arXiv subjects

Tayyab Naseer

Publications and source records attributed to Tayyab Naseer.

At least 19 recordsLinked to original sources

arXiv2025

Estimating the Role of Bag Constant and Modified Theory on Anisotropic Stellar Models

In this article, we are devoted to discuss different compact stars admitting anisotropic interiors in a particular modified theory of gravity. For this purpose, a spherically symmetric metric is adopted to formulate the field equations corresponding to two different $f(\mathcal{R},\mathcal{T},\mathcal{Q})$ models, where $\mathcal{Q}=\mathcal{R}_{αγ}\mathcal{T}^{αγ}$. Since the field equations contain extra degrees of freedom, we choose Finch-Skea metric and MIT bag model equation of state to make them solvable. We also use matching conditions to calculate a constant triplet in the chosen ansatz. The resulting solutions are then graphically analyzed for particular values of the bag constant and model parameter in the interior of 4U 1820-30 compact star. The viability and stability of the modified models are also checked through certain tests. Further, we calculate the values of model parameter through the vanishing radial pressure constraint that correspond to the observed data (radii and masses) of eight different star candidates. Finally, we conclude that our models I and II are in well-agreement with the conditions needed for physically relevant interiors to exist.

gr-qc↗

arXiv2025

Magnetized particle motion and accretion process with shock cone morphology around a decoupled hairy black holes

Relativistic accretion onto compact objects such as black holes and neutron stars is one of the most efficient known mechanisms for converting gravitational potential energy into radiation. In the case of rapidly spinning black holes, up to $40\%$ of the rest-mass energy of accreting matter can be released, far exceeding the efficiency of nuclear fusion. In this work, we investigate magnetized particle motion and relativistic accretion processes around a decoupled hairy black hole via extended geometric deformation. The developed geometry involves two hairy parameters that preserve the horizon structure with the additional feature of the fulfillment of weak energy conditions outside the event horizon. We provide the foundation with necessary formalism for magnetized particle motion around a decoupled black hole. The effective potential and innermost stable circular orbits are then derived, which demonstrate a significant reduction of the radius of the latter quantity under the hairy parameters for the magnetized particle. Afterwards, we obtain exact analytical expressions for radial velocity profiles, mass accretion rates, and a few others which reveal improved energy efficiency and emissivity as compared to the standard black hole. Furthermore, the decoupling parameter shows strong influence on oscillations, accretion presenting fantastic agreement between analytical predictions and numerical simulations, and thus offering noticeable observational signatures for future gravitational wave and X-ray astronomy.

astro-ph.HE↗

arXiv2025

Physical Existence of Relativistic Stellar Models within the context of Anisotropic Matter Distribution

Two distinct non-singular interior models that describe anisotropic spherical configurations are presented in this work. We develop the Einstein field equations and the associated mass function in accordance with a static spherical spacetime. We then discuss certain requirements that must be satisfied for compact models to be physically validated. Two distinct limitations are taken into account to solve the field equations, including different forms of the radial geometric component and anisotropy, which ultimately leads to a couple of relativistic models. In both cases, solving the differential equations result in the appearance of integration constants. By equating the Schwarzschild exterior metric and spherical interior line element on the interface, these constants are explicitly obtained. The disappearance of the radial pressure on the hypersurface is also used in this context. We further use estimated radii and masses of six different stars to graphically visualize the physical properties of new solutions. Both of our models are deduced to be well-aligned with all physical requirements, indicating the superiority of the presence of anisotropy in compact stellar interiors over the perfect isotropic fluid content.

gr-qc↗

arXiv2025

Modeling Gravastar Structure admitting Kuchowicz Spacetime in Rastall Gravity Theory

This paper investigates the gravastar model as a potential alternative to black holes, utilizing the Kuchowicz metric in the context of Rastall gravity. The model comprises three distinct regions: an interior with positive energy density and negative pressure, a thin intermediate shell made of ultra-relativistic stiff fluid, and an exterior vacuum. The negative pressure within the interior generates an outward force exerted on the shell, fulfilling the Zel'dovich criterion. This configuration eliminates the central singularity and replaces the event horizon with the shell. We then derive the radial metric functions for both the inner and thin region, yielding a non-singular solution. Furthermore, we examine the physical properties of the shell, such as its energy, proper length, entropy, equation of state parameter, gravitational redshift and adiabatic index, across a range of Rastall parameter values. We conclude that the resulting gravastar model offers a promising solution to the singularity problem of conventional black holes within the context of this non-conservative theory.

