

scholarly journals Accretion onto Relativistic Objects

1974 ◽  
Vol 64 ◽  
pp. 194-212
Author(s):  
M. J. Rees
Keyword(s):  
Black Holes ◽  
Interstellar Medium ◽  
Neutron Stars ◽  
Binary Systems ◽  
Compact Object ◽  
Stellar Mass ◽  
Supermassive Black Holes ◽  
Compact Objects ◽  
X Ray ◽  
Intergalactic Space

The physics of spherically symmetrical accretion onto a compact object is briefly reviewed. Neither neutron stars nor stellar-mass black holes are likely to be readily detectable if they are isolated and accreting from the interstellar medium. Supermassive black holes in intergalactic space may however be detectable. The effects of accretion onto compact objects in binary systems are then discussed, with reference to the phenomena observed in variable X-ray sources.

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