arXiv subjects

M. Farasat Shamir

Publications and source records attributed to M. Farasat Shamir.

At least 37 records · Page 2Linked to original sources

arXiv2020

Traversable Wormhole Solutions in $f(R)$ Gravity Via Karmarkar Condition

Motivated by recent proposals of possible wormhole shape functions, we construct a wormhole shape function by employing the Karmarkar condition for static traversable wormhole geometry. The proposed shape function generates wormhole geometry that connects two asymptotically flat regions of spacetime and satisfies the required conditions. Further, we discuss the embedding diagram in three-dimensional Euclidean space to present the wormhole configurations. The main feature of current study is to consider three well-known $f(R)$ gravity models, namely exponential gravity model, Starobinsky gravity Model and Tsujikawa $f(R)$ gravity model. Moreover, we investigate that our proposed shape function provides the wormhole solutions with less (or may be negligible) amount of exotic matter corresponding to the appropriate choice of $f(R)$ gravity models and suitable values of free parameters. Interestingly, the solutions obtained for this shape function generate stable static spherically symmetric wormhole structure in the context of non-existence theorem in $f(R)$ gravity. This may lead to a better analytical representation of wormhole solutions in other modified gravities for the suggested shape function.

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arXiv2020

Massive Compact Bardeen Stars with Conformal Motion

The main focus of this paper is to discuss the solutions of Einstein-Maxwell's field equations for compact stars study. We have chosen the MIT bag model equation of state for the pressure-energy density relationship and conformal Killing vectors are used to investigate the appropriate forms for metric coefficients. We impose the boundary conditions, by choosing the Bardeen model to describe as an exterior spacetime. The Bardeen model may provide the analysis with some interesting results. For example, the extra terms involved in the asymptotic representations as compared to the usual Reissner-Nordstrom case may influence the mass of a stellar structure. Both energy density and pressure profiles behave realistically except a central singularity. It is shown that the energy conditions are satisfied in our study. The equilibrium conditions through TOV equation and stability criteria through Adiabatic index for the charged stellar structure study are investigated. Lastly, we have also provided a little review of the case with Reissner-Nordstrom spacetime as an exterior geometry for the matching condition. In both cases, the masses obey the Andreasson's limit $\sqrt{M} \leq \sqrt{R}/3+\sqrt{R/9+q^2/3R}$ requirement for a charged star. Conclusively, the results show that Bardeen model geometry provides more massive stellar objects as compared to usual Reissner-Nordstrom spacetime. In particular, the current study supports the existence of realistic massive structures like PSR J $1614-2230$.

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arXiv2020

Bardeen Stellar Structures with Karmarkar Condition

Current study is focussed to discuss the existence of a new family of compact star solutions by adopting the Karmarkar condition in the background of Bardeen black hole geometry. For this purpose, we consider static spherically symmetric spacetime with anisotropic fluid distribution in the presence of electric charge. We consider a specific model of $g_{rr}$ metric function, to describe a new family of solutions which satisfies the Karmarkar condition. Further, we investigate the interior solutions for two different models of compact stars with observational mass and radii, i.e., $(M=1.77M_{\odot}, \;R_{b}=9.56km)$ and $(M=1.97M_{\odot}, \;R_{b}=10.3km)$. It is found that these solutions fulfill all the necessary conditions for a charged star. Through graphical discussion, it is noticed that our calculated solutions are physically arguable with a best degree of accuracy for $n\in[1.8,7)$, where parameter $n$ is involved in the model under discussion. However, it is perceived that the presented model violates all the physical conditions for $n\in\{2,4,6\}$. Finally, it is concluded that the parameter $n$ has a strong impact on the obtained solutions in the context of Bardeen stellar structures.