gr-qc↗

arXiv2025

Role of Non-conserved Gravity Theory and Electric Charge in Constructing Complexity-free Stellar Models: A Novel Approach under Non-minimal Coupling

This study explores the application of complexity factor within the context of Rastall gravity, exploring its implications on a static spacetime admitting spherical symmetry associated with anisotropic fluids under an electromagnetic field. The field equations are derived for a static charged sphere that provides a foundational framework for analyzing gravitational effects in this non-conserved theory. The mass function is formulated by incorporating both fluid and geometric parameters, offering insights into how mass distribution affects spacetime curvature. Through orthogonal decomposition of the Riemann tensor, a set of scalar quantities is obtained, referred to the structure scalars, which serve as indicators of celestial complexity. One specific scalar is then specified as the complexity factor, i.e., $\mathbb{Y}_{TF}$, facilitating further analysis on its role in characterizing complex systems. The presence of unknowns in gravitational equations necessitates the imposition of constraints to facilitate their solution. To address this, $\mathbb{Y}_{TF}=0$ alongside three distinct conditions are employed which yield diverse stellar models. A comprehensive graphical analysis is conducted using multiple values of the Rastall and charge parameters. Notably, the findings of this study align with those predicted by Einstein's theory. More appealingly, the Rastall theory demonstrates its superiority in the presence of charge under model 2 when it is compared with the general theory of relativity.

gr-qc↗

arXiv2025

Implications of Rastall Theory on Stellar Solutions admitting Vanishing Complexity: A New Perspective

In this paper, the notion of complexity factor and its implication is extended to the framework of non-conserved Rastall theory of gravity. First of all, the field equations governing a static spherical geometry associated with the anisotropic fluid are formulated. The mass function corresponding to the considered geometry is defined in terms of both matter and geometric quantities. The orthogonal decomposition of the Riemann tensor is then performed through which a family of scalar quantities, known as structure scalars, is obtained. Using the Herrera's recent definition, one of the scalars among them is claimed as the complexity factor, \emph{i.e.}, $\mathcal{Y}_{TF}$. Since there are extra degrees of freedom in the gravitational equations, some constraints are needed to make their solution possible to obtain. In this regard, a well-known vanishing complexity condition is introduced along with three different constraints which ultimately lead to distinct stellar models. In order to check their physical feasibility, a detailed graphical interpretation is provided using multiple values of the Rastall parameter. It is concluded that the obtained results in all three cases are consistent with those of general relativity. Further, the Rastall theory provides more suitable results in the case of model 2, indicating its superiority over Einstein's gravity theory.

gr-qc↗

arXiv2025

Orthogonal splitting of the Riemann curvature tensor and its implications in modeling compact stellar structures

Although the interpretation of complexity in extended theories of gravity is available in the literature, its illustration in $f(R,L_{m},\mathcal{T})$ theory is still ambiguous. The orthogonal decomposition of the Riemann tensor results in the emergence of complexity factor as recently proposed by Herrera [1]. We initiate the analysis by contemplating the interior spacetime as a static spherical anisotropic composition under the presence of charge. The modified field equations are derived along with the establishment of association between the curvature and conformal tensors that have significant relevance in evaluating complexity of the system. Furthermore, the generalized expressions for two different masses are calculated, and their link with conformal tensor is also analyzed. Moreover, we develop a particular relation between predetermined quantities and evaluate the complexity in terms of a certain scalar $Y_{TF}$. Several interior solutions admitting vanishing complexity are also determined. Interestingly, compact objects having anisotropic matter configuration along with the energy density inhomogeneity possess maximum complexity. It is concluded that the spherical distribution of matter might not manifest complexity or admitting minimal value of this factor in the framework of $f(R,L_{m},\mathcal{T})$ theory due to the appearance of dark source terms.

gr-qc↗

arXiv2025

Interpretation of complexity for spherically symmetric fluid composition within the context of modified gravity theory

