arXiv subjects

M. Sharif

Publications and source records attributed to M. Sharif.

At least 235 records · Page 13Linked to original sources

arXiv2013

Cosmological Evolution for Dark Energy Models in f(T) Gravity

In this paper, we investigate the behavior of equation of state parameter and energy density for dark energy in the framework of f(T) gravity. For this purpose, we use anisotropic LRS Bianchi type I universe model. The behavior of accelerating universe is discussed for some well-known f(T) models. It is found that the universe takes a transition between phantom and non-phantom phases for f(T) models except exponential and logarithmic models. We conclude that our results are relativity analogous to the results of FRW universe.

gr-qc↗

arXiv2013

Thermodynamics in Closed Universe with Entropy Corrections

We discuss the generalized second law of thermodynamics in three different systems by taking quantum corrections (logarithmic and power law) to cosmological horizon entropy as well as black hole entropy. Firstly, we consider phantom energy accretion onto the Schwarzschild black hole in the closed FRW universe and investigate validity of the generalized second law of thermodynamics on the apparent and event horizons. In another scenario, we evaluate the critical mass of the Schwarzschild black hole with upper and lower bounds under accretion process due to phantom-like modified generalized chaplygin gas. It is found that the generalized second law of thermodynamics is respected within these bounds and black hole cannot accrete outside them. Finally, we explore this law for a closed universe filled with interacting $n$-components of fluid (in thermal equilibrium case) and with non-interacting dark matter and dark energy components (in thermal non-equilibrium case) on the apparent and event horizons and find conditions for its validity.

gr-qc↗

arXiv2013

Cosmic Acceleration and Brans-Dicke Theory

This paper is devoted to study the accelerated expansion of the universe by exploring the Brans-Dicke parameter in different eras. For this purpose, we take FRW universe model with viscous fluid (without potential) and Bianchi type I universe model with barotropic fluid (with and without potential). We evaluate deceleration parameter as well as Brans-Dicke parameter to explore cosmic acceleration. It is concluded that accelerated expansion of the universe can also be achieved for higher values of the Brans-Dicke parameter in some cases.

gr-qc↗

arXiv2013

Dark Energy Models and Laws of Thermodynamics in Bianchi I Model

This paper is devoted to check validity of the laws of thermodynamics for LRS Bianchi type I universe model which is filled with combination of dark matter and dark energy. We take two types of dark energy models, i.e., generalized holographic dark energy and generalized Ricci dark energy. It is proved that the first and generalized second law of thermodynamics are valid on the apparent horizon for both the models. Further, we take fixed radius $L$ of the apparent horizon with original holographic or Ricci dark energy. We conclude that the first and generalized second laws of thermodynamics do not hold on the horizon of fixed radius $L$ for both the models.

gr-qc↗

arXiv2013

Singularities of Noncompact Charged Objects

We formulate a model of noncompact spherical charged objects in the framework of noncommutative field theory. The Einstein-Maxwell field equations are solved with charged anisotropic fluid. We choose the forms of mass and charge densities which belong to two parameter family of density distribution functions instead of densities as Gaussian width length. It is found that the corresponding densities and the Ricci scalar are singular at origin whereas the metric is nonsingular indicating a spacelike singularity. The numerical solution of the horizon equation implies that there are either two or one or no horizon depending on the mass. We also evaluate the Hawking temperature which implies that a black hole with two horizons is evaporated to an extremal black hole with one horizon.

physics.gen-ph↗

arXiv2013

Dynamics of Shearfree Dissipative Collapse in f(G) Gravity

In this paper, we study the dynamics of shearfree dissipative gravitational collapse in modified Gauss-Bonnet theory of gravity - so called f(G) gravity. The field equations of f(G) gravity are applied to shearfree spherical interior geometry of the dissipative star. We formulate the dynamical as well as transport equations and then couple them to investigate the process of collapse. We conclude that the gravitational force in this theory is much stronger as compared to general relativity which indicates increase in the rate of collapse. Further, the relation between the Weyl tensor and matter components is established. This shows that for constant f(G) the vanishing of the Weyl scalar leads to the homogeneity in energy density and vice versa.

