arXiv subjects

M. Sharif

Publications and source records attributed to M. Sharif.

At least 199 records · Page 11Linked to original sources

arXiv2014

Cosmological Evolution of Pilgrim Dark Energy

We study pilgrim dark energy model by taking IR cut-offs as particle and event horizons as well as conformal age of the universe. We derive evolution equations for fractional energy density and equation of state parameters for pilgrim dark energy. The phantom cosmic evolution is established in these scenarios which is well supported by the cosmological parameters such as deceleration parameter, statefinder parameters and phase space of $ω_\vartheta$ and $ω'_\vartheta$. We conclude that the consistent value of parameter $μ$ is $μ<0$ in accordance with the current Planck and WMAP$9$ results.

gr-qc↗

arXiv2014

Analysis of Generalized Ghost Version of Pilgrim Dark Energy

The proposal of pilgrim dark energy is based on the speculation that phantom-like dark energy possesses enough resistive force to preclude the black hole formation in the later universe. We explore this phenomenon by assuming the generalized ghost version of pilgrim dark energy. We find that most of the values of the interacting ($ξ^2$) as well as pilgrim dark energy ($u$) parameters push the equation of state parameter towards phantom region. The squared speed of sound shows that this model remains stable in most of the cases of $ξ^2$ and $u$. We also develop $ω_Λ-ω'_Λ$ plane and observe that this model corresponds to thawing as well as freezing regions. Finally, it is shown that the non-interacting and interacting generalized ghost versions of pilgrim dark energy correspond to $Λ$CDM limit on the statefinder plane.

gr-qc↗

arXiv2014

Energy Density Inhomogeneities with Polynomial $f(R)$ Cosmology

In this paper, we study the effects of polynomial $f(R)$ model on the stability of homogeneous energy density in self-gravitating spherical stellar object. For this purpose, we construct couple of evolution equations which relate the Weyl tensor with matter parameters. We explore different factors responsible for density inhomogeneities with non-dissipative dust, isotropic as well as anisotropic fluids and dissipative dust cloud. We find that shear, pressure, dissipative parameters and $f(R)$ terms affect the existence of inhomogeneous energy density.

gr-qc↗

arXiv2014

Study of Inflationary Generalized Cosmic Chaplygin Gas for Standard and Tachyon Scalar Fields

We consider an inflationary universe model in the context of generalized cosmic Chaplygin gas by taking matter field as standard and tachyon scalar fields. We evaluate the corresponding scalar fields and scalar potentials during intermediate and logamediate inflationary regimes by modifying the first Friedmann equation. In each case, we evaluate the number of e-folds, scalar as well as tensor power spectra, scalar spectral index and important observational parameter, i.e., tensor-scalar ratio in terms of inflatons. The graphical behavior of this parameter shows that the model remains incompatible with WMAP7 and Planck observational data in each case.

gr-qc↗

arXiv2014

Stability of a Class of Non-Static Axial Self-Gravitating Systems in $f(R)$ Gravity

In this paper, we analyze stability regions of a non-static restricted class of axially symmetric spacetime with anisotropic matter distribution. We consider $f(R)=R+εR^2$ model and assume hydrostatic equilibrium of the axial self-gravitating system at large past time. Considering perturbation from hydrostatic phase, we develop dynamical as well as collapse equations and explore dynamical instabilities at Newtonian and post-Newtonian regimes. It is concluded with the help of stiffness parameter, $Γ_1$, that radial profile of physical parameters like pressure anisotropy, energy density and higher curvature terms of the $f(R)$ model affect the instability ranges.

gr-qc↗

arXiv2014

Thermodynamics in $f(T)$ Gravity and Corrected Entropies

This paper is devoted to study the generalized second law of thermodynamics in $f(T)$ gravity. We use quantum corrections such as power-law and logarithmic corrected entropies to the horizon entropy along with Gibbs' equation in the thermal equilibrium state. We derive $f(T)$ model by taking into account a power-law scale factor through the first modified Friedmann equation which obeys the condition for a realistic model. Two types of horizons, i.e., Hubble and event horizons are used to check the validity of the generalized second law of thermodynamics with corrected entropies. We conclude that this law holds with a specific range of entropy parameter on both horizons in the case of power-law corrected entropy, while it violates for all values of entropy parameter on both horizons for logarithmic corrected entropy.

