arXiv subjects

Ahmad Sheykhi

Publications and source records attributed to Ahmad Sheykhi.

At least 91 records · Page 5Linked to original sources

arXiv2016

Effects of Backreaction on Power-Maxwell Holographic Superconductors in Gauss-Bonnet Gravity

We analytically and numerically investigate the properties of s-wave holographic superconductors by considering the effects of scalar and gauge fields on the background geometry in five dimensional Einstein-Gauss-Bonnet gravity. We assume the gauge field to be in the form of the Power-Maxwell nonlinear electrodynamics. We employ the Sturm-Liouville eigenvalue problem for analytical calculation of the critical temperature and the shooting method for the numerical investigation. Our numerical and analytical results indicate that higher curvature corrections affect condensation of the holographic superconductors with backreaction. We observe that the backreaction can decrease the critical temperature of the holographic superconductors, while the Power-Maxwell electrodynamics and Gauss-Bonnet coefficient term may increase the critical temperature of the holographic superconductors. We find that the critical exponent has the mean-field value $β=1/2$, regardless of the values of Gauss-Bonnet coefficient, backreaction and Power-Maxwell parameters.

gr-qc↗

arXiv2016

Effects of backreaction and exponential nonlinear electrodynamics on the holographic superconductors

We analytically study the properties of a $(2+1)$-dimensional $s$-wave holographic superconductor in the presence of exponential nonlinear electrodynamics. We consider the case in which the scalar and gauge fields back react on the background metric. Employing the analytical Sturm-Liouville method, we find that in the black hole background, the nonlinear electrodynamics correction will affect the properties of the holographic superconductors. We find that with increasing both backreaction and nonlinear parameters, the scalar hair condensation on the boundary will develop more difficult. We obtain the relation connecting the critical temperature with the charge density. Our analytical results support that, even in the presence of the nonlinear electrodynamics and backreaction, the phase transition for the holographic superconductor still belongs to the second order and the critical exponent of the system always takes the mean-field value $1/2$.

gr-qc↗

arXiv2016

Analytical and Numerical Study of Gauss-Bonnet Holographic Superconductors with Power-Maxwell Field

We provide an analytical as well as a numerical study of the holographic $s$-wave superconductors in Gauss-Bonnet gravity with Power-Maxwell electrodynamics. We limit our study to the case where scalar and gauge fields do not have an effect on the background metric. We use a variational method, based on Sturm-Liouville eigenvalue problem for our analytical study, as well as a numerical shooting method in order to compare with our analytical results. Interestingly enough, we observe that unlike Born-Infeld-like nonlinear electrodynamics which decrease the critical temperature compared to the linear Maxwell field, the Power-Maxwell electrodynamics is able to increase the critical temperature of the holographic superconductors in the sublinear regime. We find that requiring the finite value for the gauge field on the asymptotic boundary $r\rightarrow \infty$, restricts the power parameter, $q$, of the Power-Maxwell field to be in the range $1/2<q<{(d-1)}/{2}$. Our study indicates that it is quite possible to make condensation \emph{easier} as $q$ \emph{decreases} in its allowed range. We also find that for all values of $q$, the condensation can be affected by the Gauss-Bonnet coefficient $α$. However, the presence of the Gauss-Bonnet term makes the transition slightly harder. Finally, we obtain an analytic expression for the order parameter and thus obtain the associated critical exponent near the phase transition. We find that the critical exponent has its universal value of $β=1/2$ regardless of the parameters $q$, $α$ as well as dimension $d$, consistent with mean-field values obtained in previous studies.

gr-qc↗

arXiv2014

Charge rotating black string in gravitating nonlinear electromagnetic fields

We construct a new class of charged rotating black string solutions coupled to a nonlinear electromagnetic field in the background of anti-de Sitter spaces. We consider two types of nonlinear electromagnetic field namely, logarithmic and exponential forms. We investigate the geometric effects of nonlinearity parameter and find that for large $r$, these solutions recover the rotating back string solutions of Einstein-Maxwell theory. We calculate the conserved and thermodynamic quantities of the rotating black string. We also analyze thermodynamics of the spacetime and verify the validity of the first law of thermodynamics for the obtained solutions.

gr-qc↗

arXiv2014

Cosmological Constraints on Ghost Dark Energy in the Brans-Dicke Theory by Using MCMC Approach