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arXiv2020

Stellar Structures in $f(\mathcal{G})$ Gravity Admitting Noether Symmetries

This work aims to investigate some possible emergence of relativistic compact stellar objects in modified $f(\mathcal{G})$ gravity using Noether symmetry approach. For this purpose, we assume static spherically symmetric spacetime in the presence of isotropic matter distribution. We construct Noether symmetry generators along with associated conserved quantities by considering the standard choice of viable $f(\mathcal{G})$ gravity model i.e. $f(\mathcal{G})= α\mathcal{G}^{n}$, where $α$ is the model parameter. In particular, we use conservation relation acquired from the classical Noether approach by imposing some appropriate initial conditions to construct the metric potentials. The obtained conserved quantity play vital role in describing the stellar structure of compact stars. Moreover, by considering an appropriate numerical solution, some salient features of compact stellar structures like effective energy density, pressure, energy conditions, stability against equilibrium of the forces and speed of sound are discussed by assigning the suitable values of model parameter involved. Our study reveals that the compact objects in $f(\mathcal{G})$ gravity from Noether symmetry approach depend on the conserved quantity obtained and the model parameter $α$. In nutshell, Noether symmetries are quite helpful to generate solutions that follow physically accepted phenomena. Moreover, we observed that these obtained solutions are consistent with the astrophysical observational data, which depicts the viability of our proposed Noether symmetry scheme.

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arXiv2020

Wormhole Structures in Logarithmic-Corrected $R^2$ Gravity

This paper is devoted to find the feasible shape functions for the construction of static wormhole geometry in the frame work of logarithmic-corrected $R^2$ gravity model. We discuss the asymptotically flat wormhole solutions sustained by the matter sources with anisotropic pressure, isotropic pressure and barotropic pressure. For anisotropic case, we consider three shape functions and evaluate the null energy conditions and weak energy conditions graphically along with their regions. Moreover, for barotropic and isotropic pressures, we find shape function analytically and discuss its properties. For the formation of traversable wormhole geometries, we cautiously choose the values of parameters involved in $f(R)$ gravity model. We show explicitly that our wormhole solutions violates the non-existence theorem even with logarithmic corrections. We discuss all physical properties via graphical analysis and it is concluded that the wormhole solutions with relativistic formalism can be well justified with logarithmic corrections.

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arXiv2020

Physically viable solutions of anisotropic spheres in $f(\mathscr{R},\mathscr{G})$ gravity satisfying the Karmarkar condition

This paper is devoted to discuss compact stars in $f(\mathscr{R},\mathscr{G})$ gravity, where $\mathscr{R}$ and $\mathscr{G}$ denote the Ricci scalar and Gauss-Bonnet invariant respectively. To meet this aim, we consider spherically symmetric space-time with anisotropic fluid distribution. In particular, the Karmarkar condition is used to explore the compact star solutions. Further, we choose two specific model of compact stars namely LMC X-4 (mass =1.29$M/M_{\odot}$ \& radii=9.711 km) and EXO 1785-248 (mass =1.30$M/M_{\odot}$ \& radii=8.849 km). We develop the field equations for $f(\mathscr{R},\mathscr{G})$ gravity by employing the Karmarkar condition with a specific model already reported in literature by Lake [Phys. Rev. D 67, 104015 (2003)]. We further consider the Schwarzschild geometry for matching conditions at the boundary. It is important to mention here that we calculate the values of all the involved parameter by imposing the matching condition. We have provided a detailed graphical analysis to discuss the physical acceptability of parameters, i.e., energy density, pressure, anisotropy, and gradients. We have also examined the stability of compacts stars by exploring the energy conditions, equation of state, generalized Tolman-Oppenheimer-Volkoff equation, causality condition, and adiabatic index. For present analysis, we predict some numerical values in tabular form for central gravitational metric functions, central density and central pressures components. We have also calculated the ratio $p_{rc}/ ρ_{c}$ to check the validity of Zeldovich's condition. Conclusively, it is found that our obtained solutions are physically viable with well-behaved nature in $f(\mathscr{R},\mathscr{G})$ modified gravity for the compact star models under discussion.

