arXiv subjects

M. Sharif

Publications and source records attributed to M. Sharif.

At least 127 records · Page 7Linked to original sources

arXiv2021

Extended Black Hole Solutions in Self-interacting Brans-Dicke Theory

In this paper, we formulate black hole solutions through extended gravitational decoupling scheme in the framework of self-interacting Brans-Dicke theory. The addition of a new source in the matter distribution increases the degrees of freedom in the system of field equations. Transformations in radial as well as temporal metric functions split the system into two arrays. Each array includes the effects of only one source (either seed or additional). The seed source is assumed to be a vacuum and the corresponding system is specified through the Schwarzschild metric. In order to construct a suitable solution of the second system, constraints are applied on the metric potentials and energy-momentum tensor of the additional source. We obtain three solutions corresponding to different values of the decoupling parameter in the presence of a massive scalar field. The extra source is classified as normal or exotic through energy conditions. It is found that two solutions agree with the energy bounds and thus have normal matter as their source.

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arXiv2021

Noether Symmetry Approach in Energy-Momentum Squared Gravity

In this paper, we investigate the newly developed $f(R,\mathbf{T}^2)$ theory ($R$ is the Ricci scalar and $\mathbf{T}^2=T_{αβ}T^{αβ},~T _{αβ}$ demonstrates the energy-momentum tensor) to explore some viable cosmological models. For this purpose, we use the Noether symmetry approach in the context of flat Friedmann-Robertson-Walker (FRW) universe. We solve the Noether equations of this modified theory for two types of models and obtain the symmetry generators as well as corresponding conserved quantities. We also evaluate exact solutions and investigate their physical behavior via different cosmological parameters. For the prospective models, the graphical behavior of these parameters indicate consistency with recent observations representing accelerated expansion of the universe. In the first case, we take a special model of this theory and obtain new class of exact solutions with the help of conserved quantities. Secondly, we consider minimal and non-minimal coupling models of $f(R,\mathbf{T} ^{2})$ gravity. We conclude that conserved quantities are very useful to derive the exact solutions that are used to study the cosmic accelerated expansion.

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arXiv2021

Stability of Charged Thin-Shell Gravastars with Quintessence

This paper develops a new solution of gravitational vacuum star in the background of charged Kiselev black holes as an exterior manifold. We explore physical features and stability of thin-shell gravastars with radial perturbation. The matter thin layer located at thin-shell greatly affects stable configuration of the developed structure. We assume three different choices of matter distribution such as barotropic, generalized Chaplygin gas and generalized phantomlike equation of state. The last two models depend on the shell radius, also known as variable equation of state. For barotropic model, the structure of thin-shell gravastar is mostly unstable while it shows stable configuration for such type of matter distribution with extraordinary quintessence parameter. The resulting gravastar structure indicates stable behavior for generalized Chaplygin gas but unstable for generalized phantomlike model. It is also found that proper length, entropy and energy within the shell show linear relation with thickness of the shell.

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arXiv2021

Complexity of Dynamical Sphere in Self-interacting Brans-Dicke Gravity

This paper aims to derive a definition of complexity for a dynamic spherical system in the background of self-interacting Brans-Dicke gravity. We measure complexity of the structure in terms of inhomogeneous energy density, anisotropic pressure and massive scalar field. For this purpose, we formulate structure scalars by orthogonally splitting the Riemann tensor. We show that self-gravitating models collapsing homologously follow the simplest mode of evolution. Furthermore, we demonstrate the effect of scalar field on the complexity and evolution of non-dissipative as well as dissipative systems. The criteria under which the system deviates from the initial state of zero complexity is also discussed. It is concluded that complexity of the sphere increases in self-interacting Brans-Dicke gravity because the homologous model is not shear-free.

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arXiv2020

Impact of Kuchowicz Metric Function on Gravastars in f(R,T) Theory

This paper discusses the configuration of gravitational vacuum star or gravastar with the impact of geometry and matter coupling present in $f(R,T)$ gravity. The gravastar is also conceptualized as a substitute for a black hole which is illustrated by three geometries known as (1) the interior geometry, (2) the intermediate thin-shell and (3) the exterior geometry. For a particular $f(R,T)$ model, we analyze these geometries corresponding to Kuchowicz metric function. We evaluate another metric potential for the interior domain as well as the intermediate shell which is non-singular for both domains. The Schwarzschild metric is adopted to demonstrate the exterior geometry of gravastar, while the numerical values of unknown constants are calculated through boundary conditions. Finally, we discuss different features of gravastar regions like proper length, energy, surface redshift as well as equation of state parameter. We conclude that the gravastar model can be regarded as a successful replacement of the black hole in the context of this gravity.

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arXiv2020

Anisotropic Compact Stellar Objects in Modified Gauss-Bonnet Gravity

This paper is devoted to studying anisotropic compact stellar structures by adopting embedding class-1 technique in the background of modified Gauss-Bonnet gravity. The unknown constants are evaluated by the matching of interior spacetime with the Schwarzschild exterior geometry corresponding to $f(\mathcal{G})=χ\mathcal{G}^{n}$ model, where $χ$ and $n$ are positive constants. The observed masses of compact star candidates (SAX J1808.4-3658, Vela X-1, PSR J0348+0432, 4U 1608-52) are used with the condition of vanishing radial pressure at the stellar surface to predict their radii. We have examined viability and stability of the resulting solution through graphical behavior of matter variables, energy constraints, adiabatic index and causality condition. It is found that embedding class-1 solution for anisotropic compact stars is viable and stable in this theory.

