Anisotropic Bianchi Type-I Magnetized String Cosmological Models with Decaying Vacuum Energy Density A(t)

The present study deals with a spatially homogeneous and anisotropic Bianchi-Ⅰ cosmological modelsrepresenting massive strings with magnetic field and decaying vacuum energy density A.The energy-momentum tensor,as formulated by Letelier(1983),has been used to construct massive string cosmological models for which we assume theexpansion scalar in the models is proportional to one of the components of shear tensor.The Einstein’s field equationshave been solved by applying a variation law for generalized Hubble’s parameter in Bianchi-Ⅰ space-time.The variationlaw for Hubble’s parameter generates two types of solutions for the average scale factor,one is of power-law type andother is of the exponential form.Using these two forms,Einstein’s field equations are solved separately that correspondto expanding singular and non-singular models of the universe respectively.We have made a comparative study ofaccelerating and decelerating models in the presence of string scenario.The study reveals that massive strings dominatein the decelerating universe whereas strings dominate in the accelerating universe.The strings eventually disappear fromthe universe for sufficiently large times,which is in agreement with current astronomical observations.The cosmologicalconstant A is found to be a positive decreasing function of time which is corroborated by results from recent supernovaela observations.The physical and geometric properties of the models have been also discussed in detail.