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arXiv2020

$f(\mathcal{R},φ,χ)$ Cosmology with Noether Symmetry

This paper is devoted to explore modified $f(\mathcal{R})$ theories of gravity using Noether symmetry approach. For this purpose, Friedmann-Robertson-Walker spacetime is chosen to investigate the cosmic evolution. The study is mainly divided into two parts: Firstly Noether symmetries of metric $f(\mathcal{R})$ gravity are revisited and some new class of solutions with the help of conserved quantities are reported. It is shown that different scenarios of cosmic evolution can be discussed using Noether symmetries and one of the case indicates the chances for the existence of Big Rip singularity. Secondly, $f(\mathcal{R})$ theory coupled with scalar field has been discussed in detail. The Noether equations of modified gravity are reported with three subcases for flat Friedmann-Robertson-Walker universe. It is concluded that conserved quantities are quite helpful to find some important exact solutions in the cosmological contexts. Moreover, the scalar field involved in the modified gravity plays a vital role in the cosmic evolution and an accelerated expansion phase can be observed for some suitable choices of $f(\mathcal{R},φ,χ)$ gravity models.

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arXiv2020

Stellar Structures in $f(\mathcal{G})$ Gravity with Tolman-Kuchowicz Spacetime

This paper is devoted to explore some relativistic configurations of stellar objects for static spherically symmetric structures in the context of modified $f(\mathcal{G})$ gravity, by exploiting the Tolman-Kuchowicz spacetime [1,2]. We develop the equations of motion for spherically symmetric spacetime in the presence of anisotropic matter distribution by considering the physically valid expressions of the metric potentials, $ν=Br^2+2lnC$ and $λ=ln(1 + ar^2+br^4)$. To attain the values of the unknown constants we consider the observational data of $Cen~ X-3$, $EXO ~1785-248$ and $LMC~ X-4$ star models. Further, by using evaluated form of the solutions we provide many aspects which are described by the physical status like effective energy density, components of radial and transverse pressure, energy conditions, stability against equilibrium of the forces, speed of sound, mass-radius relation, surface redshift, compactness parameter, adiabatic index and anisotropic measurement. It is observed that all these features follow physically accepted patterns and the resulting outcome is in the experimental range which depicts the viability of our presented $f(\mathcal{G})$ gravity models.

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arXiv2018

Existence of Static Wormhole Solutions in $f(R,G)$ Gravity

This work investigates some feasible regions for the existence of traversable wormhole geometries in $f(R,G)$ gravity, where $R$ and $G$ represent the Ricci scalar and the Gauss-Bonnet invariant respectively. Three different matter contents anisotropic fluid, isotropic fluid and barotropic fluid have been considered for the analysis. Moreover, we split $f(R,G)$ gravity model into Strobinsky like $f(R)$ model and a power law $f(G)$ model to explore wormhole geometries. We select red-shift and shape functions which are suitable for the existence of wormhole solutions for the chosen $f(R,G)$ gravity model. It has been analyzed with the graphical evolution that the null energy and weak energy conditions for the effective energy-momentum tensor are usually violated for the ordinary matter content. However, some small feasible regions for the existence of wormhole solutions have been found where the energy conditions are not violated. The overall analysis confirms the existence of the wormhole geometries in $f(R,G)$ gravity under some reasonable circumstances.

