Effects of Modified Theories on Stellar Structures and Cosmic Evolution

Abstract

This thesis is devoted to explore stellar structures as well as evolution, gravitational waves and viscous cosmology in the framework of modi¯ed theories of gravity. Firstly, we study physical characteristics of stellar structures in the absence as well as presence of electromagnetic ¯eld in the framework of f(R; T) theory. We consider the compact stars whose pressure and density are related through polytropic equation of state and MIT bag model. The energy conditions are satis¯ed and stellar con¯gurations are found stable for the assumed values of free parameters. Secondly, we discuss anisotropic non-static charged spherical as well as cylindrical sources describing the phenomena of collapse and expansion in f(R; T) theory. We analyze the behavior of density, pressures, anisotropic parameter as well as mass and examine the in°uence of charge as well as model parameter on these quantities. Thirdly, we ¯nd the polarization modes of gravitational waves for some f(R) dark energy models with the help of Newman-Penrose formalism and ¯nd two extra modes than general relativity. We also investigate the propagation of axial gravita- tional waves in the background of °at FRW universe in f(R; T) theory through axial perturbations. It is found that axial waves can induce velocity memory e®ect. Finally, we consider viscous modi¯ed Chaplygin gas interacting with f(R; T) grav- ity in °at FRW universe. We investigate the behavior of total energy density, pressure and equation of state parameter for emergent, intermediate as well as logamediate scenarios of the universe with two interacting models. It is found that bulk viscos- ity enhances the expansion for the intermediate and logamediate scenarios. We also study the evolution using LRS Bianchi type-I model and discuss the behavior of scale factors as well as deceleration parameter in dark energy dominated era for di®erent bulk viscosity models. We conclude that expansion is faster when bulk viscosity is proportional to the Hubble parameter.