arXiv subjects

B. Pourhassan

Publications and source records attributed to B. Pourhassan.

At least 55 records · Page 3Linked to original sources

arXiv2016

Extended Chaplygin Gas in Horava-Lifshitz Gravity

In this paper, we investigate cosmological models of the extended Chaplygin gas in a universe governed by Horava-Lifshitz gravity. The equation of state for an extended Chaplygin gas is a $(n+2)$-variable equation determined by $A_{n}$, $α$, and $B$. In this work, we are interested to the case of second order $(n=2)$ equation of state which recovers quadratic barotropic equation of state. In that case there are four free parameters. We solve conservation equation approximately and obtain energy density in terms of scale factor with mentioned free parameters. Under some assumptions we relate free parameters to each other to have only one free independent parameter $(A_{2})$. It help us to obtain explicit expression for energy density in terms of scale factor. The allowed values of the second order extended Chaplygin gas parameter is fixed using the recent astrophysical and cosmological observational data. Thermodynamics of the model investigated based on the first and second law of thermodynamics.

gr-qc↗

arXiv2016

Logarithmic corrected Polynomial $f(R)$ inflation mimicking a cosmological constant

In this paper, we consider an inflationary model of $f(R)$ gravity with polynomial form plus logarithmic term. We calculate some cosmological parameters and compare our results with the Plank 2015 data. We find that presence of both logarithmic and polynomial corrections are necessary to yield slow-roll condition. Also, we study critical points and stability of the model to find that it is a viable model.

gr-qc↗

arXiv2015

The Klein-Gordon Equation of a Rotating Charged Hairy Black Hole in (2+1) Dimensions

In this paper, we consider the Klein-Gordon equation in a 3D charged rotating hairy black hole background to study behavior of a massive scalar field. In the general case we find periodic-like behavior for the scalar field which may be vanishes at the black hole horizon or far from the black hole horizon. For the special cases of non-rotating or near horizon approximation we find radial solution of Klein-Gordon equation in terms of hypergeometric and Kummer functions. Also for the case of uncharged black hole we find numerical solution of the Klein-Gordon equation as periodic function which may enhanced out of the black hole or vanish at horizon. We find allowed boundary conditions which yield to the identical bosons described by scalar field.

hep-th↗

arXiv2015

Thermal Fluctuations in a Charged AdS Black Hole

In this paper, we will analyze the effects of thermal fluctuations on a charged AdS black hole. This will be done by analyzing the corrections to black hole thermodynamics due to these thermal fluctuations. We will demonstrate that the entropy of this black hole get corrected by logarithmic term. We will also calculate other corrections to other important thermodynamic quantities for this black hole. Finally, we will use the corrected value of the specific heat to analyze the phase transition in this system.

gr-qc↗

arXiv2015

A Universe with a generalized ghost dark energy and Van der Waals fluid interacting with a fluid

In this paper, we consider an unusual connection between different fluids. Having established a research goal we would like to consider a toy model of the Universe and investigate its behavior, especially for later time evolution for well known facts. The main goal of the article is to consider a toy model of the Universe with generalized ghost dark energy, Van der Waals gas and a phenomenologically modified fluid. The origin of the last component can be understood as a result of interaction between some original fluid and some source of energy or matter in Universe. By unusual connection we mean an assumption that generalized ghost dark energy has its contribution to the model by an interaction term $Q$ and we suppose an interaction of the form $Q=3Hb(ρ_{\small{tot}}-ρ_{GDe})$. Graphical analysis is performed and the questions of validity of the generalized second law of thermodynamics and stability of the model also approached in this paper.

physics.gen-ph↗

arXiv2015

Unified universe history through phantom extended Chaplygin gas

We investigate the universe evolution from inflation to late-time acceleration in a unified way, using a two-component fluid constituted from extended Chaplygin gas alongside a phantom scalar field. We extract solutions for the various cosmological eras, focusing on the behavior of the scale factor, the various density parameters and the equation-of-state parameter. Furthermore, we extract and discuss bouncing solutions. Finally, we examine the perturbations of the model, ensuring about their stability and extracting the predictions for the tensor-to-scalar ratio.

gr-qc↗

arXiv2015

Constructing an Inflaton Potential by Mimicking Modified Chaplygin Gas

In this paper, we considered an inflationary model that effectively behaves as a modified Chaplygin gas in the context of quintessence cosmology. We reconstructed the inflaton potential bottom-up and using the recent observational data we fixed the free parameters of the model. We showed that the modified Chaplygin gas inspired model is suitable for both the early and the late time acceleration but has shortcomings between the two periods.

gr-qc↗

arXiv2015

STU/QCD Correspondence

In this review article we consider a special case of $D=5$, $\mathcal{N}=2$ supergravity called the STU model. We apply the gauge/gravity correspondence to the STU model to gain insight into properties of the quark-gluon plasma. Given that the quark-gluon plasma is in reality described by QCD, therefore we call our study STU/QCD correspondence. First, we investigate the thermodynamics and hydrodynamics of the STU background. Then we use dual picture of the theory, which is type IIB string theory, to obtain the drag force and jet-quenching parameter of an external probe quark.

hep-th↗

arXiv2015

The universe dominated by the extended Chaplygin gas

In this paper, we consider a universe dominated by the extended Chaplygin gas which recently proposed as the last version of Chaplygin gas models. Here, we only consider the second order term which recovers quadratic barotropic fluid equation of state. The density perturbations analyzed in both relativistic and Newtonian regimes and show that the model is stable without any phase transition and critical point. We confirmed stability of the model using thermodynamics point of view.