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arXiv2020

Phase Transition and Thermal Fluctuations of Quintessential Kerr-Newman-AdS Black Hole

This paper is devoted to analyzing the critical phenomenon and phase transition of quintessential Kerr-Newman-anti-de Sitter black hole in the framework of Maxwell equal-area law. For this purpose, we first derive thermodynamic quantities such as Hawking temperature, entropy and angular momentum in the context of extended phase space. These quantities satisfy Smarr-Gibbs-Dehum relation in the presence of quintessence matter. We then discuss the critical behavior of thermodynamic quantities through two approaches, i.e., van der Waals-like equation of state and Maxwell equal-area law. It is found that the latter approach is more effective to analyze the critical behavior of the complicated black holes. Using equal-area law, we also study phase diagram in $T-S$ plane and find an isobar which shows the coexistence region of two phases. We conclude that below the critical temperature, black holes show a similar phase transition as that of van der Waals fluid. Finally, we study the effects of thermal fluctuations on the stability of this black hole.

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arXiv2020

Static Wormhole Solutions and Noether Symmetry in Modified Gauss-Bonnet Gravity

In this paper, we analyze static traversable wormholes via Noether symmetry technique in modified Gauss-Bonnet $f(\mathcal{G})$ theory of gravity (where $\mathcal{G}$ represents Gauss-Bonnet term). We assume isotropic matter configuration and spherically symmetric metric. We construct three $f(\mathcal{G})$ models, i.e, linear, quadratic and exponential forms and examine the consistency of these models. The traversable nature of wormhole solutions is discussed via null energy bound of the effective stress-energy tensor while physical behavior is studied through standard energy bounds of isotropic fluid. We also discuss the stability of these wormholes inside the wormhole throat and conclude the presence of traversable and physically stable wormholes for quadratic as well as exponential $f(\mathcal{G})$ models.

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arXiv2020

Anisotropic Strange Stars through Embedding Technique in Massive Brans-Dicke Gravity

This paper investigates the existence and properties of anisotropic strange quark stars in the context of massive Brans-Dicke theory. The field equations are constructed in Jordan frame by assuming a suitable potential function with MIT bag model. We employ the embedding class-one approach as well as junction conditions to determine the unknown metric functions. Radius of the strange star candidate, LMC X-4, is predicted through its observed mass for different values of the bag constant. We analyze the effects of coupling parameter as well as mass of scalar field on state determinants and execute multiple checks on the stability and viability of the spherical system. It is concluded that the resulting stellar structure is physically viable and stable as it satisfies the energy conditions as well as essential stability criteria.

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arXiv2020

Extended Gravitational Decoupled Solutions in Self-interacting Brans-Dicke Theory

In this paper, we construct anisotropic spherical solutions from known isotropic solutions through extended gravitational decoupling method in the background of self-interacting Brans-Dicke theory. The field equations are decoupled into two sets by applying geometric deformations on radial as well as temporal metric components. The first array corresponds to isotropic fluid while influence of the anisotropic source is confined to the second set. The isotropic sector is determined through metric functions of isotropic solutions (Tolman IV/Krori-Barua) whereas two constraints on the anisotropic source are required to close the second system. The impact of the massive scalar field as well as the decoupling parameter on the physical characteristics of the anisotropic solutions is analyzed graphically. We also check the viability, compactness, surface redshift and stability of the obtained solutions. It is found that the resulting solutions follow accepted physical trend for some values of the decoupling parameter.

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arXiv2020

Quasi-normal Modes and Thermal Fluctuations of Charged Black Hole with Weyl Corrections

In this paper, we study thermodynamics, quasi-normal modes and thermal fluctuations of a charged black hole with Weyl corrections. We first obtain thermodynamic quantities such as Hawking temperature, entropy, and heat capacity for non-rotating as well as rotating versions of this black hole. We also evaluate temperature through quantum tunneling mechanism which is exactly the same as found through surface gravity. We then discuss the relation between Davies's point and quasi-normal modes. Finally, we investigate the effects of thermal fluctuations on the uncorrected thermodynamic quantities. It is concluded that the logarithmic corrections originated from thermal fluctuations make the system more unstable for small BHs.

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arXiv2020

Anisotropic Spherical Solutions through Extended Gravitational Decoupling Approach

This paper is devoted to evaluating exact anisotropic spherical solutions for static self-gravitating systems through extended geometric deformation decoupling technique. For this purpose, we consider an isotropic Tolman IV solution and extend it to anisotropic domain by transforming both temporal as well as radial metric potentials. To examine the physical viability and stability of interior anisotropic solutions, we plot energy bounds, TOV equation, causality condition and adiabatic index for the stars Her X-I and PSR J 1416-2230. It is found that both obtained models show realistic behavior as they fulfill all physical constraints as well as stability criterion. We conclude that the extended gravitational decoupling approach provides more proficient results to discuss the interior configuration of stellar structures.