gr-qc↗

arXiv2013

Static Cylindrically Symmetric Interior Solutions in f(R) Gravity

We investigate some exact static cylindrically symmetric solutions for a perfect fluid in the metric $f(R)$ theory of gravity. For this purpose, three different families of solutions are explored. We evaluate energy density, pressure, Ricci scalar and functional form of $f(R)$. It is interesting to mention here that two new exact solutions are found from the last approach, one is in particular form and the other is in the general form. The general form gives a complete description of a cylindrical star in $f(R)$ gravity.

gr-qc↗

arXiv2012

Structure Scalars for Charged Cylindrically Symmetric Relativistic Fluids

We investigate some structure scalars developed through Riemann tensor for self-gravitating cylindrically symmetric charged dissipative anisotropic fluid. We show that these scalars are directly related to the fundamental properties of the fluid. We formulate dynamical-transport equation as well as the mass function by including charge which are then expressed in terms of structure scalars. The effects of electric charge are investigated in the structure and evolution of compact objects. Finally, we show that all possible solutions of the field equations can be written in terms of these scalars.

gr-qc↗

arXiv2012

Cosmological Evolution of Interacting New Holographic Dark Energy in Non-flat Universe

We consider the interacting holographic dark energy with new infrared cutoff (involving Hubble parameter and its derivative) in non-flat universe. In this context, we obtain the equation of state parameter which evolutes the universe from vacuum dark energy region towards quintessence region for particular values of constant parameters. It is found that this model always remains unstable against small perturbations. Further, we establish the correspondence of this model having quintessential behavior with quintessence, tachyon, K-essence and dilaton scalar field models. The dynamics of scalar fields and potentials indicate accelerated expansion of the universe which is consistent with the current observations. Finally, we discuss the validity of the generalized second law of thermodynamics in this scenario.

gr-qc↗

arXiv2012

Analysis of F(R,T) Gravity Models Through Energy Conditions

This paper is devoted to study the energy conditions in F(R,T) gravity for FRW universe with perfect fluid, where $R$ is the Ricci scalar and $T$ is the torsion scalar. We construct the general energy conditions in this theory and reduce them in F(R) as well as F(T) theory of gravity. Further, we assume some viable models and investigate bounds on their constant parameters to satisfy the energy condition inequalities. We also plot some of the cases using present-day values of the cosmological parameters. It is interesting to mention here that the model F(R,T)=μR+ νT satisfies the energy conditions for different ranges of the parameters.

gr-qc↗

arXiv2012

Effects of Electromagnetic Field on Energy Density Inhomogeneity in Self-Gravitating Fluids

This paper is devoted to study the effects of electromagnetic field on the energy density inhomogeneity in the relativistic self-gravitating fluids for spherically symmetric spacetime. Two important equations of the Weyl tensor are formulated which help to analyze the energy density inhomogeneity in this scenario. We investigate two types of fluids, i.e., non-dissipative and dissipative. The non-dissipative fluid further includes dust, locally isotropic, and locally anisotropic charged fluids. We explore the effects of different factors on energy density inhomogeneity in all these cases, in particular, the effect of charge.

gr-qc↗

arXiv2012

Expanding and Collapsing Scalar Field Thin Shell

This paper deals with the dynamics of scalar field thin shell in the Reissner-Nordstr$\ddot{o}$m geometry. The Israel junction conditions between Reissner-Nordstr$\ddot{o}$m spacetimes are derived, which lead to the equation of motion of scalar field shell and Klien-Gordon equation. These equations are solved numerically by taking scalar field model with the quadratic scalar potential. It is found that solution represents the expanding and collapsing scalar field shell. For the better understanding of this problem, we investigate the case of massless scalar field (by taking the scalar field potential zero). Also, we evaluate the scalar field potential when $p$ is an explicit function of $R$. We conclude that both massless as well as massive scalar field shell can expand to infinity at constant rate or collapse to zero size forming a curvature singularity or bounce under suitable conditions.

