arXiv subjects

Z. Yousaf

Publications and source records attributed to Z. Yousaf.

At least 55 records · Page 3Linked to original sources

arXiv2020

Gravastars in $f(R,G)$ Gravity

In this paper, we discuss some feasible features of gravastar that were firstly demonstrated by Mazur and Mottola. It is already established that gravastar associates the de-Sitter spacetime in its inner sector with the Schwarzschild geometry at its exterior through the thin shell possessing the ultra-relativistic matter. We have explored the singularity free spherical model with a particular equation of state under the influence of $f(R,G)$ gravity, where $R$ is the Ricci scalar and $G$ is the Gauss-Bonnet term. The interior geometry is matched with a suitable exterior using Israel formalism. Also, we discussed a feasible solution of gravastar which describes the other physically sustainable factors under the influence of $f(R,G)$ gravity. Different realistic characteristics of the gravastar model are discussed, in particular, shell's length, entropy, and energy. A significant role of this particular gravity is examined for the sustainability of gravastar model.

gr-qc↗

arXiv2020

New Definition of Complexity Factor in $f(R,T,R_{μν}T^{μν})$ Gravity

This paper is devoted to present new definition of complexity factor for static cylindrically symmetric matter configurations in $f(R,T,R_{μν}T^{μν})$ gravity. For this purpose, we have considered irrotational static cylindrical spacetime coupled with a locally anisotropic relativistic fluid. After formulating gravitational field and conservation equations, we have performed orthogonal splitting of the Riemann curvature tensor. Unlike GR (for spherical case) the one of the structure scalars $X_{TF}$, has been identified to be a complexity factor. This factor contains effective forms of the energy density, and anisotropic pressure components. Few peculiar relations among complexity factor, Tolman mass and Weyl scalar are also analyzed with the modified $f(R,T,R_{μν}T^{μν})$ corrections.

gr-qc↗

arXiv2020

Gravastars in $f(R,T,R_{μν}T^{μν})$ Gravity

This work is devoted to study the analytical and regular solutions of a particular self-gravitating object (i.e., gravastar) in a particular theory of gravity. We derive the corresponding field equations in the presence of effective energy momentum tensor associated with the perfect fluid configuration of a spherical system. We then describe the mathematical formulations of the three respective regions i.e., inner, shell and exterior of a gravastar separately. Additionally, the significance and physical characteristics along with the graphical representation of gravastars are discussed in detail. It is seen that under some specific constraints, $f(R,T,R_{μν}T^{μν})$ gravity is likely to host gravastars.

gr-qc↗

arXiv2020

Construction of Charged Cylindrical Gravastar-like Structures

In this work, we extend the work of gravastars to analyze the role of electromagnetic field in $f(R,T)$ gravity. We consider the irrotational cylindrically symmetric geometry and established the $f(R,T)$ field equations and conservation laws. After considering charged exterior geometry, the mathematical quantities for evaluating Israel junction conditions are being calculated. The mass of the gravastar-like cylindrical structure is calculated through the equations of motion at the hypersurface in the presence of an electromagnetic field. The behavior of electric charge on the length of the thin shell, energy content, and entropy of gravastar is being studied graphically. We concluded that charge has an important role in the length of the thin shell, energy content, and entropy of gravastar.

physics.gen-ph↗

arXiv2020

On the Role of $f(G,T)$ Terms in Structure Scalars

This work is devoted to explore the effects of $f(G,T)$ terms on the study of structure scalars and their influences in the formulations of the Raychaudhuri, shear and Weyl scalar equations. For this purpose, we have assumed non-static spherically symmetric geometry coupled with shearing viscous locally anisotropic dissipative matter content. We have developed relations among the Misner-Sharp mass, Weyl scalar, matter and structure variables. We have also formulated set of $f(G,T)$ structure scalars after orthogonally breaking down of the Riemann curvature tensor. The influences of these scalar functions in the modeling of relativistic radiating spheres are also studied. The factor involved in the emergence of inhomogeneities is also explored for the constant and varying modified curvature corrections. We inferred that $f(G,T)$ structure scalars could lead provide an effective tool to study Penrose-Hawking singularity theorems and Newman-Penrose formalism.