gr-qc↗

arXiv2014

Viscous Dark Energy in $f(T)$ Gravity

We study the bulk viscosity taking dust matter in the generalized teleparallel gravity. We consider different dark energy models in this scenario along with a time dependent viscous model to construct the viscous equation of state parameter for these dark energy models. We discuss the graphical representation of this parameter to investigate the viscosity effects on the accelerating expansion of the universe. It is mentioned here that the behavior of the universe depends upon the viscous coefficients showing the transition from decelerating to accelerating phase. It leads to the crossing of phantom divide line and becomes phantom dominated for specific ranges of these coefficients.

gr-qc↗

arXiv2014

Cylindrical Thin-shell Wormholes in $f(R)$ gravity

In this paper, we employ cut and paste scheme to construct thin-shell wormhole of a charged black string with $f(R)$ terms. We consider $f(R)$ model as an exotic matter source at wormhole throat. The stability of the respective solutions are analyzed under radial perturbations in the context of $R+δR^2$ model. It is concluded that both stable as well as unstable solutions do exist for different values of $δ$. In the limit $δ{\rightarrow}0$, all our results reduce to general relativity.

physics.gen-ph↗

arXiv2014

Thermodynamic Behavior of particular $f(R,T)$ Gravity Models

We investigate the thermodynamics at the apparent horizon of the FRW universe in $f(R,T)$ theory under non-equilibrium description. The laws of thermodynamics have been discussed for two particular models of $f(R,T)$ theory. The first law of thermodynamics is expressed in the form of Clausius relation $T_hd\hat{S}_h=δ{Q}$, where $δ{Q}=-d\hat{E}+Wd\mathbb{V}+T_hd_{\jmath}\hat{S}$ is the energy flux across the horizon and $d_{\jmath}\hat{S}$ is the entropy production term. Furthermore, the conditions to preserve the generalized second law of thermodynamics are established with the constraints of positive temperature and attractive gravity. We have illustrated our results for some concrete models in this theory.

gr-qc↗

arXiv2013

Thermodynamics in Kaluza-Klein Universe

This paper is devoted to check the validity of laws of thermodynamics for Kaluza-Klein universe in the state of thermal equilibrium, composed of dark matter and dark energy. The generalized holographic dark energy and generalized Ricci dark energy models are considered here. It is proved that the first and generalized second law of thermodynamics are valid on the apparent horizon for both of these models. Further, we take a horizon of radius $L$ with modified holographic or Ricci dark energy. We conclude that these models do not obey the first and generalized second law of thermodynamics on the horizon of fixed radius $L$ for a specific range of model parameters.

gr-qc↗

arXiv2013

The Stability of a Shearing Viscous Star with Electromagnetic Field

We analyze the role of electromagnetic field for the stability of shearing viscous star with spherical symmetry. Matching conditions are given for the interior and exterior metrics. We use perturbation scheme to construct the collapse equation. The range of instability is explored in Newtonian and post Newtonian (pN) limits. We conclude that the electromagnetic field diminishes the effects of shearing viscosity in the instability range and makes the system more unstable at both Newtonian and post Newtonian approximations.

gr-qc↗

arXiv2013

Gravitational Binding Energy in Charged Cylindrical Symmetry

We consider static cylindrically symmetric charged gravitating object with perfect fluid and investigate the gravitational binding energy. It is found that only the localized part of the mass function provides the gravitational binding energy, whereas the non-localized part generated by the electric coupling does not contribute for such energy.

gr-qc↗

arXiv2013

Nonlinear Electrodynamics in $f(T)$ Gravity and Generalized Second Law of Thermodynamics