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arXiv2020

Stability of Gravastars with Exterior Regular Black Holes

This paper examines the stability of thin-shell gravastars in the context of regular spacetimes (Bardeen and Bardeen-de Sitter black holes). We apply cut and paste approach to construct gravastars through the matching of interior non-singular de Sitter geometry with exterior regular black hole. This model contains three regions, i.e., interior, thin-shell and exterior. The interior and exterior regions are connected at thin-shell. We investigate physical viability of the developed model by the energy conditions and explore its stability by using radial perturbation about the equilibrium shell radius. It is found that thin-shell gravastars show large stable regions for the Bardeen-de Sitter black hole as compared to the Bardeen black hole. It is concluded that stable regions exist near the formation of expected event horizon.

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arXiv2020

Inhomogeneous Perturbations and Stability Analysis of the Einstein Static Universe in f(R,T) Gravity

The purpose of this paper is to analyze the existence of static stable Einstein universe using inhomogeneous linear perturbations in the context of $f(R,T)$ gravity ($R$ and $T$ denote the scalar curvature and trace of the stress-energy tensor, respectively). The static and perturbed field equations are constructed for perfect fluid parameterized by linear equation of state parameter. We obtain solutions manifesting the Einstein static state by considering peculiar $f(R,T)$ forms for vanishing and non-vanishing conservation of the stress-energy tensor. It is observed that stable static Einstein regions exist for both closed as well as open FLRW universe models for an appropriate choice of parameters. We conclude that this theory is efficient for presenting such cosmological solutions leading to emergent universe scenario.

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arXiv2020

Greybody Factor for Quintessential Kerr-Newman Black Hole

In this paper, we formulate an analytic expression of the greybody factor for the Kerr-Newman black hole in the presence of the quintessential field. Primarily, we analyze the profile of effective potential by transforming the radial equation of motion into standard Schr$\ddot{o}$dinger form through tortoise coordinate. The two asymptotic solutions, in the form of hypergeometric functions, are computed at distinct radial regions such as a black hole and cosmological horizons determined by the quintessence. To extend the viability over the whole radial regime, we match the analytical solutions smoothly in an intermediate region by using a semi-classical approach. We also calculate the emission rates and absorption cross-section for the massless scalar fields to elaborate on the significance of our result. It is found that the electromagnetic force together with the gravitational pull of the black hole maximizes the effective potential and consequently, decreases the emission process of scalar field particles.

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arXiv2019

Role of Curvature-Matter Coupling on Anisotropic Strange Stars

In this paper, we study the physical characteristics of anisotropic spherically symmetric quark star candidates for $R+2σT$ gravity model, where $R$, $σ$ and $T$ depict scalar curvature, coupling parameter, and the trace of the energy-momentum tensor, respectively. In order to analyze the structure formation of quark stars, we consider the Heintzmann solution and assume that strange quark matter is characterized by MIT bag model equation of state. We evaluate the unknown parameters through matching conditions and obtain the values of radii of strange quark stars using modified Tolman-Oppenheimer-Volkoff equation with observed values of masses and bag constant. The feasibility of our considered solution is analyzed by graphical analysis of matter variables, energy bounds, causality condition and adiabatic index. It is found that the strange quark stars show stable structure corresponding to Heintzmann solution and their physical viability enhances with increasing values of the model parameter $σ$.

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arXiv2019

Charged Gravastars with Conformal Motion in f(R,T) Gravity

This paper studies the effects of charge on a peculiar stellar object, recognized as gravastar, under the influence of $f(R,T)$ gravity by considering the conjecture of Mazur and Mottola in general relativity. The gravastar is also known as an alternative to a black hole and is expressed by three distinct domains named as (i) the interior domain, (ii) the intermediate shell and (iii) the exterior domain. We analyze these domains for a specific $f(R,T)$ gravity model conceding the conformal Killing vectors. In the interior domain, we assume that pressure is equal to negative energy density which leads to the existence of repulsive force on the spherical shell. The intermediate shell consists of ultra-relativistic plasma and pressure which shows a direct relation with energy density and counterbalances the repulsive force applied by the interior domain. The exterior vacuum spherical domain is taken to be the de Sitter spacetime illustrated by the Reissner-Nordström metric. We conclude that non-singular solutions of charged gravastar with various physical properties such as length, energy, entropy and equation of state parameter are physically consistent.

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arXiv2019

Complexity Factor for Static Sphere in Self-interacting Brans-Dicke Gravity

In this paper, we study the complexity factor of a static anisotropic sphere in the context of self-interacting Brans-Dicke theory. We split the Riemann tensor using Bel's approach to obtain structure scalars relating to comoving congruence and Tolman mass in the presence of a scalar field. We then define the complexity factor with the help of these scalars to demonstrate the complex nature of the system. We also evaluate the vanishing complexity condition to obtain solutions for two stellar models. It is concluded that the complexity of the system increases with the inclusion of the scalar field and potential function.

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