arXiv subjects

B. Pourhassan

Publications and source records attributed to B. Pourhassan.

At least 19 recordsLinked to original sources

arXiv2024

Non-extensive Entropy and Holographic Thermodynamics: Topological Insights

In this paper, we delve into the thermodynamic topology of AdS Einstein-Gauss-Bonnet black holes, employing non-extensive entropy formulations such as Barrow, Rényi, and Sharma-Mittal entropy within two distinct frameworks: bulk boundary and restricted phase space (RPS) thermodynamics. Our findings reveal that in the bulk boundary framework, the topological charges, are influenced by the free parameters and the Barrow non-extensive parameter $(δ)$. So, we faced three topological charges $(ω= +1, -1, +1)$. When the parameter $δ$ increases to 0.9, the classification changes, resulting in two topological charges $(ω= +1, -1)$. When $δ$ is set to zero, the equations reduce to the Bekenstein-Hawking entropy structure, yielding consistent results with three topological charges. Additionally, setting the non-extensive parameter $λ$ in Rényi entropy to zero increases the number of topological charges, but the total topological charge remains (W = +1). The presence of the Rényi non-extensive parameter alters the topological behavior compared to the Bekenstein-Hawking entropy. Sharma-Mittal entropy shows different classifications and the various numbers of topological charges influenced by the non-extensive parameters $α$ and $β$. When $α$ and $β$ have values close to each other, three topological charges with a total topological charge $(W = +1)$ are observed. Varying one parameter while keeping the other constant significantly changes the topological classification and number of topological charges. In contrast, the RPS framework demonstrates remarkable consistency in topological behavior. Under all conditions and for all free parameters, the topological charge remains $(ω= +1)$ with the total topological charge $(W = +1)$. This uniformity persists even when reduced to Bekenstein-Hawking entropy.

hep-th↗

arXiv2023

de Sitter Swampland Conjecture in String Field Inflation

In this paper, we study a particular type of inflation by using non-local Friedman equations that are somehow derived from the zero levels of string field theory and express a tachyonic action. Then, we challenge it by further refining de Sitter (dS) swampland conjecture (FRdSSC) monitoring. Therefore, we investigate some quantities, such as potential and Hubble parameters. We also consider slow-roll parameters to examine quantities such as the scalar spectrum index and the tensor-to-scalar ratio. Using straightforward calculations, we investigate this model from the swampland conjecture perspective in terms of the cosmological parameters, i.e., ($n_s$), ($r$), and observable data such as Planck 2018, by constructing some structures such as $(c_1,2-n_s)$ and $(c_1,2-r_s)$. Then, we make a new restriction for this conjecture as $c_12c_22$ and get a limit for this model in the range $c0.0942$. We find this inflationary model is strongly in tension with the dS swampland conjecture (dSSC), i.e., $c_1=c_2 \neq \mathcal{O}(1)$. So, we shall challenge it with the FRdSSC, which has some free parameters, viz., $a,b>0$, $a+b=1$, and $q>2$. By setting these parameters, we examine the compatibility of the mentioned conjecture with this inflationary model. Finally, we infer from this string field inflation model that it satisfies the FRdSSC with the constraint of its free parameters $a$, $b$, and $q$.

hep-th↗

arXiv2022

Exponential corrected thermodynamics of Born-Infeld BTZ black holes in massive gravity

It is known that entropy of black hole gets correction at quantum level. Universally, these corrections are logarithmic and exponential in nature. We analyze the impacts of these quantum corrections on thermodynamics of Born-Infeld BTZ black hole in massive gravity by considering both such kinds of correction. We do comparative analysis of corrected thermodynamics with their equilibrium values. Here, we find that the exponential correction yields to the second point of the first order phase transition. Also, quantum correction effects significantly on the Helmholtz free energy of larger black holes. We study the equation of state for the exponential corrected black hole to obtain a leading order virial expansion.

