arXiv subjects

M. Sharif

Publications and source records attributed to M. Sharif.

At least 19 recordsLinked to original sources

arXiv2026

A Non-Singular Cosmic Bounce in $\mathcal{F}(Q)$ Gravity: A Reconstruction and Phase Space Analysis

The key focus of this research work is the analysis of a non-singular cosmic bounce in the context of $\mathcal{F}(Q)$ gravity, with $Q$ representing the non-metricity scalar. We consider the gravitational Lagrangian $\mathcal{F}(Q)=Q+\psi Q^n$ in the presence of a modified Chaplygin-type matter source with a flat Friedmann-Robertson-Walker spacetime and a perfect matter distribution. In order to proceed, we adopt two approaches: a reconstruction approach using scale-factor ansatz and analysis of a two-dimensional autonomous dynamical system. Our results imply that the geometry coupling parameter $\psi$ plays an important role and causes geometrical repulsion for violating the null energy condition. A numerical scan of the $(\rho_0,\psi)$ parameter space suggests that there is a critical energy density for the bounce to take place, below which the universe evolves towards the standard singular cosmological solution. Moreover, the effective equation of state slowly evolves towards the de Sitter value ($\omega_{eff}\rightarrow -1$) and the squared sound speed is bounded by the stability and causality condition ($0\leq C_s^2\leq1$). It is found that $\mathcal{F}(Q)$ gravity provides a coherent geometric picture of the non-singular bouncing universe in accordance with the cosmic accelerated expansion.

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arXiv2026

Bulk Viscosity and Thermodynamic Approaches to Singularity Resolution in Non-Metric Gravity

This manuscript investigates the impact of bulk viscosity on the viability of cosmic bounce solutions in the context of $f(\mathcal{Q})$ theory, where $\mathcal{Q}$ is a non-metricity scalar. To complete this objective, we study the behavior of an isotropic homogeneous universe filled with a perfect fluid and consider a innovative parametrization of bulk viscosity coefficient with an arbitrary constant $\zeta_0$ as $\zeta=\zeta_{0}H$. We consider the particular functional form of this modified theory to explore how this gravitational framework influences the cosmic evolution and explore a range of cosmological parameters to assess the existence and physical viability of bounce solutions. We also investigate the evolution of entropy through the second law of thermodynamics. The positive trend of energy density, negative pressure profile and violation of energy conditions support the existence of viable cosmological bounce scenario and highlights the significance of bulk viscosity in this framework. These results demonstrate that $f(\mathcal{Q})$ gravity provides a compelling alternative to standard cosmic models and provides deep insights into the nature of gravitational interaction and the early cosmos.

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arXiv2026

Stability and Physical Properties of Compact Stars Beyond Einstein Gravity

This manuscript discusses feasible features of anisotropic celestial sphere within the framework of $f(\mathbb{Q},\mathcal{L}_{m})$ gravity, where $\mathbb{Q}$ represents non-metricity scalar and $\mathcal{L}_{m}$ is the matter Lagrangian. The geometric configuration of static spherical symmetric structure is examined using a specific non-singular solution (Krori-Barua solution). A particular model of this theory is considered to derive explicit field equations. The Darmois matching conditions are used to evaluate unknown constants in the metric coefficients. To verify plausible existence of compact objects in this gravitational framework, we analyze their fundamental physical properties including fluid parameters, gradients, surface redshift, mass-radius relation, anisotropy measure, compactness factor, energy conditions and equations of state. The stability of the considered stellar objects is verified by adiabatic index and sound speed. Our results demonstrate that all required physical conditions are satisfied, confirming the existence of physically stable anisotropic celestial objects within this modified gravity.