By using a Markov Chain Monte Carlo simulation, we investigate cosmological constraints on the ghost dark energy (GDE) model in the framework of the Brans-Dicke (BD) theory. A combination of the latest observational data of the cosmic microwave background radiation data from seven-year WMAP, the baryon acoustic oscillation data form the SDSS, the supernovae type Ia data from the Union2 and the X-ray gas mass fraction data from the Chandra X-ray observations of the largest relaxed galaxy clusters are used to perform constraints on GDE in the BD cosmology. In this paper, we consider both flat and non-flat universes together with interaction between dark matter and dark energy. The main cosmological parameters are obtained as: $Ω_{\rm b}h^2= 0.0223^{+0.0016}_{-0.0013}$, $Ω_{\rm c}h^2=0.1149^{+0.0088}_{-0.0104}$ and $Ω_{\rm k}=0.0005^{+0.0025}_{-0.0073}$. In addition, the Brans-Dicke parameter $ω$ is estimated as $1/ω\simeq 0.002$.

astro-ph.CO↗

arXiv2014

Extremal Myers-Perry black holes in Born-Infeld-dilaton theory

A class of perturbative charged rotating dilaton black hole solutions coupled to the nonlinear Born-Infeld electrodynamics is presented. The strategy for obtaining these solutions is to start from Myers-Perry solutions, and then considering the effects of the dilaton field, Born-Infeld parameter as well as the electric charge as a perturbative parameter. We perform the perturbations up to 4th order in the extremal black holes of Einstein-Born-Infeld-dilaton gravity with equal angular momentum and arbitrary dilaton coupling constant. We discuss the physical properties of these black holes and study their dependence on the perturbative parameter $q$, the dilaton coupling constant $α$ and the Born-Infeld parameter $β$. One can see these black holes solutions satisfy a generalized Smarr relation.

gr-qc↗

arXiv2014

Quintessence reconstruction of interacting HDE in a non-flat universe

In this paper we consider quintessence reconstruction of interacting holographic dark energy in a non-flat background. As system's IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as $L=ar(t)$. To this end we construct a quintessence model by a real, single scalar field. Evolution of the potential, $V(ϕ)$, as well as the dynamics of the scalar field, $ϕ$, are obtained according to the respective holographic dark energy. The reconstructed potentials show a cosmological constant behavior for the present time. We constrain the model parameters in a flat universe by using the observational data, and applying the Monte Carlo Markov chain simulation. We obtain the best fit values of the holographic dark energy model and the interacting parameters as $c=1.0576^{+0.3010+0.3052}_{-0.6632-0.6632}$ and $ζ=0.2433^{+0.6373+0.6373}_{-0.2251-0.2251}$, respectively. From the data fitting results we also find that the model can cross the phantom line in the present universe where the best fit value of of the dark energy equation of state is $w_D=-1.2429$.

astro-ph.CO↗

arXiv2013

Interacting generalized ghost dark energy in non-flat universe

We investigate the generalized Quantum Chromodynamics (QCD) ghost model of dark energy in the framework of Einstein gravity. First, we study the non-interacting generalized ghost dark energy in a flat Friedmann-Robertson-Walker (FRW) background. We obtain the equation of state parameter, $w_D=p/ρ$, the deceleration parameter, and the evolution equation of the generalized ghost dark energy. We find that, in this case, $w_D$ cannot cross the phantom line ($w_D>-1$) and eventually the universe approaches a de-Sitter phase of expansion $(w_D\rightarrow-1)$. Then, we extend the study to the interacting ghost dark energy in both a flat and non-flat FRW universe. We find that the equation of state parameter of the interacting generalized ghost dark energy can cross the phantom line ($w_D<-1$) provided the parameters of the model are chosen suitably. Finally, we constrain the model parameters by using the Markov Chain Monte Carlo (MCMC) method and a combined dataset of SNIa, CMB, BAO and X-ray gas mass fraction.

gr-qc↗

arXiv2013

Sounds of instability from generalized QCD ghost dark energy

We investigate about the stability of generalized QCD ghost dark energy model against perturbations in the FRW background. For this purpose, we use the squared sound speed $v_s^2$ whose sign determines the stability of the model. We explore the stability of this model in the presence/absence of interaction between dark energy and dark matter in both flat and non-flat geometry. In all cases we find almost a same result. Based on the square sound speed analysis, due to the existence of a free parameter in this model, the model is theoretically capable to lead a dark energy dominated stable universe. However, observational constraints rule out such a chance. In conclusion, we find evidences that the generalized ghost dark energy might can not lead to a stable universe favored by observations at the present time.

