arXiv subjects

Ahmad Sheykhi

Publications and source records attributed to Ahmad Sheykhi.

123 records · Page 7Linked to original sources

arXiv2009

Generalized second law of thermodynamics in warped DGP braneworld

We investigate the validity of the generalized second law of thermodynamics on the $(n-1)$-dimensional brane embedded in the $(n+1)$-dimensional bulk. We examine the evolution of the apparent horizon entropy extracted through relation between gravitational equation and the first law of thermodynamics together with the matter field entropy inside the apparent horizon. We find that the apparent horizon entropy extracted through connection between gravity and the first law of thermodynamics satisfies the generalized second law of thermodynamics. This result holds regardless of whether there is the intrinsic curvature term on the brane or a cosmological constant in the bulk. The observed satisfaction of the generalized second law provides further support on the thermodynamical interpretation of gravity based on the profound connection between gravity and thermodynamics.

hep-th↗

arXiv2009

Interacting holographic dark energy in Brans-Dicke theory

We study cosmological application of interacting holographic energy density in the framework of Brans-Dicke cosmology. We obtain the equation of state and the deceleration parameter of the 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 that the combination of Brans-Dicke field and holographic dark energy can accommodate $w_D = -1 $ crossing for the equation of state of \textit{noninteracting} holographic dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of $w_D$ to phantom regime can be more easily accounted for than when resort to the Einstein field equations is made.

hep-th↗

arXiv2009

Interacting agegraphic dark energy models in non-flat universe

A so-called "agegraphic dark energy" was recently proposed to explain the dark energy-dominated universe. In this Letter, we generalize the agegraphic dark energy models to the universe with spatial curvature in the presence of interaction between dark matter and dark energy. We show that these models can accommodate $w_D = -1 $ crossing for the equation of state of dark energy. In the limiting case of a flat universe, i.e. $k = 0$, all previous results of agegraphic dark energy in flat universe are restored.

hep-th↗

arXiv2009

Generalized second law of thermodynamics in Gauss-Bonnet braneworld

We investigate the validity the generalized second law of thermodynamics in a general braneworld model with curvature correction terms on the brane and in the bulk, respectively. Employing the derived entropy expression associated with the apparent horizon, we examine the time evolution of the total entropy, including the derived entropy of the apparent horizon and the entropy of the matter fields inside the apparent horizon. We show that the generalized second law of thermodynamics is fulfilled on the 3-brane embedded in the 5D spacetime with curvature corrections.

hep-th↗

arXiv2009

On topological charged braneworld black holes

We study a class of topological black hole solutions in RSII braneworld scenario in the presence of a localized Maxwell field on the brane. Such a black hole can carry two types of charge, one arising from the extra dimension, the tidal charge, and the other one from a localized gauge field confined to the brane. We find that the localized charge on the brane modifies the bulk geometry and in particular the bulk Weyl tensor. The bulk geometry does not depend on different topologies of the horizons. We present the temperature and entropy expressions associated with the event horizon of the braneworld black hole and by using the first law of black hole thermodynamics we calculate the mass of the black hole.

hep-th↗

arXiv2008

Magnetic dilaton strings in anti-de Sitter spaces

With an appropriate combination of three Liouville-type dilaton potentials, we construct a new class of spinning magnetic dilaton string solutions which produces a longitudinal magnetic field in the background of anti-de Sitter spacetime. These solutions have no curvature singularity and no horizon, but have a conic geometry. We find that the spinning string has a net electric charge which is proportional to the rotation parameter. We present the suitable counterterm which removes the divergences of the action in the presence of dilaton potential. We also calculate the conserved quantities of the solutions by using the counterterm method.

hep-th↗

arXiv2008

Charged rotating dilaton black strings in AdS spaces

We derive a class of charged rotating dilaton black string solutions in the background of anti-de-Sitter spaces with an appropriate combination of three Liouville-type dilaton potentials. We also present the suitable counterterm which removes the divergences of the action in the presence of dilaton potential. The solutions are analyzed and their thermodynamics is discussed by using the counterterm method.

hep-th↗

arXiv2008

Observational constraints on the braneworld model with brane-bulk energy exchange

We investigate the viability of the braneworld model with energy exchange between the brane and bulk, by using the most recent observational data related to the background evolution. We show that this energy exchange behaves like a source of dark energy and can alter the profile of the cosmic expansion. The new Supernova Type Ia (SNIa) Gold sample, Supernova Legacy Survey (SNLS) data, the position of the acoustic peak at the last scattering surface from the Wilkinson Microwave Anisotropy Probe (WMAP) observations and the baryon acoustic oscillation peak found in the Sloan Digital Sky Survey (SDSS) are used to constrain the free parameters of this model. To infer its consistency with the age of the Universe, we compare the age of old cosmological objects with what computed using the best fit values for the model parameters. At 68% level of confidence, the combination of Gold sample SNIa, Cosmic Microwave Background (CMB) shift parameter and SDSS databases provide $Ω_m=0.29_{-0.02}^{+0.03}$, $Ω_{A}=-0.71_{-0.03}^{+0.03}$ and $μ=-0.40_{-0.26}^{+0.28}$, hence a spatially flat Universe with $Ω_K=0.00_{-0.04}^{+0.04}$. The same combination with SNLS supernova observation give $Ω_m=0.27_{-0.02}^{+0.02}$, $Ω_{A}=-0.74_{-0.02}^{+0.04}$ and $μ=0.00_{-0.30}^{+0.30}$ consequently provides a spatially flat Universe $Ω_K=-0.01_{-0.03}^{+0.04}$. These results obviously seem to be compatible with the most recent WMAP results indicating a flat Universe.