gr-qc↗

arXiv2014

Interacting Holographic Extended Chaplygin Gas and Phantom Cosmology in the Light of BICEP2

In this paper, we study the holographic dark energy density and interacting extended Chaplygin gas energy density in the Einstein gravity. We reconstruct the scalar field and the scalar potential describing the extended Chaplygin gas. In the special case, we obtain energy density and investigate some cosmological parameters. Assuming interaction between components we find energy density for some different parametrization of total EoS. We analyze tensor to scalar ratio and use recent observational data of BICEP2 to fix the model parameters.

gr-qc↗

arXiv2014

Higher order corrections of the extended Chaplygin gas cosmology with varying $G$ and $Λ$

In this paper, we study two different models of dark energy based on Chaplygin gas equation of state. The first model is the variable modified Chaplygin gas while the second one is the extended Chaplygin gas. Both models are considered in the framework of higher order $f(R)$ modified gravity. We also consider the case of time varying gravitational constant $G$ and $Λ$ for both models. We investigate some cosmological parameters such as the Hubble, the deceleration and the equation of state parameters. Then we showed that the model that we considered, extended Chaplygin gas with time-dependent $G$ and $Λ$, is consistent with the observational data. Finally we conclude with the discussion of cosmological perturbations of our model.

gr-qc↗

arXiv2014

An effective quintessence field with a power-law potential

In this paper, we consider an effective quintessence scalar field with a power-law potential interacting with a $P_{b}=ξqρ_{b}$ barotropic fluid as a first model, where $q$ is a deceleration parameter. For the second model we assume viscous polytropic gas interacting with the scalar field. We investigate problem numerically and analyze behavior of different cosmological parameter concerning to components and behavior of Universe. We also compare our results with observational data to fix parameters of the models. We find some instabilities in the first model which may disappear in the second model for the appropriate parameters. Therefore, we can propose interacting quintessence dark energy with viscous polytropic gas as a successful model to describe Universe.

gr-qc↗

arXiv2014

Higher derivative corrections of $f(R)$ gravity with varying equation of state in the case of variable $G$ and $Λ$

In this paper, we study three models of f(R) modified gravity including higher order terms based on different equation of state parameter. We also assume variable G and $Λ$. By using numerical analysis, we obtain the behavior of some important cosmological parameters, like the Hubble expansion H parameter and the deceleration parameter q. The statefinder diagnostics is also performed for all models.

gr-qc↗

arXiv2014

FRW cosmology with the extended Chaplygin gas

In this paper, we propose extended Chaplygin gas equation of state for which it recovers barotropic fluid with quadratic equation of state. We use numerical method to investigate the behavior of some cosmological parameters such as scale factor, Hubble expansion parameter, energy density and deceleration parameter. We also discuss about the resulting effective equation of state parameter. Using density perturbations we investigate the stability of the theory.

gr-qc↗

arXiv2014

Interacting Quintessence Models of Dark Energy

In this paper we consider two models of quintessence scalar fields with different potentials. Interaction with generalized cosmic Chaplygin gas is also investigated. Cosmological parameters are studied and graphical behavior is analyzed. We find that our model is agree with observational data specially $Λ$CDM model.

gr-qc↗

arXiv2014

Interacting two-component fluid models with varying EoS parameter

In this paper, we consider Universe filled with two-component fluid. We study two different models. In the first model we assume barotropic fluid with the linear equation of state as the first component of total fluid. In the second model we assume Van der Waals gas as the first component of total fluid. In both models, the second component assumed generalized ghost dark energy. We consider also interaction between component and discuss, numerically, cosmological quantities for two different parametrization of EoS which varies with time. We consider this as a toy model of our Universe. We fix parameters of the model by using generalized second law of thermodynamics. Comparing our results with some observational data suggests interacting barotropic fluid with EoS parameter $ω(t)=ω_{0}\cos(tH)+ω_{1}t\frac{\dot{H}}{H}$ and generalized ghost dark energy as an appropriate model to describe our Universe.

gr-qc↗

arXiv2013

Holograghic Brownian motion in $2+1$ dimensional hairy black holes

In this paper, we investigate the dynamics of a heavy quark under the plasmas correspond to three dimensional hairy black holes. We utilize the AdS/CFT correspondence to study the holographic Brownian motion of this particle in different kinds of hairy black holes. For uncharged black hole at low frequency limit we derive analytic expressions for correlation functions and response functions and verify that the fluctuation-dissipation theorem holds in the presence of a scalar field in the metric background. In the case of charged black hole, we think that the results are similar to what derived for uncharged black hole.

hep-th↗

arXiv2013

Thermodynamics of string black hole with hyperscaling violation

In this paper, we start with black brane and construct specific space-time which violates hyperscaling. In order to obtain the string solution we apply Null-Melvin-Twist and $KK$-reduction. By using the difference action method we study thermodynamics of system to obtain Hawking-Page phase transition. In order to have hyperscaling violation we need to consider $θ=\frac{d}{2}.$ In that case the free energy $F$ is always negative and our solution is thermal radiation without a black hole. Therefore we find that there is not any Hawking-Page transition. Also, we discuss the stability of system and all thermodynamical quantities.

hep-th↗