arXiv subjects

M. Zubair

Publications and source records attributed to M. Zubair.

At least 55 records · Page 3Linked to original sources

arXiv2017

Existence of stable wormholes on a noncommutative-geometric background in modified gravity

In this paper, we discuss spherically symmetric wormhole solutions in $f(R,T)$ modified theory of gravity by introducing well-known non-commutative geometry in terms of Gaussian and Lorentizian distributions of string theory. For some analytic discussion, we consider an interesting model of $f(R,T)$ gravity defined by $f(R,T)=f_{1}(R)+λT$. By taking two different choices for the function $f_{1}(R)$, that is, $f_{1}(R)=R$ and $f_{1}(R)=R+αR^{2}+γR^{n}$, we discuss the possible existence of wormhole solutions. In the presence of non-commutative Gaussian and Lorentizian distributions, we get exact and numerical solutions for both these models. By taking appropriate values of the free parameters, we discuss different properties of these wormhole models analytically and graphically. Further, using equilibrium condition, it is found that these solutions are stable. Also, we discuss the phenomenon of gravitational lensing for the exact wormhole model and it is found that the deflection angle diverges at wormhole throat.

physics.gen-ph↗

arXiv2017

Generalized Second Law of Thermodynamic in Modified Teleparallel Theory

This study is conducted to examine the validity of generalized second law of thermodynamics (GSLT) in flat FRW for modified teleparallel gravity involving coupling between a scalar field with the torsion scalar $T$ and the boundary term $B=2\nabla_μT^μ$. This theory is very useful since it can reproduce other important well-known scalar field theories in suitable limits. The validity of the first and second law of thermodynamics at the apparent horizon is discussed for any coupling. As examples, we have also explored the validity of those thermodynamics laws in some new cosmological solutions under the theory. Additionally, we have also considered the logarithmic entropy corrected relation and discuss the GSLT at the apparent horizon.

gr-qc↗

arXiv2017

Higher Dimensional Inhomogeneous Perfect Fluid Collapse in \emph{f(R)} Gravity

This paper is about the $n+2$-dimensional gravitational contraction of inhomogeneous fluid without heat flux in the framework of $f(R)$ metric theory of gravity. Matching conditions for two regions of a star has been derived by using the Darmois junction conditions. For the analytic solution of equations of motion in modified $f(R)$ theory of gravity, we have taken scalar curvature as constant. Hence final result of gravitational collapse in this frame work is the existence of black hole and cosmological horizons, both of these form earlier than singularity. It has been shown that constant curvature term $f(R_{0})$ ($R_0$ is constant scalar curvature) slows down the collapsing process.

gr-qc↗

arXiv2017

Relativistic space-charge-limited current for massive Dirac fermions

A theory of relativistic space-charge-limited current (SCLC) is formulated to determine the SCLC scaling, $J\propto V^α/L^β$, for a finite bandgap Dirac material of length $L$ biased under a voltage $V$. In a one-dimensional (1D) bulk geometry, our model allows ($α$, $β$) to vary from (2,3) for the non-relativistic model in traditional solids to (3/2,2) for the ultra-relativistic model of massless Dirac fermions. For a two-dimensional (2D) thin-film geometry, we obtain $α= β$ that varies between 2 and 3/2, respectively, at the non-relativistic and ultra-relativistic limits. We further provide a rigorous proof based on a Green's function approach that for uniform SCLC model described by carrier density-dependent mobility, the scaling relations of the 1D bulk model can be directly mapped into the case of 2D thin film for any contact geometries. Our simplified approach provides a convenient tool to obtain the 2D thin-film SCLC scaling relations without the need of explicitly solving the complicated 2D problems. Finally, this work clarifies the inconsistency in using the traditional SCLC models to explain the experimental measurement of 2D Dirac semiconductor. We conclude that the voltage-scaling $3/2 < α< 2$ is a distinct signature of massive Dirac fermions in Dirac semiconductor and is in agreement with experimental SCLC measurement in MoS$_2$.

cond-mat.mes-hall↗

arXiv2017

Dynamical Analysis of Cylindrically Symmetric Anisotropic Sources in $f(R,T)$ Gravity

In this paper, we have analyzed the stability of cylindrically symmetric collapsing object filled with locally anisotropic fluid in $f(R,T)$ theory, where $R$ is the scalar curvature and $T$ is the trace of stress-energy tensor of matter. Modified field equations and dynamical equations are constructed in $f(R,T)$ gravity. Evolution or collapse equation is derived from dynamical equations by performing linear perturbation on them. Instability range is explored in both Newtonian and post-Newtonian regimes with the help of adiabetic index, which defines the impact of physical parameters on the instability range. Some conditions are imposed on physical quantities to secure the stability of the gravitating sources.

