arXiv subjects

B. Eslam Panah

Publications and source records attributed to B. Eslam Panah.

At least 37 records · Page 2Linked to original sources

arXiv2023

Structure of $3D$ gravastars in the context of massive gravity

In this paper, we investigate a new model of $(2+1)-$dimensional ($3D$) gravitational vacuum stars (gravastars) with an isotropic matter distribution anti-de Sitter (AdS) spacetime in the context of massive gravity. For this purpose, we explore free singularity models with a specific equation of state. Using Mazur-Mottola's approach, we predict $3D$ gravastars as alternatives to BTZ black holes in massive gravity. We find analytical solutions to the interior of gravastars free of singularities and event horizons. For a thin shell containing an ultra-relativistic stiff fluid, we discuss length, energy, and entropy. In conclusion, the parameter of massive gravity plays a significant role in predicting the proper length, energy contents and entropyand parameters of gravastars.

gr-qc↗

arXiv2023

Effect of massive graviton on dark energy star structure

The presence of massive gravitons in the field of massive gravity is considered as an important factor in investigating the structure of compact objects. Hence, we are encouraged to study the dark energy star structure in the Vegh's massive gravity. We consider that the equation of state governing the inner spacetime of the star is the extended Chaplygin gas, and then using this equation of state, we numerically solve the Tolman-Oppenheimer-Volkoff (TOV) equation in massive gravity. In the following, assuming different values of free parameters defined in massive gravity, we calculate the properties of dark energy star such as radial pressure, transverse pressure, anisotropy parameter, and other characteristics. Then, after obtaining the maximum mass and its corresponding radius, we compute redshift and compactness. The obtained results show that for this model of dark energy star, the maximum mass and its corresponding radius depend on the massive gravity's free parameters and anisotropy parameter. These results are consistent with the observational data, and cover the lower mass gap. We also demonstrate that all energy conditions are satisfied for this model, and in the presence of anisotropy, the dark energy star is potentially unstable.

gr-qc↗

arXiv2023

Three-dimensional accelerating AdS black holes in $F(R)$ gravity

Considering a three-dimensional $C-$metric, we obtain the exact accelerating black holes in the $F(R)$ theory of gravity coupled with and without a matter field. First, we extract uncharged accelerating AdS black hole solutions in $F(R)$ gravity. Then, we study the effects of various parameters on metric function, roots, and the temperature of these black holes. The temperature is always positive for the radii less than $\frac{1}{α}$, and it is negative for the radii more than $\frac{1}{α}$. We extend our study by coupling nonlinear electrodynamics as a matter filed to $F(R)$ gravity to obtain charged black holes in this theory. Next, we evaluate the effects of different parameters such as the electrical charge, accelerating parameter, angular, $F(R)$ gravity, and scalar curvature on the obtained solutions, roots, and temperature of three-dimensional charged accelerating AdS black holes. The results indicate that there is a root in which it depends on various parameters. The temperature of these black holes is positive after this root.

gr-qc↗

arXiv2023

Three-dimensional energy-dependent $C$-metric: black hole solutions

Considering a three-dimensional $C$-metric and adding energy-dependent to this spacetime, we first create a three-dimensional energy-dependent $C$-metric. Then, we extract accelerating BTZ black hole solutions in gravity's rainbow. Besides, we show that (A)dS black holes cover by an event horizon that depends on all the parameters of this theory. Using the definition of Hawking temperature, we obtain the temperature of these black holes and study the effects of various parameters on this quantity. We find a critical radius in which the temperature is always positive (negative) before (after) it. Then, we obtain the entropy of such black holes. Our analysis indicates that there is the same behavior for entropy, similar to the temperature. Indeed, before (after) the critical radius, the entropy is positive (negative). In order to study the local stability of such black holes, we calculate the heat capacity. We find two different behaviors for the heat capacity, which depend on the cosmological energy-dependent constant. As a final result, accelerating AdS BTZ black holes can satisfy the physical condition and local stability at the same time.