In this paper, we study the nonlinear electrodynamics in the framework of $f(T)$ gravity for FRW universe along with dust matter, magnetic and torsion contributions. We evaluate the equation of state and deceleration parameters to explore the accelerated expansion of the universe. The validity of generalized second law of thermodynamics for Hubble and event horizons is also investigated in this scenario. For this purpose, we assume pole-like and power-law forms of scale factor and construct $f(T)$ models. The graphical behavior of the cosmological parameters versus smaller values of redshift $z$ represent the accelerated expansion of the universe. It turns out that the generalized second law of thermodynamics holds for all values of $z$ with event horizon for power-law scale factor whereas it holds in a specific range of $z$ with Hubble horizon for power-law and both horizons in pole-like scale factors.

gr-qc↗

arXiv2013

Noether Symmetries of Some Homogeneous Universe Models in Curvature Corrected Scalar-Tensor Gravity

We explore Noether gauge symmetries of FRW and Bianchi I universe models for perfect fluid in scalar-tensor gravity with extra term $R^{-1}$ as curvature correction. Noether symmetry approach can be used to fix the form of coupling function $ω(ϕ)$ and the field potential $V(ϕ)$. It is shown that for both models, the Noether symmetries, the gauge function as well as the conserved quantity, i.e., the integral of motion exist for the respective point like Lagrangians. We determine the form of coupling function as well as the field potential in each case. Finally, we investigate solutions through scaling or dilatational symmetries for Bianchi I universe model without curvature correction and discuss its cosmological implications.

gr-qc↗

arXiv2013

Warm Anisotropic Inflationary Universe Model

This paper is devoted to study the warm inflation using vector fields in the background of locally rotationally symmetric Bianchi type I universe model. We formulate the field equations, slow-roll and perturbation parameters (scalar and tensor power spectra as well as their spectral indices) under slow-roll approximation. We evaluate all these parameters in terms of directional Hubble parameter during intermediate and logamediate inflationary regimes by taking the dissipation factor as a function of scalar field as well as a constant. In each case, we calculate the observational parameter of interest, i.e., tensor-scalar ratio in terms of inflation. The graphical behavior of these parameters shows that the anisotropic model is also compatible with WMAP7 and Planck observational data.

gr-qc↗

arXiv2013

Energy Conditions in a Generalized Second-Order Scalar-Tensor Gravity

The study of energy conditions has many significant applications in general relativistic and cosmological contexts. This paper explores the energy conditions in the framework of the most general scalar-tensor theory with field equations involving second-order derivatives. For this purpose, we use flat FRW universe model with perfect fluid matter contents. By taking power law ansatz for scalar field, we discuss the strong, weak, null and dominant energy conditions in terms of deceleration, jerk and snap parameters. Some particular cases of this theory like k-essence model and modified gravity theories etc. are analyzed with the help of the derived energy conditions and the possible constraints on the free parameters of the presented models are determined.

gr-qc↗

arXiv2013

Generalized Teleparallel Gravity Via Some Scalar Field Dark Energy Models

We consider generalized teleparallel gravity in the flat FRW universe with a viable power-law f(T) model. We construct its equation of state and deceleration parameters which give accelerated expansion of the universe in quintessence era for the obtained scale factor. Further, we develop correspondence of f(T) model with scalar field models such as, quintessence, tachyon, K-essence and dilaton. The dynamics of scalar field as well as scalar potential of these models indicate the expansion of the universe with acceleration in the f(T) gravity scenario.

physics.gen-ph↗

arXiv2013

Expansion-free Cylindrically Symmetric Models

This paper investigates cylindrically symmetric distribution of an-isotropic fluid under the expansion-free condition, which requires the existence of vacuum cavity within the fluid distribution. We have discussed two family of solutions which further provide two exact models in each family. Some of these solutions satisfy Darmois junction condition while some show the presence of thin shell on both boundary surfaces. We also formulate a relation between the Weyl tensor and energy density.

gr-qc↗