physics.gen-ph↗

arXiv2017

Reconstruction of Scalar Potentials in $f(R,R_{αβ} R^{αβ},ϕ)$ theory of gravity

In this paper, we explore the nature of scalar field potential in $f(R, R_{αβ} R^{αβ},ϕ)$ gravity using a well-motivated reconstruction scheme for flat FRW geometry. The beauty of this scheme lies in the assumption that the Hubble parameter can be expressed in terms of scalar field and vice versa. Firstly, we develop field equations in this gravity and present some general explicit forms of scalar field potential via this technique. In the first case, we take De Sitter universe model and construct some field potentials by taking different cases for coupling function. In the second case, we derive some field potentials using power law model in the presence of different matter sources like barotropic fluid, cosmological constant and Chaplygin gas for some coupling functions. From graphical analysis, it is concluded that using some specific values of the involved parameters, the reconstructed scalar field potentials are cosmologically viable in both cases.

gr-qc↗

arXiv2016

Thermodynamics in $f(R,R_{αβ}R^{αβ},ϕ)$ theory of gravity

First and second laws of black hole thermodynamics are examined at the apparent horizon of FRW spacetime in $f(R,R_{αβ}R^{αβ} ,ϕ)$ gravity, where $R$, $R_{αβ}R^{αβ}$ and $ϕ$ are the Ricci scalar, Ricci invariant and the scalar field respectively. In this modified theory, Friedmann equations are formulated for any spatial curvature. These equations can be presented into the form of first law of thermodynamics for $T_{h}d\hat{S}_{h}+ T_{h}d_{i}\hat{S}_{h}+W dV=dE$, where $d_{i}\hat{S}_{h}$ is an extra entropy term because of the non-equilibrium presentation of the equations and $T_{h}d\hat{S}_{h}+W dV=dE$ for the equilibrium presentation. The generalized second law of thermodynamics (GSLT) is expressed in an inclusive form where these results can be represented in GR $f(R)$ and $f(R, ϕ)$ gravities. Finally to check the validity of GSLT, we take some particular models and produce constraints of the parameters.

gr-qc↗

arXiv2016

Interior Solutions of Fluid Sphere in f(R,T) Gravity Admitting Conformal Killing Vectors

We discuss the interior solutions of fluid Sphere in f(R,T) gravity admitting conformal killing vectors, where R is Ricci scalar and T is trace of energy momentum tensor. The solutions corresponding to isotropic and anisotropic configurations have been investigated explicitly. Further, the anisotropic case has been dealt by the utilization of linear equation of state. The results for both cases have been interpreted graphically. The equation of state parameter, integration constants and other parameters of the theory have been chosen to find the central density equal to standard value of central density of the compact objects. The energy conditions as well as stability of the solutions have been investigated in the background of f(R,T) gravity.

gr-qc↗

arXiv2016

Static Spherically Symmetric Wormholes in $f(R,T)$ Gravity

In this work, we explore wormhole solutions in $f(R,T)$ theory of gravity, where $R$ is the scalar curvature and $T$ is the trace of stress-energy tensor of matter. To investigate this, we consider static spherically symmetric geometry with matter contents as anisotropic, isotropic and barotropic fluids in three separate cases. By taking into account Starobinsky $f(R)$ model , we analyze the behavior of energy conditions for these different kind of fluids. It is shown that the wormhole solutions can be constructed without exotic matter in few regions of spacetime. We also give the graphical illustration of obtained results and discuss the equilibrium picture for anisotropic case only. It is concluded that the wormhole solutions with anisotropic matter are realistic and stable in this gravity.

gr-qc↗

arXiv2016

Axially Symmetric Shear-free Fluids in $f(R,T)$ Gravity

In this work we have discussed the implications of shear-free condition on axially symmetric anisotropic gravitating objects in $f(R,T)$ theory. Restricted axial symmetry ignoring rotation and reflection enteries is taken into account for establishment of instability range. Implementation of linear perturbation on constitutive modified dynamical equations yield evolution equation. This equation associates adiabatic index $Γ$ with material and dark source components defining stable and unstable regions in Newtonian (N) and post-Newtonian (pN) approximations.

gr-qc↗

arXiv2016

Study of Anisotropic Compact Stars in Starobinsky Model

The aim of this paper is to study the formation of anisotropic compact stars in modified $f(R)$ theory of gravity, which is the generalization of the Einstein's gravity. To this end, we have used the solution of Krori and Barua to the anisotropic distribution of matter in $f(R)$ gravity. Further, we have matched the interior solution with the exterior solution to determine the constants of Krori and Barua solution. Finally the constant have been determined by using the data of compact compact stars like 4$U1820-30, Her X-1, SAX J 1808-3658$. Using the evaluated form of the solutions, we have discussed the regularity of matter components at the center as well as on the boundary, energy conditions, anisotropy, stability analysis and mass-radius relation of the compact stars 4$U1820-30$, $Her X-1$, $SAX J 1808-3658.$

physics.gen-ph↗

arXiv2016

Cosmological reconstruction and energy bounds in $f(R,R_{αβ}R^{αβ},ϕ)$ gravity

We discuss the cosmological reconstruction of $f(R,R_{αβ}R^{αβ},ϕ)$ (where $R$, $R_{αβ}R^{αβ}$ and $ϕ$ represents the Ricci scalar, Ricci invariant and scalar field) corresponding to power law and de Sitter evolution in the framework of FRW universe model. We derive the energy conditions for this modified theory which seem to be more general and can be reduced to some known forms of these conditions in general relativity, $f(R)$ and $f(R,ϕ)$ theories. We have presented the general constraints in terms of recent values of snap, jerk, deceleration and Hubble parameters. The energy bounds are analyzed for reconstructed as well as known models in this theory. Finally, the free parameters are analyzed comprehensively.