gr-qc↗

arXiv2022

Cosmic evolution of the logarithmic f(R) model and the dS swampland conjecture

In this paper, we study the inflationary scenario in logarithmic f(R) gravity, where the rate of inflation roll is constant. On the other hand, our gravitational f(R) model is a polynomial plus a logarithmic term. We take advantage of constant-roll conditions and investigate the cosmic evolution of the logarithmic f(R) gravity. Therefore, we plot some figures such as the scalar spectrum index $n_{s}$ and tensor-to-scaler ratio $r$ concerning $n$, $β$ and model's constant parameters, i.e., $α$, $θ$ and $γ$ respectively. Also, we obtain the potential by using the constant roll condition. We know that the potential value obtained with this condition has an exact value. Next, we challenge it with refined swampland conjecture with respect to the Planck data. Finally, we compare our results with the experimental data, especially Planck 2018.

gr-qc↗

arXiv2022

The emergence of universal relations in the AdS black holes thermodynamics

Our primary goal in this paper is to confirm new universal relations in black hole thermodynamics. We investigate the universal relations by selecting different black holes. First, we obtain the black holes' thermodynamic relations assuming a new minor correction is added to the AdS part of the action. Then we confirm the universal relations by performing a series of direct calculations. It is noteworthy that according to each of the properties related to black holes, a new universal relation can be obtained according to this method. We confirm two different types of these universal relations for various block holes. Furthermore, we also consider black holes in AdS space surrounded by perfect fluid. We use the small correction to the action and obtain the modified thermodynamic quantities. We achieve two new universal relations which correspond to the parameters of perfect fluid and magnetic charge of the Bardeen AdS Black Hole. Finally, the new universal relation leads us to understand the charge-to-mass ratio, i.e., WGC-like behavior. We also find that the weak gravity conjecture condition is satisfied for the black hole surrounded by perfect fluid.

physics.gen-ph↗

arXiv2022

The effect of the WGC condition on the maximal energy extracted from black holes

In this paper, we study the Penrose process and the maximum energy extracted from the collision of two particles near the Kerr-Newman black hole with WGC condition. We consider the collision process when two particles collide in the ergosphere region of a black hole, scattering two new particles; one of them falls into the black hole and the other escapes to infinity. Our calculations also show an increasing in the energy. We also checked how much energy could be received from a black hole for particles with different spins. Results of this paper will help us to identify and study the black holes of astrophysics and the particles with different spins. Also, it states how black holes work in WGC conditions.

gr-qc↗

arXiv2022

Traversable wormhole in logarithmic $f(R)$ gravity by various shape and redshift functions

We study the traversable wormhole solutions for a logarithmic corrected $f(R)$ model by considering two different statements of shape $b(r)$ and redshift $Φ(r)$ functions. We calculate the parameters of the model including energy density $ρ$, tangential pressure $P_{t}$ and radial pressure $P_{r}$ for the corresponding forms of the functions. Then, we investigate different energy conditions such as null energy condition, weak energy condition, dominant energy condition and strong energy condition for our considered cases. Finally, we explain the satisfactory conditions of energy of the models by related plots.

gr-qc↗

arXiv2022

Smeared Mass Source Wormholes in Modified $f(R)$ Gravity with the Lorentzian Density Distribution Function

Wormholes are speculative structures linking disparate space-time points. Their geometry can be obtained by solving Einstein equations with tolerating the violation of null energy conditions. Recently, many researchers have studied different wormholes according to different criteria, and they achieved remarkable results. In this paper, we investigate a series of exact solutions of the static wormhole with smeared mass source geometry in modified $f(R)$ gravity theories. In fact, we consider the Lorentzian density distribution which is coming from a particle-like source. To be more specific, the modified gravity models we consider here are some power laws. We compute resulting solutions according to the wormhole field equations. We also specify parameters such as the radial pressure and transverse pressure as well as various energy conditions such as null energy conditions, weak energy conditions and strong energy conditions. Finally, by plotting some figures, in addition to identifying the wormhole throat, we describe the results of either the violation or the satisfaction of the energy conditions completely.

gr-qc↗

arXiv2022

Pleasant behavior of swampland conjectures in the face of specific inflationary models

