arXiv subjects

J. Sadeghi

Publications and source records attributed to J. Sadeghi.

At least 73 records · Page 4Linked to original sources

arXiv2016

Holographic Schwinger effect with a deformed AdS background

In this paper, we consider a deformed AdS background and study effect of deformation parameter on the pair production rate of the Schwinger effect. The electrostatic potential is important for the pair production in the holographic Schwinger effect. In this paper, we analyze the electrostatic potential in a deformed AdS background and investigate the effect of deformation parameter which may be useful to test of AdS/QCD. In the case of zero temperature we find that larger value of the deformation parameter leads to a smaller value of separation length of the test particles on the probe. Also we find a finite maximum of separation length in presence of modification parameter.

hep-th↗

arXiv2016

P - V Criticality of Logarithmic Corrected Dyonic Charged AdS Black Hole

In this paper, we consider dyonic charged AdS black hole which is holographic dual of a van der Waals fluid. We use logarithmic corrected entropy and study thermodynamics of the black hole and show that holographic picture is still valid. Critical behaviors and stability also discussed. Logarithmic corrections arises due to thermal fluctuations which are important when size of black hole be small. So, thermal fluctuations interpreted as quantum effect. It means that we can see quantum effect of a black hole which is a gravitational system.

gr-qc↗

arXiv2016

Logarithmic corrected $F(R)$ gravity in the light of Planck 2015

In this letter, we consider the theory of $F(R)$ gravity with the lagrangian density $ £= R+αR^2 + βR^2 \ln βR $. We obtain the constant curvature solutions and find the scalar potential of the gravitational field. We also obtain the mass squared of a scalaron in the Einstein$^,$s frame. We find cosmological parameters corresponding to the recent Plank 2015 results. Finally, we analyze the critical points and stability of the new modified theory of gravity and find that logarithmic correction is necessary to have successful model.

gr-qc↗

arXiv2016

Interacting Ghost Dark Energy Models in the Higher Dimensional Cosmology

We investigate interacting ghost dark energy models in higher dimensional cosmology. We attempt to model dark matter within a barotropic fluid with $P_{b}=ω(t)_{b}ρ$. In this work we consider four different models based on choosing equation of state parameter and interaction term. We confirm that our models agree with observational data.

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 effects of deformation parameter on thermal width of moving quarkonia in plasma

In general we can say that the thermal width of quarkonia corresponds to imaginary part of its potential. Gravity dual of theories give explicit form of potential as $V_{Q\bar{Q}}$ . Since there is an explicit formula for $Im V_{Q\bar{Q}}$ one can consider different gravity duals and study the results of contribution of various parameters. Variable gravity duals of moving pair in plasma have different results for potential. Our paper shows that deformation parameter c in warp factor leads to new results that we present them for arbitrary angles of the pair with respect to its velocity. We compare our results with the case that no deformation parameter is in metric background. We will see that the thermal width of the pair increases with increasing deformation parameter. Also, for nonzero values of deformation parameter the pair feels moving plasma in all distances. In addition our results indicate that contribution of deformation parameter leads to larger dissociation length for the moving pair reverse to the effect of the pairs velocity in the plasma.

hep-th↗

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↗

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

Logarithmic curvature correction to Lifshitz and hyperscaling violation geometries

In this note we consider the logarithmic curvature correction to Lifshitz and hyper scaling violation geometries. We investigate the effect of this correction to the gauge kinetic function f and the effective potential V . For the case of hyper scaling violating we show that the coupling between dilaton and the correction terms exhibit the logarithmic behavior for dilaton like for the case of squared curvature correction. We find the unexpected form of gauge kinetic function and potential in the presence of logarithmic terms. We try to resolve the IR singularity of Lifshitz and hyperscaling violation geometries by adding the logarithmic term.

hep-th↗

arXiv2014

Interacting quintessence dark energy models in Lyra manifold

In this paper, we consider two-component dark energy models in Lyra manifold. The first component is assumed as a quintessence field while the second model may be a viscous polytropic gas, a viscous Van der Waals gas or a viscous modified Chaplygin gas. We also consider the possibility of interaction between components. By using numerical analysis, we study some cosmological parameters of the models and compare them with observational data.

gr-qc↗

arXiv2014

Mutually interacting Tachyon dark energy with variable $G$ and $Λ$

In this paper we consider Tachyonic scalar field as dark energy with interaction between components in the case of variable $G$ and $Λ$. We assume a flat Universe with specific form of scale factor and study cosmological parameters numerically and graphically. Statefinder analysis also performed as well. In the special choice of interaction parameters we succeed to obtain analytical expression of densities. We find that our model will be stable in the late stage but there is an instability at the early Universe.