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arXiv2026

Feasible Stellar Interiors Beyond Einstein Gravity: Insights from Non-Metricity-Matter Coupled Gravitational Theory

This manuscript examines viability and stability of anisotropic compact objects in the framework of $f(Q,L_m)$ gravity ($Q$ is the non-metricity and $L_m$ is the matter Lagrangian). We assume a particular functional form of this theory to get explicit expressions for the field equations which govern the behavior of matter and geometry in this context. The configuration of static spherically symmetric structures is evaluated using the two innovative non-singular solutions. We use smooth matching conditions to evaluate the values of unknown constants in the metric coefficients. The viability of considered compact stars is assessed using a graphic analysis of various important physical characteristics. We also investigate stability of the considered stellar objects through sound speed method. It is found that these stellar objects are viable and stable, as all the required conditions are satisfied.

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arXiv2026

Analysis of Charged Compact Stars with Bardeen Black Hole in $f(\mathfrak{Q}, \mathcal{T})$ Gravity

This study investigates the behavior of charged compact stars within the $f(\mathfrak{Q}, \mathcal{T})$ gravitational framework, where $\mathfrak{Q}$ denotes the non-metricity scalar and $\mathcal{T}$ represents the trace of the energy-momentum tensor. Recognized as a promising model for explaining the accelerated expansion of the universe, this approach offers a strong theoretical foundation. A central focus of this research is the application of Bardeen's model to describe the exterior spacetime. Additionally, the study examines the internal structure of compact stars using a solution based on the Finch-Skea metric potential. Various physical properties, including energy density, pressure components, anisotropy, energy conditions and equation of state parameters are analyzed through graphical representations. Equilibrium conditions are explored via the Tolman-Oppenheimer-Volkoff equation while key characteristics such as the mass-radius relationship, compactness, surface redshift and stability criteria based on the causality condition and adiabatic index are thoroughly evaluated. The analysis concludes that the proposed solutions for charged compact stars in this framework are both theoretically consistent and physically valid.

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arXiv2026

Regular Bardeen Black Hole Solutions in Rastall Theory: A Gravitational Decoupling Approach

This research applies the generalized technique of gravitational decoupling to the Bardeen black hole, producing novel black hole solutions in the context of Rastall theory. We proceed by decomposition of the field equations corresponding to an additional matter source into two sets, for further considerations. The metric functions of the Bardeen black hole are adopted to specify the first set. The second one, which is subject to an extra source, is resolved considering a linear equation of state of matter. Through the integration of the solutions of these sets, we develop two expanded models and conduct an in-depth analysis of their distinct physical characteristics, governed by specific parameters. We investigate thermodynamic quantities like density, anisotropic pressure, energy bounds, asymptotic flatness, and thermodynamical properties like the Hawking temperature, entropy, and specific heat, etc. Both models are asymptotically flat but violate the energy bounds. Furthermore, the density, radial pressure, and Hawking temperature demonstrate consistent and acceptable behavior. Ultimately, the thermodynamic stability is affirmed through the analysis of specific heat and the Hessian matrix.

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arXiv2026

Comparative Analysis of Holographic Dark Energy Models in $f(R,T^2)$ Gravity

This study investigates the Renyi Holographic dark energy, Sharma-Mittal Holographic dark energy and Generalized Holographic dark energy models in the framework of $f(R,T^2)$ gravity, where $R$ denotes the Ricci scalar and $T^2$ represents the self-contraction of the stress-energy tensor. For this purpose we employed two horizons as infrared cut-offs, such as Hubble horizon and Ricci horizon. The analysis is conducted for a non-interacting scenario in a spatially flat Friedmann-Robertson-Walker universe. By considering a specific form of this modified gravity, we reconstruct the corresponding gravitational models based on these selected dark energy formulations. Additionally, a stability analysis is performed for all cases and the evolution of the equation of state parameter is examined. Our finding indicates that the reconstructed $f(R,T^2)$ models effectively describe both the phantom and quintessence phases of cosmic evolution, aligning with the observed accelerated expansion of the universe. This study highlights the deep interconnections between holographic dark energy models and modified gravity theories, offering valuable insights into the large scale dynamics of the cosmos.