gr-qc↗

arXiv2019

Charged Gravastars in Modified Gravity

In this paper, we investigate the effects of electromagnetic field on the isotropic spherical gravastar models in metric $f(R,T)$ gravity. For this purpose, we have explored singularity-free exact models of relativistic spheres with a specific equation of state. After considering Reissner Nordström spacetime as an exterior region, the interior charged manifold is matched at the junction interface. Several viable realistic characteristics of the spherical gravastar model are studied in the presence of electromagnetic field through graphical representations. It is concluded that the electric charge has a substantial role in the modeling of proper length, energy contents, entropy and equation of state parameter of the stellar system. We have also explored the stable regions of the charged gravastar structures.

gr-qc↗

arXiv2019

Dissipative Self-Gravitating Systems in Modified Gravity

We discuss the gravitational collapse of spherical compact objects in the background of $f(R,T,Q)$ theory, where $R$ represent the Ricci scalar, $T$ is the trace of energy momentum tensor while $Q\equiv R_{μν}T^{μν}$, and investigate the influence of anisotropy and heat dissipation in this scenario. We provide an analysis on the role of distinct material terms considered while studying the dynamical equation. The dynamical equation is coupled with a heat transport equation and discussed in the background of $f(R,T,Q)$ theory of gravity. The reduction element in the density of inertial mass, is re-acquired which is based on the internal position of thermodynamics. In collation with the equivalence relation, the reduction quantity in density is similar as appeared with gravitational force. We formulate the connection of Weyl tensor with different matter variables to see the non-identical outcomes. The inhomogeneous nature of energy density is also analyzed in the framework of modified gravity.

gr-qc↗

arXiv2019

Compression properties of polymeric syntactic foam composites under cyclic loading

Syntactic foams are composite materials frequently used in applications requiring the properties of low density and high damage tolerance. In the present work, polymer-based syntactic foams were studied under cyclic compression in order to investigate their compressibility, recoverability, energy dissipation and damage tolerance. These syntactic foams were manufactured by adding hollow polymer microspheres of various sizes and wall thicknesses into a polyurethane matrix. The associated loading and unloading curves during cyclic testing were recorded, revealing the viscoelastic nature of the materials. SEM images of the samples were obtained in order to study potential damage mechanisms during compression. It was observed that these syntactic foams exhibit high elastic recovery and energy dissipation over a wide range of compressional strains and the addition of polymer microspheres mitigate the damage under compressional loading.

cond-mat.mtrl-sci↗

arXiv2019

Bounds on Higher Derivative $f(R,\square R,T)$ Models from Energy Conditions

This paper studies the viable regions of some cosmic models in a higher derivative $f(R,\square R, T)$ theory with the help of energy conditions (where $R$ and $T$ are the Ricci scalar, and trace of energy momentum tensor, respectively). For this purpose, we assume a flat Friedmann-Lemaître-Robertson-Walker metric which is assumed to be filled with perfect fluid configurations. We take two distinct realistic models, that might be helpful to explore stable regimes of cosmological solutions. After taking some numerical values of cosmic parameters, like crackle, snap, jerk (etc) as well as viable constraints from energy conditions, the viable zones for the under observed $f(R,\square R, T)$ models are examined.

gr-qc↗

arXiv2018

Circular Motion and Energy Extraction in a Rotating Black Hole

This paper explores the circular geodesics of neutral test particles on an equatorial plane around a rotating black hole. After using equations of motion of scalar-tensor-vector gravity with the circular geodesics of null-like particles, we find the equation of photon orbit. With the help of an effective potential form, we have examined the stable regimes of photons orbits. The Lyapunov exponent, as well as the effective force acting on photons, is also investigated. We examine the energy extraction from a black hole via Penrose process. Furthermore, we discuss the negative energy state and the efficiency of energy extraction. We have made compare our results with that obtained for some well known black holes models. We concluded that the efficiency of the energy extraction decreases with the increase of dimensionless parameter of theory and increases as spin parameter increases.

gr-qc↗

arXiv2018

Dynamical Instability of Spherical Anisotropic Sources in $f(R,T,R_{μν}T^{μν})$ Gravity

In this paper, we study the effects of modification of gravity on the problem of dynamical instability of the spherical relativistic anisotropic interiors. We have considered non-zero influence of expansion scalar throughout during the evolutionary phases of spherical geometry that led to the use of fluid stiffness parameter. The modified hydrostatic equation for the stellar anisotropic matter distributions is constructed and then solved by using radial perturbation scheme. Such a differential equation can be further used to obtain instability constraints at both weak field and post-Newtonian approximations after considering a particular Harrison-Wheeler equation of state. This approach allows us to deal with the effects of usual and effective matter variables on the stability exotic stellar of self-gravitating structures.

gr-qc↗

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.