Recently researchers have been studying various conditions as swampland criteria in cosmological implications. They have studied many inflation models with different swampland conditions. Occasionally these conjectures are modified and lead to fascinating points. The swampland conjecture is in contradiction with the slow-roll single-field inflation model. So in this paper, we will briefly describe resolving this contradiction with the special method; we consider a small parameter λ with respect to a coupling function in inflation and resolve this important antithesis. Then we will discuss a new point of view in studying inflation models according to the swampland criteria. Therefore we introduce some inflation models and challenge the swampland criteria. We specify the allowable range of cosmological parameters as the tensor-to-scalar ratio (r) the scalar spectral index (ns) according to the latest observable data. Next, we study the new constraints to determine the compatibility or incompatibility of the corresponding model with the swampland conjectures. Finally, we express the results and compare them with the latest observational data by plotting some figures.

astro-ph.CO↗

arXiv2022

Weak Gravity Conjecture, Black Branes and Violations of Universal Thermodynamic Relation

The universal thermodynamic relations between corrections to entropy and extremality for various black holes solutions have been studied. In this regard, we hereby consider a number of different black brane solutions in different structures for perturbative corrections to general relativity. These are, namely, black brane solution in Rastall AdS massive gravity, Einstein-Yang-Mills AdS black brane solution in massive gravity and general anisotropic black brane in Horava-Lifshitz gravity. We calculate both the entropy and extremality bound by introducing a small constant correction to the action. Remarkably, we find that black brane violates the universal thermodynamic relations. In other words, a universal relation between corrections to entropy and extremality are not valid in the black brane structure.

gr-qc↗

arXiv2021

Swampland Conjecture and Inflation Model from Brane Perspective

Over the past few decades, inflation models have been studied by researchers from different perspectives and conditions in order to introduce a model for the expanding universe. In this paper, we introduce a modified $f(R)$ gravitational model as ($R+γR^{p}$) in order to examine a new condition for inflation models. Given that our studies are related to a modified $f(R)$ gravitational model on the brane, therefore we will encounter modified cosmological parameters. So, we first introduce these modified cosmological parameters such as spectral index, a number of e-folds and etc. Then, we apply these conditions to our modified $f(R)$ gravitational model in order to adapt to the swampland criteria. Finally, we determine the range of each of these parameters by plotting some figures and with respect to observable data such as Planck 2018.

gr-qc↗

arXiv2021

Quantum corrected thermodynamics of nonlinearly charged BTZ black holes in massive gravity's rainbow

In this paper, we consider three-dimensional massive gravity's rainbow and obtain black hole solutions in three different cases of Born-Infeld, logarithmic, and exponential theories of nonlinear electrodynamics. We discuss the horizon structure and geometrical properties. Then, we study thermodynamics of these models by considering the first-order quantum correction effects, which appears as a logarithmic term in the black hole entropy. We discuss such effects on the black hole stability and phase transitions. We find that, due to the quantum corrections, the second-order phase transition happens in Born-Infeld and logarithmic models. We obtain the modified first law of black hole thermodynamics in the presence of logarithmic corrections.

gr-qc↗

arXiv2021

Kiselev/CFT correspondence and black hole thermodynamics

In this paper, we study the thermodynamic properties of Kiselev black hole and its holographic dual. We obtain the thermodynamic product formula for the Kiselev black hole. We consider Kiselev Black hole surrounded by both dust and radiation. We find that the area (or entropy) product formula for both cases is mass-independent as well as the case in the Einstein gravity, which interpreted as universal quantity. Moreover, we calculate the black hole entropy bound for both the inner and outer horizons. Furthermore, we show that the central charges of the left and right moving sectors are not the same via universal thermodynamic relations. Such universal relations lead us to calculate the central charge of conformal field theory (CFT). Finally, we see that the left and right of central charge of CFT are same.

hep-th↗

arXiv2021

Holographic nuclear matter in presence of chemical potential at finite temperature