Regardless of the adequate descriptions of complexity in distinct alternative gravity theories, its elaboration in the framework of $f(R,\mathcal{L}_{m},\mathcal{T})$ theory remains uncertain. The orthogonal splitting of the curvature tensor yields the complexity factor as suggested by Herrera \cite {herrera2018new}. To commence our study, the inner spacetime is assumed to be spherically symmetric static composition comprised of the anisotropic fluid. In this context, we derive the modified field equations for the considered theory and take into account the established relationship between the conformal and curvature tensors to interpret the complexity. Furthermore, we determine the correspondence of the mass functions with the complexity factor, represented by a specific scalar $Y_{TF}$. Certain solutions complying with the precedent of diminishing $Y_{TF}$ are also evaluated. It is noted that celestial formations having anisotropic and non-uniform compositions of matter assert the utmost complexity. Nevertheless, the spherically symmetric matter distribution may not exhibit complexity in the scenario of vanishing impacts of non-homogenous energy density and anisotropic pressure due to the presence of dark source terms associated with this extended gravity theory.

gr-qc↗

arXiv2025

Non-singular anisotropic solutions for strange star model in $f(\mathcal{R},\mathcal{T},\mathcal{R}_{ζγ}\mathcal{T}^{ζγ})$ gravity theory

This article focuses on different anisotropic models within the framework of a specific modified $f(\mathcal{R},\mathcal{T},\mathcal{R}_{ζγ}\mathcal{T}^{ζγ})$ gravity theory. The study adopts a static spherically symmetric spacetime to determine the field equations for two different modified models: (i) $f(\mathcal{R},\mathcal{T},\mathcal{R}_{ζγ}\mathcal{T}^{ζγ})=\mathcal{R}+η\mathcal{R}_{ζγ}\mathcal{T}^{ζγ}$, and (ii) $f(\mathcal{R},\mathcal{T},\mathcal{R}_{ζγ}\mathcal{T}^{ζγ})=\mathcal{R}(1+η\mathcal{R}_{ζγ}\mathcal{T}^{ζγ})$, where $η$ is a constant parameter. To address the additional degrees of freedom in the field equations and obtain their corresponding unique solution, the Durgapal-Fuloria spacetime geometry and MIT bag model are utilized. Matching conditions are applied to determine unknown constants within the chosen spacetime geometry. We adopt a certain range of model parameters to analyze the physical characteristics of the developed models in the interior distribution of a particular compact star candidate 4U 1820-30. Energy conditions and some other tests are also implemented to ensure their viability and stability. Additionally, the disappearing radial pressure constraint is employed to find the values of the model parameter, aligning with the observed information of an array of stars. The study concludes that both of our models are well-behaved and satisfy all necessary conditions, and thus we observe them suitable for the modeling of astrophysical objects.

gr-qc↗

arXiv2025

Possible Existence of Ghost Stars in the context of Electromagnetic Field

In this paper, we discuss the existence of ghost star models in the Einstein-Maxwell framework. In order to explore these objects, we put forward the idea of Zeldovich and Novikov by keeping in mind that the energy density of such models lie in the negative range in some regions of the spacetime geometry. We proceed by taking into account a static sphere and develop the field equations for a charged anisotropic fluid configuration. The two generating functions are then considered and we rewrite the field equations in terms of the mass and these physical quantities. Afterwards, we formulate two different models using the conformally flatness condition along with the considered generating functions. Further, we adopt the vanishing complexity constraint as well as null active gravitational mass to find two more solutions. The energy density for all developed models is also graphically shown. We conclude that the ghost stars exist in the presence of charge as the energy density for all the resulting solutions lie in the negative region for a particular range of the radial coordinate.

gr-qc↗

arXiv2025

Physical validity of anisotropic models derived from isotropic fluid dynamics in $f(R,T)$ theory: An implication of gravitational decoupling

In this paper, we derive multiple anisotropic analogs from the established isotropic model by means of the gravitational decoupling approach in a fluid-geometry interaction based theory. To accomplish this, we initially consider a static spherical perfect-fluid configuration and then introduce a new matter source to induce anisotropic behavior in the system. The resulting field equations encapsulate the entire matter distribution and thus become much complicated. We then split these equations into two sets through implementing a particular transformation, each set delineating characteristics attributed to their original fluid sources. We adopt the Heintzmann's ansatz and some constraints on extra gravitating source to deal with the first and second systems of equations, respectively. Furthermore, the two fundamental forms of the matching criteria are used to make the constant in the considered solution known. By utilizing the preliminary information of a star candidate LMC X-4, we assess the physical validity of the developed models. Our analysis indicates that both our models exhibit characteristics which are well-agreed with the acceptability criteria for certain parametric values.