gr-qc↗

arXiv2016

Reconstructing QCD ghost $f(R,T)$ models

We reconstruct $f(R,T)$ theory (where $R$ is the scalar curvature and $T$ is the trace of energy-momentum tensor) in the framework of QCD ghost dark energy models. In this study, we concentrate on particular models of $f(R,T)$ gravity which permits the standard continuity equation in this theory. It is found that reconstructed function can represent phantom and quintessence regimes of the universe in the background of flat FRW universe. In addition, we explore the stability of ghost $f(R,T)$ models.

gr-qc↗

arXiv2015

Possible Formation of Compact Stars in $f(R,T)$ Gravity

This paper constitutes the investigations regarding possible formation of compact stars in $f(R,T)$ theory of gravity, where $R$ is Ricci scalar and $T$ is trace of energy momentum tensor. In this connection, we use analytic solution of Krori and Barua metric (Krori and Barua 1975)to spherically symmetric anisotropic star in context of $f(R,T)$ gravity. The masses and radii of compact stars models namely model $1$, model$ 2$ and model $3$ are employed to incorporate with unknown constants in Krori and Barua metric. The physical features such as regularity at center, anisotropy measure, casuality and well-behaved condition of above mentioned class of compact starts are analyzed. Moreover, we have also discussed energy conditions, stability and surface redshift in $f(R,T)$ gravity.

physics.gen-ph↗

arXiv2015

Anisotropic Strange Quintessence Stars in $f(R)$ Gravity

In this paper, we have formulated the new exact model of quintessence anisotropic star in $f(R)$ theory of gravity. The dynamical equations in $f(R)$ theory with the anisotropic fluid and quintessence field have been solved by using Krori-Barua solution. In this case, we have used the Starobinsky model of $f(R)$ gravity. We have determined that all the obtained solutions are free from central singularity and potentially stable. The observed values of mass and radius of the different strange stars PSR J 1614-2230, SAXJ1808.4-3658(SS1), 4U1820- 30, PSR J 1614-2230 have been used to calculate the values of unknown constants in Krori and Barua metric. The physical parameters like anisotropy, stability and redshift of the stars have been investigated in detail.

physics.gen-ph↗

arXiv2015

Shearfree Condition and dynamical Instability in $f(R,T)$ gravity

The implications of shearfree condition on instability range of anisotropic fluid in $f(R,T)$ are studied in this manuscript. A viable $f(R, T)$ model is chosen to arrive at stability criterion, where $R$ is Ricci scalar and $T$ is the trace of energy momentum tensor. The evolution of spherical star is explored by employing perturbation scheme on modified field equations and contracted Bianchi identities in $f(R, T)$. The effect of imposed shearfree condition on collapse equation and adiabatic index $Γ$ is studied in Newtonian and post-Newtonian regimes.

gr-qc↗

arXiv2015

Generalized Second Law of Thermodynamics in $f(T,T_{G})$ gravity

An equilibrium picture of thermodynamics is discussed at the apparent horizon of FRW universe in $f(T,T_G)$ gravity, where $T$ represents the torsion invariant and $T_G$ is the teleparallel equivalent of the Gauss-Bonnet term. It is found that one can translate the Friedmann equations to the standard form of first law of thermodynamics. We discuss GSLT in the locality of assumption that temperature of matter inside the horizon is similar to that of horizon. Finally, we consider particular models in this theory and generate constraints on the coupling parameter for the validity of GSLT in terms of recent cosmic parameters and power law solutions.

gr-qc↗

arXiv2015

Evolution of Axially Symmetric Anisotropic Sources in $f(R,T)$ Gravity

We discuss the dynamical analysis in $f(R,T)$ gravity (where $R$ is Ricci scalar and $T$ is trace of energy momentum tensor) for gravitating sources carrying axial symmetry. The self gravitating system is taken to be anisotropic and line element describes axially symmetric geometry avoiding rotation about symmetry axis and meridional motions (zero vorticity case). The modified field equations for axial symmetry in $f(R,T)$ theory are formulated, together with the dynamical equations. Linearly perturbed dynamical equations lead to the evolution equation carrying adiabatic index $Γ$ that defines impact of non-minimal matter to geometry coupling on range of instability for Newtonian (N) and post-Newtonian (pN) approximations.

gr-qc↗