gr-qc↗

arXiv2023

Charged Accelerating BTZ black holes

In this paper, we first extract general uncharged accelerating BTZ black hole solutions and study some of their properties. Our analysis shows that spacetime's asymptotic behavior depends on four parameters: the cosmological constant, mass, acceleration, and topological constant. Then, we study the temperature of these black holes and find that the temperature is always positive for AdS spacetime. Next, we extend our study for extracting charged accelerating BTZ black hole solutions in the presence of a nonlinear electrodynamics field known as conformally invariant Maxwell. Our findings indicate a coupling between the electrical charge and other quantities of the accelerating BTZ black holes. The asymptotic behavior of charged accelerating BTZ black holes depends on five parameters: the cosmological constant, the electrical charge, mass, the acceleration parameter, and the topological constant. Then, we studied the effects of charge, acceleration parameters, and the topological constant on the root of these black holes. Finally, we investigate the temperature of these black holes in AdS spacetime. For these black holes, the temperature depends on the electrical charge, accelerating parameter, and the cosmological constant. Our analysis indicates that the temperature of charged accelerating BTZ AdS black holes is always positive.

gr-qc↗

arXiv2023

Topological phantom AdS black holes in $F(R)$ gravity

In this paper, we obtain exact phantom (A)dS black hole solutions in the context of $F(R)$ gravity with topological spacetime in four dimensions. Then, we study the effects of different parameters on the event horizon. In the following, we calculate the conserved and thermodynamic quantities of the system and check the first law of thermodynamics for these kinds of black holes. Next, we evaluate the local stability of the topological phantom (A)dS black holes in $F(R)$ gravity by studying the heat capacity and the geometrothemodynamic, where we show that the two approaches agrees. We extend our study and investigate global stability by employing the Gibbs potential and the Helmholtz free energy. In addition, the effects of different parameters on local and global stabilities will be highlighted.

gr-qc↗

arXiv2023

Stable three-dimensional (un)charged AdS gravastars in gravity's rainbow

In this work, we study the three-dimensional AdS gravitational vacuum stars (gravastars) in the context of gravity's rainbow theory. Then we extend it by adding the Maxwell electromagnetic field. We compute the physical features of gravastars, such as proper length, energy, entropy, and junction conditions. Our results show that the physical parameters for charged and uncharged states depend significantly on rainbow functions. Besides from charged state, they also depend on the electric field. Finally, we explore the stability of thin shell of three-dimensional (un)charged AdS gravastars in gravity's rainbow. We show that the structure of thin shell of these gravastars may be stable and is independent of the type of matter.

gr-qc↗

arXiv2023

Black holes in dRGT massive gravity with the signature of EHT observations of M87*

The recent Event Horizon Telescope (EHT) observations of the M87* have led to a surge of interest in studying the shadow of black holes. Besides, investigation of time evolution and lifetime of black holes helps us to veto/restrict some theoretical models in gravitating systems. Motivated by such exciting properties, we study optical features of black holes, such as the shadow geometrical shape and the energy emission rate in modified gravity. We consider a charged AdS black hole in dRGT massive gravity and look for criteria to restrict the free parameters of the theory. The main goal of this paper is to compare the shadow of the mentioned black hole in a rotating case with the EHT data to obtain the allowed regions of the model parameters. Therefore, we employ the Newman-Janis algorithm to build the rotating counterpart of static solution in dRGT massive gravity. We also calculate the energy emission rate for the rotating case and discuss how the rotation factor and other parameters affect the emission of particles around the black holes.

gr-qc↗

arXiv2022

Dark Energy Star in Gravity's Rainbow

The concept of dark energy can be a candidate for preventing the gravitational collapse of compact objects to singularities. According to the usefulness of gravity's rainbow in UV completion of general relativity (by providing a new description of spacetime), it can be an excellent option to study the behavior of compact objects near phase transition regions. In this work, we obtain a modified Tolman-Openheimer-Volkof (TOV) equation for anisotropic dark energy as a fluid by solving the field equations in gravity's rainbow. Next, to compare the results with general relativity, we use a generalized Tolman-Matese-Whitman mass function to determine the physical quantities such as energy density, radial pressure, transverse pressure, gravity profile, and anisotropy factor of the dark energy star. We evaluate the junction condition and investigate the dynamical stability of dark energy star thin shell in gravity's rainbow. We also study the energy conditions for the interior region of this star. We show that the coefficients of gravity's rainbow can significantly affect this non-singular compact object and modify the model near the phase transition region.