physics.gen-ph↗

arXiv2018

Stellar Hydrostatic Equilibrium Compact Structures in $f(\mathcal{G},T)$ Gravity

In this paper, stellar hydrostatic equilibrium configuration of the compact stars (neutron stars and strange stars) has been studied for $f(\mathcal{G},T)$ gravity model, with $\mathcal{G}$ and $T$ being the Gauss-Bonnet invariant and the trace of energy momentum tensor, respectively. After having derived the hydrostatic equilibrium equations for $f(\mathcal{G},T)$ gravity, the fluid pressure for the neutron stars and the strange stars has been computed by implying two equation of state models corresponding to two different existing compact stars. For the $f(\mathcal{G},T)=α{\mathcal{G}^n+λ{T}}$ gravity model, with $α$, $n$, and $λ$ being some specific constants, substantial change in the behavior of the physical attributes of the compact stars like the energy density, pressure, stellar mass, and total radius has been noted with the corresponding change in $λ$ values. Meanwhile, it has been shown that for some fixed central energy density and with increasing values of $λ$, the stellar mass both for the neutron stars and the strange stars increases, while the total stellar radius $R$ exhibits the opposite behavior for both of the compact stars. It is concluded that for this $f(\mathcal{G},T)$ stellar model, the maximum stellar mass can be boosted above the observational limits.

physics.gen-ph↗

arXiv2018

Gravastars in $f(\mathcal{G},T)$ Gravity

This work proposes a stellar model under Gauss-Bonnet $f(\mathcal{G}, T)$ gravity with the conjecture theorised by Mazur and Mottola, well known as the gravitational vacuum stars (gravastars). By taking into account the $f(\mathcal{G},T)$ stellar model, the structure of the gravastar with its exclusive division of three different regions namely, (i) the core interior region (ii) the junction region (shell), and (iii) the exterior region, has been investigated with reference to the existence of energy density, pressure, ultra-relativistic plasma, and repulsive forces. The different physical features like, the equation of the state parameter, length of the shell, entropy, energy-thickness relation of the gravastar shell model have been discussed. Also, some other physically valid aspects have been presented with the connection to non-singular and event-horizon free gravastar solutions, which in contrast to a black hole solution, might be stable without containing any information paradox.

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arXiv2018

Analysis of Charged Compact Stars in Modified Gravity

Current study highlights the physical characteristics of charged anisotropic compact stars by exploring some exact solutions of modified field equations in $f(R,G)$ gravity. A comprehensive analysis is performed from the obtained solutions regarding stability, energy conditions, regularity, sound velocity and compactness. These solutions can be referred to model the compact celestial entities. In particular, a compact star named, $Her X-1$ has been modeled which indicates that current solution fits and is in conformity to the observational data as well. A useful and interesting fact from this model arises that relative difference between two forces of anisotropic pressure and electromagnetic force may occur inside the aforementioned compact star. This is another mechanism which is essential for stability of the compact object and prevent stellar object to annihilate.

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arXiv2018

Modified Gauss-Bonnet Gravity with Radiating Fluids

The main purpose of this paper is to investigates structure scalars in the context of $f(\mathcal{G}, T)$ gravity, where $\mathcal{G}$ is the Gauss-Bonnet invariant and $T$ is the trace of stress energy tensor. For this aim, we have considered the spherically symmetric spacetime and dissipative anisotropic fluid coupled with radiation and heat ejecting shearing matter distributions. We have found these scalar variables by orthogonally decomposing the Riemann curvature tensor in $f(\mathcal{G}, T)$ gravity. Moreover, the evolution equations of shear and expansion are also developed with the help of these scalar functions. We have also analyzed these scalars by taking constant $\mathcal{G}$ and $T$ for dust cloud. The physical behavior of structure scalars for radiating matter distributions has been examined in the presence of modified gravity. It is shown that the evolutionary stages of relativistic stellar structures can be explored via modified scalar functions.

