arXiv subjects

M. Sharif

Publications and source records attributed to M. Sharif.

At least 145 records · Page 8Linked to original sources

arXiv2019

Greybody Factor for a Rotating Bardeen Black Hole

In this paper, we formulate an analytical expression of greybody factor in the context of rotating Bardeen black hole which is valid in low energy and low angular momentum region. Primarily, we analyze the profile of effective potential which originates the absorption probability. We then derive two asymptotic solutions by solving the radial part of Klein-Gordon equation in two different regions, namely, black hole and far-field horizons. We match these solutions smoothly to an intermediate regime to extend our results over the whole radial coordinate. In order to elaborate on the significance of the analytical solution, we compute the energy emission rate and absorption cross-section for the massless scalar field. It is found that the rotation parameter increases the emission rate of scalar field particles while the orbital angular momentum minimizes the emission process.

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arXiv2019

Ghost Dark Energy Model in f(G) Gravity

In this paper, we investigate the cosmological consequences of ghost dark energy model in modified Gauss-Bonnet gravity. We construct ghost dark energy $f(G)$ model by using correspondence scenario for both interacting and non-interacting schemes. For this purpose, we consider the FRW universe with pressureless matter and power-law scale factor. We examine the behavior of the equation of state parameter and check the stability of ghost dark energy model through the squared speed of sound parameter. We also analyze the behavior of phase planes like $ω_{eff}-ω'_{eff}$ and $r-s$ graphically. It is found that the equation of state parameter represents the quintessence era for non-interacting while the phantom phase of the universe for an interacting case. The squared speed of sound indicates stable ghost dark energy model for both cases. The $ω_{eff}-ω'_{eff}$ plane shows thawing region for non-interacting while freezing region for an interacting case. The $r-s$ plane corresponds to the Chaplygin gas model in both scenarios. We conclude that ghost dark energy model describes the evolution of the universe for the appropriate choice of parameters.

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arXiv2019

Stability of the Universe Model Coupled with Phantom and Tachyon Fields

In this paper, we study the phase space analysis of locally rotationally symmetric Bianchi type I universe model by taking interactions between dark matter and scalar field models. We define normalized dimensionless variables to develop an autonomous system of equations. We also find the corresponding critical points in order to study the dynamics of the system. The dynamical analysis indicates that all the critical points correspond to accelerated cosmic expansion for tachyon coupled field. We observe that positive values of $m$ provide more stable future attractors as compared to its negative values. We also analyze the behavior of power-law scale factor which shows different cosmological phases. It is found that the region for decelerated expansion gets larger for the phantom coupled matter by increasing $m$ while this region decreases for tachyon coupled field.

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arXiv2019

Propagation of Polar Gravitational Waves in f(R,T) Scenario

This paper investigates the propagation of polar gravitational waves in the spatially flat FRW universe consisting of a perfect fluid in the scenario of $R+2λT$ model of $f(R,T)$ gravity ($λ$ being the model parameter). The spatially flat universe model is perturbed via Regge-Wheeler perturbations inducing polar gravitational waves and the field equations are formulated for both unperturbed as well as perturbed spacetimes. We solve these field equations simultaneously for the perturbation parameters introduced in the metric, matter, and velocity in the radiation, as well as dark energy, dominated phases. It is found that the polar gravitational waves can produce changes in the background matter distribution as well as velocity components in the radiation era similar to general relativity case. Moreover, we have discussed the impact of model parameter on the amplitude of gravitational waves.

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arXiv2019

Effects of Charge on Gravitational Decoupled Anisotropic Solutions in f(R) Gravity

This paper is devoted to studying charged anisotropic static spherically symmetric solutions through gravitationally decoupled minimal geometric deformation technique in $f(R)$ gravity. For this purpose, we first consider the known isotropic Krori-Barua solution for $f(R)$ Starobinsky model in the interior of a charged stellar system and then include the effects of two types of anisotropic solutions. The corresponding field equations are constructed and the unknown constants are obtained from junction conditions. We analyze the physical viability and stability of the resulting solutions through effective energy density, effective radial/tangential pressure, energy conditions, and causality condition. It is found that both solutions satisfy the stability range as well as other physical conditions for specific values of charge as well as model parameter and anisotropic constant. We conclude that the modified theory under the influence of charge yields more stable behavior of the self-gravitating system.