gr-qc↗

arXiv2012

Solvable K-essence Cosmologies and Modified Chaplygin Gas Unified Models of Dark Energy and Dark Matter

This paper is devoted to the investigation of modified Chaplygin gas model in the context of solvable k-essence cosmologies. For this purpose, we construct equations of state parameter of this model for some particular values of the parameter $n$. The graphical behavior of these equations are also discussed by using power law form of potential. The relationship between k-essence and modified Chaplygin gas model shows viable results in the dark energy scenario. We conclude that the universe behaves as a cosmological constant, quintessence phase or phantom phase depending upon $n$.

physics.gen-ph↗

arXiv2012

Dynamics of Magnetized Bulk Viscous Strings in Brans-Dicke Gravity

We explore locally rotationally symmetric Bianchi I universe in Brans-Dicke gravity with self-interacting potential by using charged viscous cosmological string fluid. We use a relationship between the shear and expansion scalars and also take the power law for scalar field as well as self-interacting potential. It is found that the resulting universe model maintains its anisotropic nature at all times due to the proportionality relationship between expansion and shear scalars. The physical implications of this model are discussed by using different parameters and their graphs. We conclude that this model corresponds to an accelerated expanding universe for particular values of the parameters.

gr-qc↗

arXiv2012

Fermions Tunneling from Charged Accelerating and Rotating Black Holes with NUT Parameter

This paper is devoted to the study of Hawking radiation as a tunneling of charged fermions through event horizons of a pair of charged accelerating and rotating black holes with NUT parameter. We evaluate tunneling probabilities of outgoing charged particles by using the semiclassical WKB approximation to the general covariant Dirac equation. The Hawking temperature corresponding to this pair of black holes is also investigated. For the zero NUT parameter, we find results consistent with those already available in the literature.

gr-qc↗

arXiv2012

Modified Holographic Dark Energy in Non-flat Kaluza-Klein Universe with Varying G

The purpose of this paper is to discuss the evolution of modified holographic dark energy with variable $G$ in non-flat Kaluza$-$Klein universe. We consider the non-interacting and interacting scenarios of the modified holographic dark energy with dark matter and obtain the equation of state parameter through logarithmic approach. It turns out that the universe remains in different dark energy eras for both cases. Further, we study the validity of the generalized second law of thermodynamics in this scenario. We also justify that the statefinder parameters satisfy the limit of $Λ$CDM model.

gr-qc↗

arXiv2012

Thermodynamics in f(R,T) Theory of Gravity

A non-equilibrium picture of thermodynamics is discussed at the apparent horizon of FRW universe in $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor. We take two forms of the energy-momentum tensor of dark components and demonstrate that equilibrium description of thermodynamics is not achievable in both cases. We check the validity of the first and second law of thermodynamics in this scenario. It is shown that the Friedmann equations can be expressed in the form of first law of thermodynamics $T_hdS'_h+T_hd_{\jmath}S'=-dE'+W'dV$, where $d_{\jmath}S'$ is the entropy production term. Finally, we conclude that the second law of thermodynamics holds both in phantom and non-phantom phases.

gr-qc↗

arXiv2012

Non-Commutative Correction to Thin Shell Collapse in Reissner Nordstr$\ddot{o}$m Geometry

This paper investigates the polytropic matter shell collapse in the non-commutative Reissner-Nordstr$\ddot{o}$m geometry. Using the Israel criteria, equation of motion for the polytropic matter shell is derived. In order to explore the physical aspects of this equation, the most general equation of state, $p=kρ^{({1+\frac{1}{n}})}$, has been used for finite and infinite values of $n$. The effective potentials corresponding to the equation of motion have been used to explain different states of the matter shell collapse. The numerical solution of the equation of motion predicts collapse as well as expansion depending on the choice of initial data. Further, in order to include the non-commutative correction, we modify the matter components and re-formulate the equation of motion as well as the corresponding effective potentials by including non-commutative factor and charge parameter. It is concluded that charge reduces the velocity of the expanding or collapsing matter shell but does not bring the shell to static position. While the non-commutative factor with generic matter favors the formation of black hole.

gr-qc↗