gr-qc↗

arXiv2022

Two-dimensional Lifshitz-like AdS black holes in $F(R)$ gravity

Two-dimensional ($2D$) Lifshitz-like black holes in special $F(R)$ gravity cases are extracted. We indicate an essential singularity at $r=0$, covered with an event horizon. Then conserved and thermodynamic quantities such as temperature, mass, entropy, and the heat capacity of $2D$ Lifshitz-like black holes in $F(R)$ gravity are evaluated. Our analysis shows that $2D$ Lifshitz-like black hole solutions can be physical solutions, provided that the cosmological constant is negative ($Λ<0$). Indeed, there is a phase transition between stable and unstable cases by increasing the radius of AdS black holes. In other words, the $2D$ Lifshitz-like AdS black holes with large radii are physical and enjoy thermal stability. The obtained $2D$ Lifshitz-like AdS-black holes in $F(R)$ gravity turn to the well-known $2D$ Schwarzschild AdS-black holes when the Lifshitz-like parameter is zero ($s=0$). Also, correspondence between these black hole solutions and the $2D$ rotating black hole solutions is found by adjusting the Lifshitz-like parameter.

gr-qc↗

arXiv2022

Is the remnant of GW190425 a strange quark star?

This study investigates the effects of different QCD models on the structure of strange quark stars (SQS). In these models, the running coupling constant has a finite value in the infrared region of energy. By imposing some constraints on the strange quark matter (SQM) and exploiting the analytic and background perturbation theories, the equations of states for the SQM are obtained. Then, the properties of SQSs in general relativity are evaluated. By using component masses of GW190425 \cite{Abbott2020ApJL} as well as some conversion relations between the baryonic mass and the gravitational mass, the remnant mass of GW190425 is obtained. Our results for the maximum gravitational mass of SQS are then compared with the remnant mass of GW190425. The results indicate that the obtained maximum gravitational masses are comparable to the remnant mass of GW190425. Therefore, it is proposed that the remnant mass of GW190425 might be a SQS.

gr-qc↗

arXiv2022

Structure of magnetized strange quark star in perturbative QCD

We have performed the leading order perturbative calculation to obtain the equation of state (EoS) of the strange quark matter (SQM) at zero temperature under the magnetic field B = 10^18 G. The SQM comprises two massless quark flavors (up and down) and one massive quark flavor(strange). Consequently, we have used the obtained EoS to calculate the maximum gravitational mass and the corresponding radius of the magnetized strange quark star (SQS). We have employed two approaches, including the regular perturbation theory (RPT) and the background perturbation theory (BPT). In RPT the infrared (IR) freezing effect of the coupling constant has not been accounted for, while this effect has been included in BPT. We have obtained the value of the maximum gravitational mass to be more than three times the solar mass. The validity of isotropic structure calculations for SQS has also been investigated. Our results show that the threshold magnetic field from which an anisotropic approach begins to be significant lies in the interval 2*10^18G < B < 3*10^18G. Furthermore, we have computed the redshift, compactness and Buchdahl-Bondi bound of the SQS to show that this compact object cannot be a black hole.

hep-ph↗

arXiv2022

The structure of hybrid neutron star in Einstein-$Λ$ gravity

In this paper, we investigate the structure of neutron stars by considering both the effects of the cosmological constant and the existence of quark matter for neutron stars in Einstein's gravity. For this purpose, we use a suitable equation of state (EoS) which includes a layer of hadronic matter, a mixed phase of quarks and hadrons, and a quark matter in the core. To investigate the effect of the cosmological constant on the structure of hybrid neutron stars, we utilize the modified TOV equation in Einstein-$Λ$ gravity. Then we drive the mass-radius relation for different values of the cosmological constant. Our results show that for small values of the cosmological constant ($Λ$), especially for the cosmological constant from the cosmological perspective $(Λ=10^{-52}$ $m^{-2})$, $Λ$ has no significant effect on the structure of hybrid neutron stars. But for higher values, for example, by considering $Λ>10^{-14}$ $m^{-2}$, this quantity affects the maximum mass and radius of these stars. We find an upper limit for the cosmological constant as $Λ<9\times 10^{-13}m^{-2}$, based on the fact that the gravitational redshift cannot be more than $1$ for stars. The maximum mass and radius of these stars decrease by increasing the cosmological constant $Λ$. Also, by determining and analyzing radius, the compactness, Kretschmann scalar, and gravitational red shift of the hybrid neutron stars with $M=1.4M_{\,\odot }$ in the presence of the cosmological constant, we find that by increasing $Λ$, they are contracted. Also, our results for dynamical stability show that these stars satisfy this condition.

physics.gen-ph↗

arXiv2022

Thermal Stability, P-V Criticality and Heat Engine of Charged Rotating Accelerating Black Holes