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arXiv2026

Exploring Hybrid Star Models with Quark and Hadronic Matter in $f(Q)$ Gravity

In this paper, we develop a model for a static anisotropic hybrid star that includes strange quark matter and hadronic matter. We solve the field equations in the $f(Q)$ gravity framework (where $Q$ is the non-metricity) using the Finch-Skea metric. The relationship between density and pressure for strange quark matter is described using the MIT bag model equation, while for hadronic matter, the radial pressure and density are related by a linear equation of state. We select the compact star EXO 1785-248 and analyze five different values of the coupling constant. To evaluate the physical feasibility of the model, we perform a graphical analysis of key properties, including the metric components, energy density, radial and tangential pressures, anisotropy, gradients, quark matter density and pressure, the equation of state parameter, energy conditions and the mass function. We further examine the stability and equilibrium of the star through parameters such as compactness, redshift, causality conditions, Herrera cracking, the adiabatic index and the Tolman-Oppenheimer-Volkoff equation. We observe that $f(Q)$ gravity effectively describes the macroscopic properties of hybrid stars.

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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 $\mathcal{Q}=\mathcal{R}_{\alpha\gamma}\mathcal{T}^{\alpha\gamma}$. 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.

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arXiv2025

Cosmological Implications and Stability of $f\mathbb{(Q,T)}$ Gravity with Pilgrim Dark Energy Model

This manuscript endeavors to construct a pilgrim dark energy framework within the $f\mathbb{(Q,T)}$ gravity theory, employing a correspondence approach aligned with a non-interacting model that incorporates pressureless matter alongside a power-law scale factor. Here $\mathbb{Q}$ and $\mathbb{T}$ represent the non-metricity and trace of the energy-momentum tensor, respectively. This extended modified gravity framework accurately replicates various epochs in the cosmological history. The $f\mathbb{(Q,T)}$ gravity models are utilized to derive the equation of state parameter, phase planes and squared speed of sound. The analysis reveals that the reconstructed model exhibits an increasing or decreasing trend with the pilgrim dark energy parameter. The equation of state parameter characterizes the phantom regime, while the squared speed of sound parameter provides a stable framework for examining the ongoing cosmic evolution. The $\omega_{DE}-\omega'_{DE}$ plane trajectories reveal the freezing region, while the $r-s$ phase plane shows the Chaplygin gas model. It is important to highlight that our findings align with the most recent observational data.

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arXiv2025

Non-Interacting New Agegraphic Dark Energy Model in $f(Q)$ Gravity

In this study, we explore the reconstruction of a new agegraphic dark energy model in a flat Friedmann-Robertson-Walker spacetime by $f(Q)$ gravity framework, where $Q$ represents non-metricity. We assume that the scale factor follows a power-law and explore how this model aligns with the expanding universe. In this perspective, we develop a new agegraphic $f(Q)$ model and analyze the graphical behavior for cosmic evolution. We analyze physical characteristics of the model using the equation of state parameter, $(\omega_{D}-\omega^{\prime}_{D})$ and the $(r-s)$ planes. The equation of state parameter indicates a quintessence era characterized by accelerated expansion. The $(\omega_{D}-\omega^{\prime}_{D})$-plane identifies the freezing region and the Chaplygin gas model is represented in the $(r-s)$-plane. Finally, we examine the stability of the non-interacting model by evaluating the squared speed of sound. Our findings show that the non-interacting new agegraphic dark energy model effectively resolves the cosmic coincidence problem.

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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.

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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.

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arXiv2025

Impact of Pulsar SAX J1748.9-2021 Observations on $f(\mathcal{Q}, \mathbb{T})$ Gravity

The main objective of this study is to investigate the viability and stability of a pulsar filled with anisotropic matter in $f(\mathcal{Q}, \mathbb{T})$ gravity, where $\mathcal{Q}$ represents non-metricity and $\mathbb{T}$ is the trace of the energy-momentum tensor. In this context, we employ non-singular solution and a particular model of this gravity. We use junction conditions to evaluate unknown constants in the metric coefficients. Observations from the pulsar SAX J1748.9-2021 star are employed to validate the model, producing stable configurations that address both geometric and physical characteristics. This framework establishes relationships between various physical quantities, including fluid parameters, anisotropy, mass-radius relation, redshift, the Zeldovich condition, energy conditions, causality conditions, adiabatic index, Tolman-Oppenheimer-Volkoff equation, the equation of state parameter, and compactness. Our findings confirm the viability and stability of the proposed pulsar star in this theoretical framework.