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arXiv2019

Phase Space Analysis for Anisotropic Universe with Nonlinear Bulk Viscosity

In this paper, we discuss the phase space analysis of locally rotationally symmetric Bianchi type I universe model by taking a noninteracting mixture of dust like and viscous radiation like fluid whose viscous pressure satisfies a nonlinear version of the Israel-Stewart transport equation. An autonomous system of equations is established by defining normalized dimensionless variables. In order to investigate the stability of the system, we evaluate corresponding critical points for different values of the parameters. We also compute the power-law scale factor whose behavior indicates different phases of the universe model. It is found that our analysis does not provide a complete immune from fine-tuning because the exponentially expanding solution occurs only for a a particular range of parameters. We conclude that stable solutions exist in the presence of nonlinear model for bulk viscosity with different choices of the constant parameter $m$ for anisotropic universe.

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arXiv2019

Stellar Evolution of Compact Stars in Curvature-Matter Coupling Gravity

This paper is devoted to the study of stellar evolution of compact objects whose energy density and pressure of the fluid are interlinked by means of MIT bag model and a realistic polytropic equation of state in the scenario of $f(R,T,Q)$ gravity, where $Q=R_{ab}T^{ab}$. We derive the field equations as well as the hydrostatic equilibrium equation and analyze their solutions numerically for $R+δQ$ functional form with $δ$ being a coupling parameter. We discuss the dependence of various physical properties such as pressure, energy density, total mass and surface redshift on the chosen values of the model parameter. The physical acceptability of the proposed model is examined by checking the validity of energy conditions, causality condition, and adiabatic index. We also study the effects arising due to matter-curvature coupling on the compact stellar system. It is found that maximum mass point lies within the observational range which indicates that our model is appropriate to describe dense stellar objects.

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arXiv2019

Cosmic Evolution of Holographic Dark Energy in f(\mathcal{G},T) Gravity

The aim of this paper is to analyze the cosmological evolution of holographic dark energy in $f(\mathcal{G},T)$ gravity ($\mathcal{G}$ and $T$ represent the Gauss-Bonnet invariant and trace of the energy-momentum tensor, respectively). We reconstruct $f(\mathcal{G},T)$ model through correspondence scheme using power-law form of the scale factor. The qualitative analysis of the derived model is investigated with the help of evolutionary trajectories of equation of state, deceleration as well as state-finder diagnostic parameters and $ω_{_{\mathcal{G}T}}-ω'_{_{\mathcal{G}T}}$ cosmological phase plane. It is found that the equation of state parameter represents phantom epoch of the Universe whereas the deceleration parameter illustrates the accelerated phase. The state-finder plane corresponds to Chaplygin gas model while the freezing region is attained in $ω_{_{\mathcal{G}T}}-ω'_{_{\mathcal{G}T}}$ plane.

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arXiv2019

Collapse and Expansion of Scalar Thin-Shell for a Class of Black Holes

This paper investigates the dynamics of thin-shell in the presence of perfect fluid as well as the scalar field. We formulate the equations of motion using Israel thin-shell formalism by taking the interior and exterior regions of Schwarzschild, Kerr as well as Kerr-Newmann black hole. We find numerical solutions of equations of motion and effective potential to analyze the scalar shell for collapse and expansion. It is found that the rate of collapse and expansion of scalar shell through shell's radius depends on charge and rotation parameters. We conclude that the massive scalar shell leads to a collapse of thin-shell, while massless scalar shell indicates both collapses as well as expansion.

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arXiv2019

Models of Collapsing and Expanding Cylindrical Source in f(R,T) Theory

We discuss the collapsing and expanding solutions of anisotropic charged cylinder in the context of $f(R,T)$ theory ($R$ represents the Ricci scalar and $T$ denotes the trace of energy-momentum tensor). For this purpose, we take an auxiliary solution of Einstein-Maxwell field equations and evaluate expansion scalar whose negative values lead to collapse and positive values give expansion. For both cases, the behavior of density, pressure, anisotropic parameter as well as mass is explored and the effects of charge as well as model parameter on these quantities are examined. The energy conditions are found to be satisfied for both solutions.

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arXiv2018

Role of $σR^{2}+γR_{μν}T^{μν}$ Model on Anisotropic Polytropes

This paper analyzes the anisotropic stellar evolution governed by a polytropic equation of state in the framework of $f(R,T,Q)$ gravity, where $Q=R_{ab}T^{ab}$. We construct the field equations, hydrostatic equilibrium equation and trace equation to obtain their solutions numerically under the influence of $σR^{2}+γQ$ gravity model, where $σ$ and $γ$ are arbitrary constants. We examine the dependence of various physical characteristics such as radial/tangential pressure, energy density, anisotropic factor, total mass and surface redshift for specific values of the model parameters. The physical acceptability of the considered model is discussed by verifying the validity of energy conditions, causality condition, and adiabatic index. We also study the effects arising due to the strong non-minimal matter-curvature coupling on anisotropic polytropes. It is found that the polytropic stars are stable and their maximum mass point lies within the required observational Chandrasekhar limit.