In this paper, we study thermodynamic features of the charged rotating accelerating black holes in anti-de Sitter spacetime. First, we consider these black holes as the thermodynamic systems and analyze thermal stability/instability through the use of heat capacity in the canonical ensemble. We also investigate the effects of angular momentum, electric charge and string tension on the thermodynamic quantities and stability of the system. Considering the known relation between pressure and the cosmological constant, we extract the critical quantities and discuss how the mentioned parameters affect them. Then, we construct a heat engine by taking into account this black hole as the working substance, and obtain the heat engine efficiency by considering a rectangle heat cycle in the $P-V$ plane. We examine the effects of black hole parameters on the efficiency and analyze their effective roles. Finally, by comparing the engine efficiency with Carnot efficiency, we investigate conditions in order to have a consistent thermodynamic second law.

hep-th↗

arXiv2021

Geometrical thermodynamics and P-V criticality of charged accelerating AdS black holes

The unusual asymptotic structure of the accelerating black holes led to ambiguity in their geometric characteristics and thermodynamic behavior. Motivated by the interesting properties of such black holes and the significant role of electric charge and string tension on their structure, we study the thermodynamic behavior of these black holes by two methods and examine the changes of free parameters on the thermal behavior of the black holes. First, we investigate phase transition and thermal stability of the system through the use of heat capacity in the non-extended phase space. We examine the effects of electric charge, string tension and the cosmological constant on the phase transition and stability of the system. We also find that to have a phase transition, we have to apply some constraints on the free parameters. Then, we employ the geometrical thermodynamic (GT) method to study phase transition and compare the obtained results with those of the heat capacity. Next, we work in the extended phase space by considering the cosmological constant as a dynamical pressure and evaluate the existence of van der Waals like phase transition. We obtain critical quantities and study the effective role of electric charge and string tension on these quantities. Finally, we make use of the GT method in the extended phase space and find that the results of the GT method, heat capacity and $P-V$ diagram lead to a consistent conclusion.

hep-th↗

arXiv2020

Charged 4D Einstein-Gauss-Bonnet-AdS Black Holes: Shadow, Energy Emission, Deflection Angle and Heat Engine

Recently, there has been a surge of interest in the 4D Einstein-Gauss-Bonnet (4D EGB) gravity theory which bypasses the Lovelock theorem and avoids Ostrogradsky's instability. Such a novel theory has nontrivial dynamics and presents several predictions for cosmology and black hole physics. Motivated by recent astrophysical observations and the importance of anti-de Sitter spacetime, we investigate shadow geometrical shapes and deflection angle of light from the charged AdS black holes in 4D EGB gravity theory. We explore the shadow behaviors and photon sphere around such black holes, and inspect the effect of different parameters on them. Then, we present a study regarding the energy emission rate of such black holes and analyze the significant role of the Gauss-Bonnet (GB) coupling constant in the radiation process. Then, we perform a discussion of holographic heat engines of charged 4D EGB-AdS black holes by obtaining the efficiency of a rectangular engine cycle. Finally, by comparing heat engine efficiency with the Carnot efficiency, we indicate that the ratio $\frac{η}{η_{c}}$ is always less than one which is consistent with the thermodynamic second law.

hep-th↗

arXiv2020

Simulation of geodesic trajectory of charged BTZ black holes in massive gravity

In order to classify and understand the spacetime structure, investigation of the geodesic motion of massive and massless particles is a key tool. So the geodesic equation is a central equation of gravitating systems and the subject of geodesics in the black hole dictionary attracted much attention. In this paper, we give a full description of geodesic motions in three-dimensional spacetime. We investigate the geodesics near charged BTZ black holes and then generalize our prescriptions to the case of massive gravity. We show that electric charge is a critical parameter for categorizing the geodesic motions of both lightlike and timelike particles. In addition, we classify the type of geodesics based on the particle properties and geometry of spacetime.

gr-qc↗

arXiv2020

Black Hole Remnant in Massive Gravity

The possibility of a nonzero graviton mass has been widely pursued in the literature. In this work we investigate a black hole solution in massive gravity with a degenerate fiducial metric often used in the literature. We find that the end state of Hawking evaporation leads to black hole remnant, which could help to ameliorate the information paradox. We prove that these remnants only exist in anti-de Sitter spacetime. Nevertheless, we speculate on their possible relevance to our Universe as dark matter candidate, in view of the possibility that our Universe could be inherently anti-de Sitter-like, with a transient accelerated expansion phase.

gr-qc↗