Browsing by Author "Hina Azmat"

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    Dynamical Analysis of Self-gravitating Cylindrical Objects in f (R, T ) Gravity
    (Library Information Services, COMSATS University Islamabad, Lahore Campus, 2017) Hina Azmat; FA15-RMT-010; Dr. Muhammad Zubair; LHR TP 6823
    The exploration of stability or instability range in modified theories provide deep insight of gravitational interaction in current era which is based on the ex pansion of the universe. In this thesis, we have analyzed cylindrically symmet ric self-gravitating objects for stability analysis within the framework of f(R,T) gravity. This alternative theory provides substitute to dark energy (DE) which assumes high negative pressure and is considered to be responsible for cosmic expansion. In this discussion, we have considered cylindrically symmetric systems with anisotropic matter distribution. The modified field equations and dynamical equations are constructed in f(R,T) theory of gravity. First order perturbation is applied on the modified field equations and dynamical equations which facil itates the construction of the collapse equation. Instability ranges are explored in both Newtonian(N)andpost-Newtonian(pN)eraswiththehelpofadiabetic index Γ which measures the pressure variation with changing energy density. Some conditions are imposed on material variables that are required for stable configuration. Cylindrically symmetric sources evolving under expansion-free condition are analyzed with locally anisotropic distribution . The collapse equation of cylin drical star is obtained by adopting perturbation approach for general solutions of gravitational field equations and conservation equations. Dynamical insta bility is discussed in N and pN regimes, stability constraints have also been ix developed. The adiabatic index ‘Γ’ is found to be meaningless for the discus sion of stability of gravitating sources carrying expansion-free condition, while stability variations are determined by physical properties of the fluid. Stability analysis for shearing viscous anisotropic fluid with cylindrical symmetry has also been made in f(R,T) theory
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    Dynamics of Stellar Structures in Curvature Based Gravitational Theories
    (Library Information Services, CUI Lahore, 2022) Hina Azmat; FA18-PMT-002; Dr. Muhammad Zubair
    The main objective of this thesis is to explore different astrophysical structures in the background of general relativity as well as in the framework of modified theories. Firstly, we discuss the minimal geometric deformation (MGD) decoupling approach to develop new exact anisotropic solutions for compact stellar systems in the absence as well as in the presence of the electromagnetic field. We consider two well-known solutions, namely Tolman V and charged Durgapal V, and extend them to anisotropic domains by transforming their radial metric functions. The physical analysis of the new interior anisotropic solutions is performed by examining the graphical behavior of energy conditions, equilibrium condition, causality condition, adiabatic index and gravitational redshift for different compact star candidates. Secondly, we develop two analytical models of ultra-compact stellar configurations with non-uniform anisotropic features in the framework of 𝑓(𝑅, 𝑇) theory. In order to introduce anisotropy in the system, we again consider gravitational decoupling by means of MGD and obtain two families of interior solutions for both charged and uncharged scenarios. Dark energy EoS is considered in the interior of the configuration. In the exterior region, we consider both standard vacuum and conformally deformed black hole solutions. The junction conditions at the stellar boundary for both charged and uncharged stellar sources are presented in detail. Some fundamental properties of a stable configuration including regularity at the center, positive energy density with monotonically decreasing behavior from the center outwards, and non-uniform pressure with the monotonic profile are checked for the new anisotropic solutions. Finally, the complexity analysis of non-static spherically and cylindrically symmetric self-gravitating sources surrounding in the locally anisotropic environment and suffering dissipation in the form of heat flux is presented in the light of 𝑓(𝑅, 𝑇) gravity. Our study is based on the definition of complexity for dynamical self-gravitating sources, proposed by Herrera et al. [68]. The scalar functions through the orthogonal splitting of the Riemann tensor are defined in 𝑓(𝑅, 𝑇) gravity, and structure scalars are worked out for both geometries. Our main objective is to analyze the behavior of