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arXiv2025

The Dynamics of Cosmic Evolution: Insights from Bouncing Cosmology

The primary aim of this work is to explore feasible bouncing cosmological solutions in the framework of $f(\mathcal{Q}, \mathcal{C})$ gravity, where $\mathcal{Q}$ denotes non-metricity and $\mathcal{C}$ indicates the boundary term. To achieve this, we analyze the dynamics of a Bianchi type-I spacetime with perfect fluid distribution. We consider various functional forms of $f(\mathcal{Q,C})$ theory to assess how this modified gravity framework influences cosmic evolution. Additionally, we examine the dynamics of different cosmological parameters to explore non-singular bounce solutions. We also use linear perturbation to study the stability analysis. Our findings reveal the breach of the null energy conditions, which is required for the existence of viable bounce solutions. The equation of state parameter demonstrates either a quintessence phase or a phantom regime of the universe, demonstrating that the cosmos is undergoing accelerating expansion. This gravitational framework presents a promising alternative to the standard cosmological model, presenting an innovative viewpoint on gravitational interactions and the dynamics of the early universe.

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arXiv2025

Cosmography of Non-Interacting Ghost and Generalized Ghost Dark Energy Models in f(Q) Gravity

This paper aims to develop non-interacting ghost dark energy and generalized ghost dark energy models within the framework of $f(Q)$ theory using the correspondence scheme. We use pressureless matter and a power-law scale factor. The cosmic implications of the resulting models are studied through the equation of state parameter and the phase planes. We also check the stability of the reconstructed models through the squared speed of sound parameter. The equation of state parameter exhibits a phantom era, the $(\omega_{D}-\omega^{\prime}_{D})$-plane indicates a freezing region, while the $(r-s)$-plane corresponds to the Chaplygin gas model for both models. It is also found that only the generalized ghost dark energy model remains stable throughout cosmic evolution. We conclude that our findings align well with current observational data.

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arXiv2025

Investigating Strange Stars in Rastall Theory

This study explores the structural formation of various spherically symmetric anisotropic stars within the framework of Rastall theory. To achieve this, we derive modified field equations which are then resolved using the Finch-Skea ansatz, which involve unknown parameters $(A_1,A_2,A_3)$. These parameters are found by using appropriate constraints given by the junction conditions, in addition to observational data from some selected stars. The $\mathbb{EOS}$ given by the $\mathbb{MIT}$ bag model is employed to examine the interior structure and various physical properties of these compact objects. For calculated values of the bag constant $\mathcal{B}$ and two values of the Rastall parameter, $\xi = 0.3, 0.5$, we investigate the regularity and viability of the state variables. Additionally, we analyze stability of the developed model by employing three distinct criteria. We find that the obtained model is stable and provides an accurate approximation for the mass and radius of strange stars when the Rastall parameter $\xi = 0.3$ is considered.

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arXiv2025

Charged Anisotropic Pulsar SAX J1748.9-2021 in Non-Riemannian Geometry

In this paper, we investigate the impact of charge on the stability of the pulsar star SAX J1748.9-2021 within the context of $f(\mathbb{Q}, \mathcal{T})$ theory, where $\mathbb{Q}$ and $\mathcal{T}$ represent the non-metricity scalar and the energy-momentum tensor, respectively. To achieve this, we employ the Krori-Barua metric ansatz with anisotropic fluid. We obtain exact relativistic solutions for the corresponding field equations. Additionally, we examine its physical and geometric properties using astrophysical observations of the pulsar SAX J1748.9-2021. This approach provides a basis for connecting several physical quantities, including fluid parameters, anisotropy, mass-radius relationship, compactness, redshift, \emph{energy}, the Zeldovich and causality conditions, equation of state parameter, adiabatic index and the Tolman-Oppenheimer-Volkoff equation. Our findings align with observational evidence, indicating that the pulsar SAX J1748.9-2021 remains both feasible and stable under this modified theory.

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