gr-qc↗

arXiv2014

Strong Gravitational Lensing with Gauss-Bonnet correction

In this paper we investigate the strong gravitational lensing with Gauss-Bonnet gravity. By considering the logarithmic term for deflection angle, we obtain the deflection angle $\hatα$ and corresponding parameters $\bar{a}$ and $\bar{b}$. Finally, by using the relations between the observables and lensing parameters, we estimate the observables such as, $θ_{\infty}$, $s$ and $r_m$, for the relativistic images in the Gauss-Bonnet background.

gr-qc↗

arXiv2014

Dark Energy and Tachyon Field in Bianchi Type-V Space-time

In this paper, we consider Bianchi type-V space-time and study a cosmological model of dark energy based on Tachyon scalar field. We assumed three different kinds of matter without possibility of interaction with scalar dark energy. Assuming power law Hubble parameter in terms of scale factor we obtain evolution of scalar field, scalar potential and equation of state parameter.

gr-qc↗

arXiv2014

Hubble parameter corrected interactions in cosmology

In this paper we make steps in a new direction by considering fluids with EoS of more general form $F(ρ,P)=0$. It is thought that there should be interaction between cosmic fluids, but this assumption for this stage carries only phenomenological character opening a room for different kind of manipulations. In this article we will consider a modification of an interaction $Q$, where we accept that interaction parameter $b_{1}$ (order of unity) in $Q=3Hb_{1}ρ$ is time dependent and presented as a linear function of Hubble parameter $H$ of the form $b_{0}+btH$, where $b$ and $b_{0}$ are constants. We consider two different models include modified Chaplygin gas and palotropic gas which have bulk viscosity. Then, we investigate problem numerically and analyze behavior of different cosmological parameters concerning to fluids and behavior of Universe.

gr-qc↗

arXiv2014

Phenomenologically varying $Λ$ and a toy model for the Universe

We consider a model of the Universe with variable G and Λ. Subject of our interest is a phenomenological model for Λ proposed and considered in this article first time (up to our knowledge). Modification based on an assumption that ghost dark energy exists and Universe will feel it through Λ. In that case we would like to consider possibility that there exist some unusual connections between different components of the fluids existing in Universe. We would like to stress, that this is just an assumption and could be very far from the reality. We are interested by this model as a phenomenological and mathematical and unfortunately, we will not discuss about physical conditions and possibilities of having such modifications. To test our assumption and to observe behavior of the Universe, we will consider toy models filled by a barotropic fluid and modified Chaplyagin gas. To complete the logic of the research we will consider interaction between barotropic fluid or Chaplygin gas with ghost dark energy as a separate scenario. Statefinder diagnostic also provided with stability analysis of the models. All free parameters of the model fixed to satisfy generalized second law of thermodynamics.

gr-qc↗

arXiv2014

Interacting Ricci dark energy models with an effective $Λ$-term in Lyra manifold

In this paper we consider a universe filled with barotropic dark matter and Ricci dark energy in Lyra geometry with varying Lambda. We assume two different kinds of interactions between dark matter and dark energy. Then, by using numerical analysis, we investigate some cosmological parameters of the models such as equation of state, Hubble and deceleration parameters.

gr-qc↗

arXiv2014

Interaction between modified Chaplygin gas and ghost dark energy in presence of extra dimensions

In this paper, we consider three different models of dark energy in higher dimensional space-time and discuss about some cosmological parameters numerically. The first model is a single component universe including viscous varying modified Chaplygin gas. In the second model, we consider two-component universe including viscous varying modified Chaplygin gas and ghost dark energy. In the third model, we consider another two-component universe including viscous modified cosmic Chaplygin gas and ghost dark energy. In the cases of two-component fluids we also consider possibility of interaction between components.

gr-qc↗

arXiv2014

Quintessence Cosmology with an Effective $Λ$-Term in Lyra Manifold

In this paper, we study quintessence cosmology with an effective $Λ$-term in Lyra manifold. We consider three different models by choosing variable $Λ$ depend on time, the Hubble parameter and the energy density of dark matter and dark energy. Dark energy assumed as quintessence which interacts with the dark matter. By using numerical analysis we investigate behavior of cosmological parameters in three different models and compare our results with observational data. Statefinder diagnostic is also performed for all models.

gr-qc↗