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arXiv2017

Anisotropic Universe in $f(\mathcal{G},\textit{T})$ Gravity

This paper is devoted to investigate the recently introduced $f(\mathcal{G},\textit{T})$ theory of gravity, where $\mathcal{G}$ is the Gauss-Bonnet term, and ${\textit{T}}$ is the trace of the energy-momentum tensor. For this purpose, anisotropic background is chosen and a power law $f(\mathcal{G},\textit{T})$ gravity model is used to find the exact solutions of field equations. In particular, a general solution is obtained which is further used to reconstruct some important solutions in cosmological contexts. The physical quantities like energy density, pressure, and equation of state parameter are calculated. A Starobinsky Like $f_2(\textit{T})$ model is proposed which is used to analyze the behavior of universe for different values of equation of state parameter. It is concluded that presence of term $\textit{T}$ in the bivariate function $f(\mathcal{G},\textit{T})$ may give many cosmologically important solutions of the field equations.

physics.gen-ph↗

arXiv2017

Emerging Anisotropic Compact Stars in $f(\mathcal{G},T)$ Gravity

The possible emergence of compact stars has been investigated in the recently introduced modified Gauss-Bonnet $f(\mathcal{G},T)$ gravity, where $\mathcal{G}$ is the Gauss-Bonnet term and ${T}$ is the trace of the energy-momentum tensor. Specifically, for this modified $f(\mathcal{G}, T)$ theory, the analytic solutions of Krori and Barua have been applied to anisotropic matter distribution. To determine the unknown constants appearing in Krori and Barua metric, the well-known three models of the compact stars namely 4U1820-30, Her X-I, and SAX J 1808.4-3658 have been used. The analysis of the physical behavior of the compact stars has been presented and the physical features like energy density and pressure, energy conditions, static equilibrium, stability, measure of anisotropy, and regularity of the compact stars, have been discussed.

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arXiv2017

Energy Bounds in $f(R,G)$ Gravity with Anisotropic Background

This paper investigates the energy bounds in modified Gauss-Bonnet gravity with anisotropic background. Locally rotationally symmetric Bianchi type ${I}$ cosmological model in $f(R,G)$ gravity is considered to meet this aim. Primarily, a general $f(R,G)$ model is used to develop the field equations. In this aspect, we investigate the viability of modified gravitational theory by studying the energy conditions. We take in account four $f(R,G)$ gravity models commonly discussed in the literature. We formulate the inequalities obtained by energy conditions and investigate the viability of the above mentioned models using the Hubble, deceleration, jerk and snap parameters. Graphical analysis shows that for first two $f(R,G)$ gravity models, NEC, WEC and SEC are satisfied under suitable values of anisotropy and model parameters involved. Moreover, SEC is violated for the third and fourth models which predicts the cosmic expansion.

physics.gen-ph↗

arXiv2017

Dissipative Cylindrical Collapse of a Charged Anisotropic Fluid in $f(R)$ Gravity

This paper is devoted to investigate the cylindrical collapse of an anisotropic fluid in $f(R)$ gravity. For this purpose, the viscous charged anisotropic fluid dissipating energy with heat flow and shear is assumed. We use the perturbation scheme to develop the dynamical equations for the variables that ultimately lead to the disturbance of the physical variables and the Starobinksy like $f(R)$ model chosen. The evolution of the matter variables is discussed with the help of these equations. It can be concluded that the range of dynamic instabilities depends on the field strength, density distribution, pressure and the curvature term of the $f(R)$ model. We find that our results of Newtonian and post-Newtonian regimes reduce asymptotically to general relativity solutions in the limiting case.

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arXiv2017

Some Exact Solutions in $f(\mathcal{G},T)$ Gravity via Noether Symmetries

This paper is devoted to investigate the recently proposed modified Gauss-Bonnet $f(\mathcal{G},T)$ gravity, with $\mathcal{G}$, the Gauss-Bonnet term, coupled with ${T}$, the trace of energy-momentum tensor. We have used the Noether symmetry methodology to discuss some cosmologically important $f(\mathcal{G},T)$ gravity models with anisotropic background. In particular, the Noether symmetry equations for modified $f(\mathcal{G},T)$ gravity are reported for locally rotationally symmetric Bianchi type $I$ universe. Explicitly, two models have been proposed to explore the exact solutions and the conserved quantities. It is concluded that the specific models of modified Gauss-Bonnet gravity may be used to reconstruct $Λ$CDM cosmology without involving any cosmological constant.

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