arXiv subjects

Z. Yousaf

Publications and source records attributed to Z. Yousaf.

68 records · Page 4Linked to original sources

arXiv2017

Energy Conditions in Modified $f(G)$ Gravity

In this paper, we have considered flat Friedmann-Lemaître-Robertson-Walker metric in the framework of perfect fluid models and modified $f(G)$ gravity (where $G$ is the Gauss Bonnet invariant). Particularly, we have considered particular realistic $f(G)$ configurations that could be used to cure finite-time future singularities arising in the late-time cosmic accelerating epochs. We have then developed the viability bounds of these models induced by weak and null energy conditions, by using the recent estimated numerical figures of the deceleration, Hubble, snap and jerk parameters.

gr-qc↗

arXiv2017

Role of $f(R,T,R_{μν}T^{μν})$ Model on the Stability of Cylindrical Stellar Model

The aim of this paper is to investigate the stable/unstable regimes of the non-static anisotropic filamentary stellar models in the framework of $f(R,T,R_{μν}T^{μν})$ gravity. We construct the field equations and conservation laws in the perspective of this gravity. The perturbation scheme is applied to analyze the behavior of a particular $f(R,T,R_{μν}T^{μν})$ cosmological model on the evolution of cylindrical system. The role of adiabatic index is also checked in the formulations of instability regions. We have explored the instability constraints at Newtonian and post-Newtonian limits. Our results reinforce the significance of adiabatic index and dark source terms in the stability analysis of celestial objects in modified gravity.

physics.gen-ph↗

arXiv2017

Role of Tilted Congruence and $f(R)$ Gravity on Regular Compact Objects

The purpose of this paper is to check the impact of observer and Palatini $f(R)$ terms in the formulations of inhomogeneity factors of spherical relativistic systems. We consider Lemaître-Tolman-Bondi dynamical model as a compact object and studied its evolution with both tilted and non-tilted observers. We performed our analysis for particular cases of fluid distribution in tilted frame and found some energy density irregularity variables. We found that these variables are drastically different from those observed by non-tilted observer. The conformal flat dust and perfect matter contents are homogeneous as long as they impregnate vacuum core. However, this restriction is relaxed, when the complexity in the fluid description is increased. The radial fluid velocity due to tilted congruences and Palatini $f(R)$ curvature terms tend to produce hindrances in the appearance of energy-density inhomogeneities in the initially regular spherical stellar populations.

gr-qc↗

arXiv2016

Electromagnetic Field and Cylindrical Compact Objects in Modified Gravity

In this paper, we have investigated the role of different fluid parameters particularly electromagnetic field and $f(R)$ corrections on the evolution of cylindrical compact object. We have explored the modified field equations, kinematical quantities and dynamical equations. An expression for the mass function has been found in comparison with the Misner-Sharp formalism in modified gravity, after which different mass radius diagrams are drawn. The coupled dynamical transport equation have been formulated to discuss the role of thermoinertial effects on the inertial mass density of the cylindrical relativistic interior. Finally, we have presented a framework, according to which all possible solutions of the metric $f(R)$-Maxwell field equations coupled with static fluid can be written through set of scalar functions. It is found that modified gravity induced by Lagrangians $f(R)=αR^2,~f(R)=αR^2-βR$ and $f(R)=\frac{αR^2-βR}{1+γR}$ are likely to host more massive cylindrical compact objects with smaller radii as compared to GR.

physics.gen-ph↗

arXiv2016

Causes of Irregular Energy Density in $f(R,T)$ Gravity

We investigate irregularity factors for a self-gravitating spherical star evolving in the presence of imperfect fluid. We explore the gravitational field equations and the dynamical equations with the systematic construction in $f(R,T)$ gravity, where $T$ is the trace of the energy-momentum tensor. Furthermore, we analyze two well-known differential equations (which occupy principal importance in the exploration of causes of energy density inhomogeneities) with the help of the Weyl tensor and the conservation laws. The irregularity factors for a spherical star are examined for particular cases of dust, isotropic and anisotropic fluids in dissipative and non-dissipative regimes in the framework of $f(R,T)$ gravity. It is found that as the complexity in the matter with the anisotropic stresses increases, the inhomogeneity factor has more correspondences to one of the structure scalars.

gr-qc↗

arXiv2016

Cavity Evolution and Instability Constraints of Relativistic Interiors

In this manuscript, we have identified the dynamical instability constraints of a self-gravitating cylindrical object within the framework of $f(R,T)$ theory of gravity. We have explored the modified field equations and corresponding dynamical equations for the systematic constructions of our analysis. We have imposed the linear perturbations on metric and material variables with some known static profile up to first order in the perturbation parameter. The role of expansion scalar is also examined in this scenario. The instability regimes have been discussed in the background of Newtonian and post-Newtonian limits. We found that the dark source terms due to the influence of modification in the gravity model is responsible for the instability of the system.

physics.gen-ph↗

arXiv2016

Influence of Electric Charge and Modified Gravity on Density Irregularities

This work aims to identify some inhomogeneity factors for plane symmetric topology with anisotropic and dissipative fluid under the effects of both electromagnetic field as well as Palatini $f(R)$ gravity. We construct the modified field equations, kinematical quantities and mass function to continue our analysis. We have explored the dynamical quantities, conservation equations and modified Ellis equations with the help of a viable $f(R)$ model. Some particular cases are discussed with and without dissipation to investigate the corresponding inhomogeneity factors. For non-radiating scenario, we examine such factors with dust, isotropic and anisotropic matter in the presence of charge. For dissipative fluid, we investigate the inhomogeneity factor with charged dust cloud. We conclude that electromagnetic field increases the inhomogeneity in matter while the extra curvature terms make the system more homogeneous with the evolution of time.

gr-qc↗

arXiv2016

The Influence of Modification of Gravity on the Dynamics of Radiating Spherical Fluids

We explore the evolutionary behaviors of compact objects in a modified gravitational theory with the help of structure scalars. Particularly, we consider the spherical geometry coupled with heat and radiation emitting shearing viscous matter configurations. We construct structure scalars by splitting the Riemann tensor orthogonally in $f(R,T)$ gravity with and without constant $R$ and $T$ constraints, where $R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor. We investigate the influence of modification of gravity on the physical meaning of scalar functions for radiating spherical matter configurations. It is explicitly demonstrated that even in modified gravity, the evolutionary phases of relativistic stellar systems can be analyzed through the set of modified scalar functions.

gr-qc↗

arXiv2015

Instability of Meridional Axial System in f(R) Gravity

We analyze dynamical instability of non-static reflection axial stellar structure by taking into account generalized Euler's equation in metric $f(R)$ gravity. Such an equation is obtained by contracting Bianchi identities of usual anisotropic and effective stress-energy tensors, which after using radial perturbation technique gives modified collapse equation. In the realm of $R+εR^n$ gravity model, we investigate instability constraints at Newtonian and post-Newtonian approximations. We find that instability of meridional axial self-gravitating system depends upon static profile of structure coefficients while $f(R)$ extra curvature terms induce stability to the evolving celestial body.

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

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

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↗

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↗

arXiv2013

Shearfree Spherically Symmetric Fluid Models

We try to find some exact analytical models of spherically symmetric spacetime of collapsing fluid under shearfree condition. We consider two types of solutions: one is to impose a condition on the mass function while the other is to restrict the pressure. We obtain totally of five exact models, and some of them satisfy the Darmois conditions.

gr-qc↗