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arXiv2018

Gravitational Decoupled Anisotropic Solutions in $f(\mathcal{G})$ Gravity

In this paper, we investigate anisotropic static spherically symmetric solutions in the framework of $f(\mathcal{G})$ gravity through gravitational decoupling approach. For this purpose, we consider Krori and Barua (known solution) isotropic interior solution for static spherically symmetric self-gravitating system and extend it to two types of anisotropic solutions. We examine the physical viability of our models through energy conditions, squared speed of sound and anisotropy parameter. It is found that the first solution is physically viable as it fulfills the energy bounds as well as stability criteria while the second solution satisfies all energy bounds but is unstable at the core of the compact star.

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arXiv2018

Thermodynamics of Modified Cosmic Chaplygin Gas

In this paper, we examine the thermodynamic features of an exotic fluid known as modified cosmic Chaplygin gas in the context of homogeneous isotropic universe model. For this purpose, the behavior of physical parameters are discussed that help to analyze nature of the universe. Using specific heat formalism, the validity of third law of thermodynamics is checked. Furthermore, with the help of thermodynamic entities, the thermal equation of state is also discussed. The thermodynamic stability is explored by means of adiabatic, specific heat and isothermal conditions from classical thermodynamics. It is concluded that the considered fluid configuration is thermodynamically stable and expands adiabatically for an appropriate choice of parameters.

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arXiv2018

Curvature-Matter Coupling Effects on Axial Gravitational Waves

In this paper, we investigate the propagation of axial gravitational waves in the background of the flat FRW universe in $f(R,T)$ theory. The field equations are obtained for unperturbed as well as axially perturbed FRW metric. These field equations are solved simultaneously to obtain the unknown perturbation parameters. We find that the assumed perturbations can affect matter as well as four-velocity. Moreover, ignoring the material perturbations we explicitly obtain an expression for four-velocity. It is concluded that axial gravitational waves in the curvature-matter coupling background can produce cosmological rotation or have memory effect if the wave profile has a discontinuity at the wavefront.

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arXiv2018

Complexity Factor for Charged Spherical System

In this paper, we study the complexity factor for a charged anisotropic self-gravitating object. We formulate the Einstein-Maxwell field equations, Tolman-Opphenheimer-Volkoff equation, and the mass function. We form the structure scalars by the orthogonal splitting of the Riemann tensor and then find the complexity factor with the help of these scalars. Finally, we investigate some astrophysical objects for the vanishing of complexity condition. It is found that the presence of the electromagnetic field decreases the complexity of the system.

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arXiv2018

Study of Homogeneous and Isotropic Universe in $f(R,T^φ)$ Gravity

This paper is devoted to study the cosmological behavior of homogeneous and isotropic universe model in the context of $f(R,T^φ)$ gravity where $φ$ is the scalar field. For this purpose, we follow the first order formalism defined by $H=W(φ)$. We evaluate Hubble parameter, effective equation of state parameter $(ω^{eff})$, deceleration parameter and potential of scalar field for three different values of $W(φ)$. We obtain phantom era in some cases for the early times. It is found that exponential expression of $W(φ)$ yields $ω^{eff}$ independent of time for flat universe and independent of model parameter otherwise. It is concluded that our model corresponds to $Λ$CDM for both initial as well as late times.

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arXiv2018

Energy Conditions in Higher Derivative $f(R,\Box R,T)$ Gravity

In this paper, we examined the viability bounds of a higher derivative $f(R,\Box R, T)$ theory through analyzing energy conditions (where $R$ and $T$ are the Ricci scalar and trace of energy momentum tensor, respectively). We take flat Friedmann-Lemaître-Robertson-Walker spacetime coupled with ideal configurations of matter content. We consider three different realistic models of this gravity, that could be utilized to understand the stability of cosmological solutions. After constructing certain bounds mediated by energy conditions, more specifically weak energy condition, we discuss viable zones of the under considered modified models in an environment of recent estimated numerical choices of the cosmic parameters.

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arXiv2018

Study of Static Wormhole Solutions in $F(T,T_\mathcal{G})$ Gravity

In this paper, we investigate static spherically symmetric wormhole solutions in the background of $F(T,T_\mathcal{G})$ gravity ($T$ is the torsion scalar and $T_{\mathcal{G}}$ represents teleparallel equivalent of the Gauss-Bonnet term). We study the wormhole solutions by assuming four different matter contents, a specific redshift function and a particular $F(T,T_\mathcal{G})$ model. The behavior of null/weak energy conditions for these fluids is analyzed graphically. It turns out that wormhole solutions can be obtained in the absence of exotic matter for some particular regions of spacetime. We also explore stability of wormhole solutions through equilibrium condition. It is concluded that there exist physically acceptable wormhole solutions for anisotropic, isotropic and traceless fluids.

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