We study the dense nuclear matter within the holographic Sakai-Sugimoto model. The nuclear matter is described via instantons in bulk, whose size has the new temperature dependency. Then, properties of nuclear matter have been studied for different temperatures. For example, free energy and baryon density are examined for different temperature values. Also, nuclear matter properties, like the speed of sound and connection of nuclear matter and quark matter, are discussed. As we have increased the temperature, the phase transition value has been changed from baryon to quark, which is non-physical. Also, we can see continuity between the baryonic and quark phases. Speed of sound at the low chemical potential in the presence of temperature is different, but when the potential increases, the effect of temperature on the speed of sound will remain unchanged. In the phase diagram, for significant chemical potential, chiral symmetry is restored and provides all things that we have the realistic model.

hep-th↗

arXiv2021

Gravitational partition function modified by super-light brane world perturbative modes

In this paper, we will analyze the effects of super-light brane world perturbative modes on clustering of galaxies. In present manuscript, we use the Boltzmann and Tsallis statistical approaches to study the large distance modification of gravity in the brane world. The impact of modified potential on clustering of galaxies is analyzed in both the approaches. The infinities associated with Newtonian point mass approximation of gravity models is removed using the analytical extensions. The regularized and finite partition function is obtained for the system of galaxies in the brane world model and is used to evaluate the regularized thermodynamic properties of the system in the form of equations of state. Hence, we study thermodynamic quantities and discuss about the thermodynamic stability of the model. We find that large number of galaxies may lead to the thermodynamic instability.

hep-th↗

arXiv2021

Two approaches that prove divergence free nature of non-local gravity

This paper is an attempt to study the thermodynamics of the structure formation in the large scale universe in the non local gravity using Boltzmann statistics and the Tsallis statistics. The partition function is obtained in both the approaches and the corresponding thermodynamics properties are evaluated. The important thing about the paper is that we surprisingly get the divergence free integrals and thus stress upon the fact that the nonlocal gravity is the singularity free model of gravity.

gr-qc↗

arXiv2021

Energy spectrum of massive Dirac particles in gapped graphene with Morse potential

In this paper, we study the massive Dirac equation with the presence of the Morse potential in polar coordinate. The Dirac Hamiltonian is written as two second-order differential equations in terms of two spinor wavefunctions. Since the motion of electrons in graphene is propagated like relativistic fermionic quasi-particles, then one is considered only with pseudospin symmetry for aligned spin and unaligned spin by arbitrary $k$. Next, we use the confluent Heun's function for calculating the wavefunctions and the eigenvalues. Then, the corresponding energy spectrum obtains in terms of $N$ and $k$. Afterward, we plot the graphs of the energy spectrum and the wavefunctions in terms of $k$ and $r$, respectively. Moreover, we investigate the graphene band structure by a linear dispersion relation which creates an energy gap in the Dirac points called gapped graphene. Finally, we plot the graph of the valence and conduction bands in terms of wavevectors.

quant-ph↗

arXiv2021

Effect of electromagnetic permeability on transition temperature of superconductivity

The description of superconductivity at high temperature is a problem that has recently been addressed. Transition temperature of superconductivity, $T_c$, depends on the lattice structure type, size, and room pressure. In super-lattices and low dimensional layered nanostructures, $T_c$ is increased, by increasing the complexity of the structure and internal pressures in solid lattice. In this paper, we investigate the relation between physical parameters ($ε, μ$) of matter and superconductivity properties as well as transition temperature ($T_c$), and explain the superconductivity at high temperature. In this study, a semi-classical electromagnetic description along with vortex topologic theory and quantum dynamic models with experimental data considered to justify the relation between superconductivity phenomena and magnetic monopole properties. We find that the electromagnetic energy of magnetic monopole is in agreement with vortex energy in topological theory and it can get close to thermal energy at high temperature. These models suggest the superconductivity is related to the mobile monopole or vortices. we show that the electrical permittivity ($ε$) and magnetic permeability ($μ$) of matter have a key role in the superconductive properties.

cond-mat.supr-con↗