gr-qc↗

arXiv2025

Complexity and Isotropization based Extended Models in the context of Electromagnetic Field: An Implication of Minimal Gravitational Decoupling

This paper formulates three different analytical solutions to the gravitational field equations in the framework of Rastall theory by taking into account the gravitational decoupling approach. For this, the anisotropic spherical interior fluid distribution is assumed as a seed source characterized by the corresponding Lagrangian. The field equations are then modified by introducing an additional source which is gravitationally coupled with the former fluid setup. Since this approach makes the Rastall equations more complex, the MGD scheme is used to tackle this, dividing these equations into two systems. Some particular ansatz are taken into account to solve the first system, describing initial anisotropic fluid. These metric potentials contain multiple constants which are determined with the help of boundary conditions. On the other hand, the solution for the second set is calculated through different well-known constraints. Afterwards, the estimated data of a pulsar $4U 1820-30$ is considered so that the feasibility of the developed models can be checked graphically. It is concluded that all resulting models show physically acceptable behavior under certain choices of Rastall and decoupling parameters.

gr-qc↗

arXiv2024

Extending Finch-Skea Isotropic Model to Anisotropic Domain in Modified $f(\mathcal{R},\mathcal{T})$ Gravity

This paper considers the Finch-Skea isotropic solution and extends its domain to three different anisotropic interiors by using the gravitational decoupling strategy in the context of $f(\mathcal{R},\mathcal{T})$ gravitational theory. For this, we consider that a static spherical spacetime is initially coupled with the perfect matter distribution. We then introduce a Lagrangian corresponding to a new gravitating source by keeping in mind that this new source produces the effect of pressure anisotropy in the parent fluid source. After calculating the field equations for the total matter setup, we apply a transformation on the radial component, ultimately providing two different systems of equations. These two sets are solved independently through different constraints that lead to some new solutions. Further, we consider an exterior spacetime to calculate three constants engaged in the seed Finch-Skea solution at the spherical interface. The estimated radius and mass of a star candidate LMC X-4 are utilized to perform the graphical analysis of the developed models. It is concluded that only the first two resulting models are physically relevant in this modified theory for all the considered parametric choices.

gr-qc↗

arXiv2024

Applicability of Modified Gauss-Bonnet Gravity Models on the Existence of Stellar Structures

In this paper, we explore the existence of spherically symmetric strange quark configurations coupled with anisotropic fluid setup in the framework of modified Gauss-Bonnet theory. In this regard, we adopt two models such as \emph{(i)} $f(\mathcal{G})=β\mathcal{G}^2$, and \emph{(ii)} $f(\mathcal{G})=δ_{1}\mathcal{G}^{x}(δ_{2}\mathcal{G}^{y}+1)$, and derive the field equations representing a static sphere. We then introduce bag constant in the gravitational equations through the use of MIT bag model, so that the quarks' interior can be discussed. Further, we work out the modified equations under the use of Tolman IV ansatz to make their solution possible. Junction conditions are also employed to find the constants involved in the considered metric potentials. Afterwards, different values of model parameters and bag constant are taken into account to graphically exploring the resulting solutions. This analysis is done by considering five strange quark objects like Her X-I, LMC X-4, 4U 1820-30, PSR J 1614-2230, and Vela X-I. Certain tests are also applied on the developed models to check their physical feasibility. It is much interesting that this modified gravity under its both considered functional forms yield physically viable and stable results for certain parametric values.

gr-qc↗

arXiv2024

Anisotropic Stellar Models with Tolman IV Spacetime in Non-minimally Coupled Theory