physics.gen-ph↗

arXiv2013

Friedmann equations from emergence of cosmic space

Padmanabhan [arXiv:1206.4916] argues that the cosmic acceleration can be understood from the perspective that spacetime dynamics is an emergence phenomena. By calculating the difference between the surface degrees of freedom and the bulk degrees of freedom in a region of space, he also arrives at the Friedmann equation in a flat universe. In this paper, by modification of his proposal, we are able to derive the Friedmann equation of the Friedmann-Robertson-Walker universewith any spatial curvature.We also extend the study to higher-dimensional spacetime and derive successfully the Friedmann equations not only in Einstein gravity but also in Gauss-Bonnet and more general Lovelock gravity with any spatial curvature. This is the first derivation of Friedmann equations in these gravity theories in a nonflat Friedmann-Robertson-Walker universe by using the novel idea proposed by Padmanabhan. Our study indicates that the approach presented here is powerful enough and further supports the viability of Padmanabhan's perspective of emergence gravity.

gr-qc↗

arXiv2013

Extremal Myers-Perry black holes coupled to Born-Infeld electrodynamics in odd dimensions

Employing higher order perturbation theory, we find a new class of perturbative extremal rotating black hole solutions with Born-Infeld electric charge in odd D-dimensional spacetime. The seed solution is an odd dimensional extremal Myers-Perry black hole with equal angular momenta to which a perturbative nonlinear electric Born-Infeld field charge q is added maintaining the extremality condition. The perturbations are performed up to third order. We also study some physical properties of these black holes. In particular, it is shown that the values of the gyromagnetic ratio of the black holes are modified by the perturbative parameter q and the Born-Infeld parameter β.

gr-qc↗

arXiv2013

Extremal Myers-Perry black holes coupled to Born-Infeld electrodynamics in five dimensions

We construct a new class of perturbative extremal charged rotating black hole solutions in five dimensions in the Einstein--Born-Infeld theory. We start with an extremal five-dimensional Myers-Perry black hole seed in which the two possible angular momenta have equal magnitude, and add Born-Infeld electrical charge keeping the extremality condition as a constraint. The perturbative parameter is assumed to be the electric charge $q$ and the perturbations are performed up to 4th order. We also study some physical properties of these black holes. It is shown that the perturbative parameter $q$ and the Born-Infeld parameter $β$ modify the values of the physical quantities of the black holes. The solution suggests that the magnetic moment and gyromagnetic ratio of the black hole spacetime change sign when the Born-Infeld character of the solution starts to depart strongly from the Maxwell limit. We venture an interpretation for the effect.

gr-qc↗

arXiv2013

Thermodynamics of the apparent horizon in infrared modified Horava-Lifshitz gravity

It is well known that by applying the first law of thermodynamics to the apparent horizon of a Friedmann-Robertson-Walker universe, one can derive the corresponding Friedmann equations in Einstein, Gauss-Bonnet, and more general Lovelock gravity. Is this a generic feature of any gravitational theory? Is the prescription applicable to other gravities? In this paper we would like to address the above questions by examining the same procedure for Horava-Lifshitz gravity. We find that in Horava-Lifshitz gravity, this approach does not work and we fail to reproduce a corresponding Friedmann equation in this theory by applying the first law of thermodynamics on the apparent horizon, together with the appropriate expression for the entropy in Horava-Lifshitz gravity. The reason for this failure seems to be due to the fact that Horava-Lifshitz gravity is not diffeomorphism invariant, and thus, the corresponding field equation cannot be derived from the first law around horizon in the spacetime. Without this, it implies that the specific gravitational theory is not consistent, which shows an additional problematic feature of Horrava-Lifshitz gravity. Nevertheless, if we still take the area formula of geometric entropy and regard Horava-Lifshitz sector in the Friedmann equation as an effective dark radiation, we are able to extract the corresponding Friedmann equation from the first law of thermodynamics.

