Browsing by Author "MUHAMMAD AWAIS SADIQ"

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    Dynamics of stellar evolution in modified Gauss-Bonnet gravity
    (UMT Lahore, 2025-05-14) MUHAMMAD AWAIS SADIQ
    This thesis investigates the dynamics of stellar evolution by focusing on the implications of modified Gauss-Bonnet gravity as an exotic energy candidate. In this setting, we explore the evolution of axial and spherical stellar systems in context of f(G, T) gravity. First of all, we examine the evolution of dissipative, axially symmetric collapsing fluid in the presence of dark sources. We use the modified Gauss-Bonnet gravity in order to formulate the dynamical variables and investigate the impact of dark sources on heat dissipation and pressure anisotropy. We derive scalar functions through the orthogonal decomposition of the Riemann tensor and assess the physical behavior of these scalars in both matter and dark source configurations. The evolution equations related to dynamical variables, heat transport, the Weyl tensor and the super-Poynting vector are developed, highlighting features such as thermodynamics, density inhomogeneity and gravitational radiations in the presence of exotic terms. These findings reveal that dark source terms influence the system's thermodynamics, kinematical variables' evolution and density inhomogeneity, potentially generating repulsive radiations that induce cosmic expansion. Further we explore the effects of dark sources on the evolution of anisotropic, dissipative and shear-free fluid with axial symmetry within the modified Gauss-Bonnet gravity framework. In this configuration, governing equations and heat transfer equations are derived to investigate shear-free evolution, exploring different fluid models, including dissipative non-geodesic and geodesic fluids. The non-geodesic model requires rotational distribution for a radiating scenario, while the geodesic model is non-radiating and irrotational, resembling the FRW model for positive expansion parameters. In this thesis, we also analyze the dynamical impacts of the f(G, T) gravity model on star clusters by considering spherically symmetric interior geometry with anisotropic fluid. We express the modified field equations by using a specific f(G, T) model, with the observational data of the compact star 4U 1820-30 which we utilize to explore the evolutionary behavior of the stellar. We use scalar functions to find factors causing density irregularities. We also calculate the evolution parameters and investigate structure scalars for dust balls, finding that the Gauss-Bonnet parameter significantly governs the dynamics of star clusters. The expansion-free model of star clusters in modified Gauss-Bonnet gravity is investigated, considering dissipative anisotropic viscous models. We examine field equations, junction conditions and dynamical equations to explain the physical meaning of expansion and shear effects through radial and relative radial velocities. It is deduced that cavities appear in the expansion-free evolution of star clusters, with shear-free and expansion-free collapse determined by the relative velocity between neighboring fluid layers. The Skripin model, corresponding to a non-dissipative, expansion-free isotropic cluster, exhibits homologous evolution in the shear-free case. Lastly, we analyze the evolution of cavities in star clusters within the framework of modified Gauss-Bonnet gravity. For this purpose, we consider the spherically symmetric geometry with locally anisotropic fluid distribution, assuming that the proper radial distance among neighboring stellar components remains unchanged during the purely areal evolution stage. We provide the analytical solutions by using general formalism in f(G, T) gravitation theory, with thick-shell cavities at boundary surfaces satisfying the Darmois conditions. It is concluded that dark matter significantly impacts the evolution of cavities in star clusters.