astro-ph↗

arXiv2008

Rotating Black Holes in Einstein-Maxwell-Dilaton Gravity

We present a new class of slowly rotating black hole solutions in $(n+1)$-dimensional $(n\geq3)$ Einstein-Maxwell-dilaton gravity in the presence of Liouville-type potential for the dilaton field and an arbitrary value of the dilaton coupling constant. Because of the presence of the dilaton field, the asymptotic behaviour of these solutions are neither flat nor (A)dS. In the absence of a dilaton field, our solution reduces to the $(n+1)$-dimensional Kerr-Newman modification thereof for small rotation parameter \cite{Aliev2}. We also compute the angular momentum and the gyromagnetic ratio of these rotating dilaton black holes.

hep-th↗

arXiv2008

Topological Born-Infeld-dilaton black holes

We construct a new analytic solution of Einstein-Born-Infeld-dilaton theory in the presence of Liouville-type potentials for the dilaton field. These solutions describe dilaton black holes with nontrivial topology and nonlinear electrodynamics. Black hole horizons and cosmological horizons in these spacetimes, can be a two-dimensional positive, zero or negative constant curvature surface. The asymptotic behavior of these solutions are neither flat nor (A)dS. We calculate the conserved and thermodynamic quantities of these solutions and verify that these quantities satisfy the first law of black hole thermodynamics.

hep-th↗

arXiv2008

Thermodynamical properties of topological Born-Infeld-dilaton black holes

We examine the $(n+1)$-dimensional $(n\geq3)$ action in which gravity is coupled to the Born-Infeld nonlinear electrodynamic and a dilaton field. We construct a new $(n+1)$-dimensional analytic solution of this theory in the presence of Liouville-type dilaton potentials. These solutions which describe charged topological dilaton black holes with nonlinear electrodynamics, have unusual asymptotics. They are neither asymptotically flat nor (anti)-de Sitter. The event horizons of these black holes can be an $(n-1)$-dimensional positive, zero or negative constant curvature hypersurface. We also analyze thermodynamics and stability of these solutions and disclose the effect of the dilaton and Born-Infeld fields on the thermal stability in the canonical ensemble.

hep-th↗

arXiv2007

Thermodynamics of charged topological dilaton black holes

A class of $(n+1)$-dimensional topological black hole solutions in Einstein-Maxwell-dilaton theory with Liouville-type potentials for the dilaton field is presented. In these spacetimes, black hole horizon and cosmological horizon can be an $(n-1)$-dimensional positive, zero or negative constant curvature hypersurface. Because of the presence of the dilaton field, these topological black holes are neither asymptotically flat nor (anti)-de Sitter. We calculate the charge, mass, temperature, entropy and electric potential of these solutions. We also analyze thermodynamics of these topological black holes and disclose the effect of the dilaton field on the thermal stability of the solutions.

hep-th↗

arXiv2007

String inspired explanation for the super-acceleration of our universe

We investigate the effect of the bulk content in the general Gauss-Bonnet braneworld on the evolution of the universe. We find that the Gauss-Bonnet term and the combination of the dark radiation and the matter content of the bulk play a crucial role in the universe evolution. We show that our model can describe the super-acceleration of our universe with the equation of state of the effective dark energy in agreement with observations.

hep-th↗

arXiv2007

Deep Connection Between Thermodynamics and Gravity in Gauss-Bonnet Braneworld

We disclose the deep connection between the thermodynamics and gravity in a general braneworld model with curvature correction terms on the brane and in the bulk, respectively. We show that the Friedmann equations on the 3-brane embedded in the 5D spacetime with curvature correction terms can be written directly in the form of the first law of thermodynamics on the apparent horizon. Using the first law, we extract the entropy expression of the apparent horizon on the brane, which is useful in studying the thermodynamical properties of the black hole horizon on the brane in Gauss-Bonnet gravity.

hep-th↗

arXiv2007

Thermodynamical Properties of Apparent Horizon in Warped DGP Braneworld

In this paper we first obtain Friedmann equations for the $(n-1)$-dimensional brane embedded in the $(n+1)$-dimensional bulk, with intrinsic curvature term of the brane included in the action (DGP model). Then, we show that one can always rewrite the Friedmann equations in the form of the first law of thermodynamics, $dE=TdS+WdV$, at apparent horizon on the brane, regardless of whether there is the intrinsic curvature term on the brane or a cosmological constant in the bulk. Using the first law, we extract the entropy expression of the apparent horizon on the brane. We also show that in the case without the intrinsic curvature term, the entropy expressions are the same by using the apparent horizon on the brane and by using the bulk geometry. When the intrinsic curvature appears, the entropy of apparent horizon on the brane has two parts, one part follows the $n$-dimensional area formula on the brane, and the other part is the same as the entropy in the case without the intrinsic curvature term. As an interesting result, in the warped DGP model, the entropy expression in the bulk and on the brane are not the same. This is reasonable, since in this model gravity on the brane has two parts, one induced from the $(n+1)$-dimensional bulk gravity and the other due to the intrinsic curvature term on the brane.

hep-th↗