hep-th↗

arXiv2013

Thermodynamics of Fractal Universe

We investigate the thermodynamical properties of the apparent horizon in a fractal universe. We find that one can always rewrite the Friedmann equation of the fractal universe in the form of the entropy balance relation $ δQ=T_h d{S_h}$, where $ δQ $ and $ T_{h} $ are the energy flux and Unruh temperature seen by an accelerated observer just inside the apparent horizon. We find that the entropy $S_h$ consists two terms, the first one which obeys the usual area law and the second part which is the entropy production term due to nonequilibrium thermodynamics of fractal universe. This shows that in a fractal universe, a treatment with nonequilibrium thermodynamics of spacetime may be needed. We also study the generalized second law of thermodynamics in the framework of fractal universe. When the temperature of the apparent horizon and the matter fields inside the horizon are equal, i.e. $T=T_h$, the generalized second law of thermodynamics can be fulfilled provided the deceleration and the equation of state parameters ranges either as $-1 \leq q < 0 $, $- 1 \leq w < - 1/3$ or as $q<-1$, $w<-1$ which are consistent with recent observations. We also find that for $T_h=bT$, with $b<1$, the GSL of thermodynamics can be secured in a fractal universe by suitably choosing the fractal parameter $β$.

physics.gen-ph↗

arXiv2012

Higher dimensional charged $f(R)$ black holes

We construct a new class of higher dimensional black hole solutions of $f(R)$ theory coupled to a nonlinear Maxwell field. In deriving these solutions the traceless property of the energy-momentum tensor of the matter filed plays a crucial role. In $n$-dimensional spacetime the energy-momentum tensor of conformally invariant Maxwell field is traceless provided we take $n=4p$, where $p$ is the power of conformally invariant Maxwell lagrangian. These black hole solutions are similar to higher dimensional Reissner-Nordstrom AdS black holes but only exist for dimensions which are multiples of four. We calculate the conserved and thermodynamic quantities of these black holes and check the validity of the first law of black hole thermodynamics by computing a Smarr-type formula for the total mass of the solutions. Finally, we study the local stability of the solutions and find that there is indeed a phase transition for higher dimensional $f(R)$ black holes with conformally invariant Maxwell source.

hep-th↗

arXiv2011

Holographic Scalar Fields Models of Dark Energy

Many theoretical attempts toward reconstructing the potential and dynamics of the scalar fields have been done in the literature by establishing a connection between holographic/agegraphic energy density and a scalar field model of dark energy. However, in most of these cases the analytical form of the potentials in terms of the scalar field have not been reconstructed due to the complexity of the equations involved. In this paper, by taking Hubble radius as system's IR cutoff, we are able to reconstruct the analytical form of the potentials as a function of scalar field, namely $V=V(ϕ)$ as well as the dynamics of the scalar fields as a function of time, namely $ϕ=ϕ(t)$ by establishing the correspondence between holographic energy density and quintessence, tachyon, K-essence and dilaton energy density in a flat FRW universe. The reconstructed potentials are quite reasonable and have scaling solutions. Our study further supports the viability of the holographic dark energy model with Hubble radius as IR cutoff.

physics.gen-ph↗

arXiv2011

Holographic dark energy in Brans-Dicke theory with log correction

In the derivation of holographic dark energy density, the area law of the black hole entropy plays a crucial role. However, the entropy-area relation can be modified from the inclusion of quantum effects, motivated from the loop quantum gravity, string theory and black hole physics. In this paper, we study cosmological implication of the interacting entropy-corrected holographic dark energy model in the framework of Brans-Dicke cosmology. We obtain the equation of state and the deceleration parameters of the entropy-corrected holographic dark energy in a non-flat Universe. As system's IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as $L=ar(t)$. We find out that when the entropy-corrected holographic dark energy is combined with the Brans-Dicke field, the transition from normal state where $w_D >-1 $ to the phantom regime where $w_D <-1 $ for the equation of state of interacting dark energy can be more easily achieved for than when resort to the Einstein field equations is made.

physics.gen-ph↗

arXiv2011

Holographic dark energy in Brans-Dicke cosmology with Granda-Oliveros cut-off

Motivated by the recent works of one of us [1,2] we study the holographic dark energy in Brans-Dicke gravity with the Granda-Oliveros cut-off proposed recently in literature. We find out that when the present model is combined with Brans-Dicke field the transition from normal state where $w_D >-1 $ to the phantom regime where $w_D <-1 $ for the equation of state of dark energy can be more easily achieved for than when resort to the Einstein field equations is made. Furthermore, the phantom crossing is more easily achieved when the matter and the holographic dark energy undergo an exotic interaction. We also calculate some relevant cosmological parameters and their evolution.

physics.gen-ph↗