gr-qc↗

arXiv2018

Thermodynamics and Glassy Phase Transition of Regular Black Holes

The present paper is aimed to study thermodynamical properties of phase transition for regular charged black holes. In this context, we have considered two different forms of black hole metrics supplemented with exponential and logistic distribution functions and investigated the recent expansion of phase transition through grand canonical ensemble. After exploring the corresponding Ehrenfest's equation, we found the second order background of phase transition at critical points. In order to check the critical behavior of regular black holes, we have evaluated some corresponding explicit relations for the critical temperature, pressure and volume and draw certain graphs with constant values of Smarr's mass. We found that for the BH metric with exponential configuration function, the phase transition curves are divergent near the critical points, while glassy phase transition has been observed for the Ayón-Beato-García-Bronnikov BH in $n=5$ dimensions.

gr-qc↗

arXiv2018

Existence of Compact Structures in $f(R,T)$ Gravity

The present paper is devoted to investigate the possible emergence of relativistic compact stellar objects through modified $f(R,T)$ gravity. For anisotropic matter distribution, we used Krori and Barura solutions and two notable and viable $f(R,T)$ gravity formulations. By choosing particular observational data, we determine the values of constant in solutions for three relativistic compact star candidates. We have presented some physical behavior of these relativistic compact stellar objects and some aspects like energy density, radial as well as transverse pressure, their evolution, stability, measure of anisotropy and energy conditions.

physics.gen-ph↗

arXiv2017

Stability of Anisotropic Stellar Filaments

The study of perturbation of self-gravitating celestial cylindrical object have been carried out in this paper. We have designed a framework to construct the collapse equation by formulating the modified field equations with the background of $f(R,T)$ theory as well as dynamical equations from the contracted form of Bianchi identities with anisotropic matter configuration. We have encapsulated the radial perturbations on metric and material variables of the geometry with some known static profile at Newtonian and post-Newtonian regimes. We examined a strong dependence of unstable regions on stiffness parameter which measures the rigidity of the fluid. Also, the static profile and matter variables with $f(R,T)$ dark source terms control the instability of compact cylindrical system.

physics.gen-ph↗

arXiv2017

Influence of $f(R)$ Models on the Existence of Anisotropic Self-Gravitating Systems

This paper aims to explore some realistic configurations of anisotropic spherical structures in the background of metric $f(R)$ gravity, where $R$ is the Ricci scalar. The solutions obtained by Krori and Barua are used to examine the nature of particular compact stars with three different modified gravity models. The behavior of material variables is analyzed through plots and the physical viability of compact stars is investigated through energy conditions. We also discuss the behavior of different forces, equation of state parameter, measure of anisotropy and Tolman-Oppenheimer-Volkoff equation in the modeling of stellar structures. The comparison from our graphical representations may provide evidences for the realistic and viable $f(R)$ gravity models at both theoretical and astrophysical scale.

gr-qc↗

arXiv2017

Evolution of Compact Stars and Dark Dynamical Variables

This work is aimed to explore the dark dynamical effects of $f(R,T)$ modified gravity theory on the dynamics of compact celestial star. We have taken the interior geometry as spherical star which is filled with imperfect fluid distribution. The modified field equations are explored by taking a particular form of $f(R,T)$ model, i.e., $f(R,T)=f_1(R)+f_2(R)f_3(T)$. These equations are then utilized to formulate the well-known structure scalars under the dark dynamical effects of this higher order gravity theory. Also, the evolution equations for expansion and shear are formulated with the help of these scalar variables. Further, all this analysis have been made under the condition of constant $R$ and $T$. We found a crucial significance of dark source terms and dynamical variables on the evolution and density inhomogeneity of compact objects.

gr-qc↗

arXiv2017

Stability Analysis of Stellar Radiating Filaments

The aim of this paper is to perform stability analysis of anisotropic dissipative cylindrical collapsing model in $f(R,T,R_{μν} T^{μν})$ gravity. In this context, the modified version of hydrodynamical equation is explored by means of dynamical equations and radial perturbation scheme. We examined the role of adiabatic index, dissipation as well as the particular cosmological model on the onset of dynamical instability of the evolving cylindrical system that was initially in hydrostatic equilibrium with Newtonian and post Newtonian approximations. It is pointed out that extra curvature terms of $f(R,T,R_{μν}T^{μμν})$ gravity tends to increase the stability, while that heat radiations push the system to enter into unstable window. Further, our results reveal the significance of adiabatic index in the stability analysis of cylindrical celestial model.

gr-qc↗