This article aims to investigate various anisotropic stellar models in the background of $f(\mathcal{R},\mathcal{T},\mathcal{Q})$ gravity, where $\mathcal{Q}=\mathcal{R}_{φ\vartheta}\mathcal{T}^{φ\vartheta}$. In this regard, we adopt two standard models as $\mathcal{R}+ζ\mathcal{Q}$ and $\mathcal{R}+ζ\mathcal{R}\mathcal{Q}$, where $ζ$ symbolizes an arbitrary coupling parameter. We take spherical interior geometry and find solution to the modified gravitational field equations corresponding to each model by employing the `Tolman IV' spacetime. We need an additional constraint to close the system of field equations, thus the $\mathbb{MIT}$ bag model equation of state is chosen. The effects of modified theory on physical properties of six compact stars like PSR J 1614 2230,~SMC X-1,~Cen X-3,~PSR J 1903+327,~SAX J 1808.4-3658 and 4U 1820-30 are analyzed by using their respective masses and radii. We also determine the values of three unknowns involving in Tolman IV solution as well as the bag constant for each star at the hypersurface. Furthermore, various characteristics of the resulting solutions are examined through graphical interpretation for $ζ=\pm5$. Finally, we explore the stability of the compact objects through two different approaches. We conclude that our model-I produces physically acceptable structures corresponding to each star candidate for both values of $ζ$ whereas model-II is stable only for $ζ=5$.

gr-qc↗

arXiv2024

Existence of Non-singular Stellar Solutions within the context of Electromagnetic Field: A Comparison between Minimal and Non-minimal Gravity Models

In this paper, we explore the existence of various non-singular compact stellar solutions influenced by the Maxwell field within the matter-geometry coupling based modified gravity. We start this analysis by considering a static spherically symmetric spacetime which is associated with the isotropic matter distribution. We then determine the field equations corresponding to two specific functions of this modified theory. Along with these models, we also adopt different forms of the matter Lagrangian. We observe several unknowns in these equations such as the metric potentials, charge and fluid parameters. Thus, the embedding class-one condition and a particular realistic equation of state is used to construct their corresponding solutions. The former condition provides the metric components possessing three constants, and we calculate them through junction conditions. Further, four developed models are graphically analyzed under different parametric values. Finally, we find all our developed solutions well-agreeing with the physical requirements, offering valuable insights for future explorations of the stellar compositions in this theory.

gr-qc↗

arXiv2024

Decoupled Anisotropic Buchdahl's Relativistic Models in $f(\mathbb{R},\mathbb{T})$ Theory

This paper constructs three different anisotropic extensions of the existing isotropic solution to the modified field equations through the gravitational decoupling in $f(\mathbb{R},\mathbb{T})$ theory. For this, we take a static sphere that is initially filled with the isotropic fluid and then add a new gravitational source producing anisotropy in the system. The field equations now correspond to the total matter configuration. We transform the radial metric component to split these equations into two sets characterizing their parent sources. The unknowns comprising in the first set are determined by considering the Buchdahl isotropic solution. On the other hand, we employ different constraints related to the additional gravitational source and make the second system solvable. Further, the constant triplet in Buchdahl solution is calculated by means of matching criteria between the interior and exterior geometries at the spherical boundary. The mass and radius of a compact star LMC X-4 are used to analyze the physical relevancy of the developed models. We conclude that our resulting models II and III are in well-agreement with acceptability conditions for the considered values of the parameters.

gr-qc↗

arXiv2024

Anisotropic Durgapal-Fuloria Neutron Stars in $f(\mathcal{R},\mathrm{T}^{2})$ Gravity

The main purpose of this paper is to obtain physically stable stellar models coupled with anisotropic matter distribution in the context of $f(\mathcal{R},\mathrm{T}^{2})$ theory. For this, we consider a static spherical geometry and formulate modified field equations containing various unknowns such as matter determinants and metric potentials. We then obtain a unique solution to these equations by employing Durgapal-Fuloria ansatz possessing a constant doublet. We also use matching criteria to calculate the values of these constants by considering the Schwarzschild exterior spacetime. Two different viable models of this modified theory are adopted to analyze the behavior of effective matter variables, anisotropy, energy conditions, compactness and redshift in the interiors of Her X-1, PSR J0348-0432, LMC X-4, SMC X-1, Cen X-3, and SAX J 1808.4-3658 star candidates. We also check the stability of these models by using three different physical tests. It is concluded that our considered stars satisfy all the physical requirements and are stable in this modified gravity for the considered parametric values.

gr-qc↗