A New Class of LRS Bianchi Type ${\rm VI}_{0}$ Universes with Free Gravitational Field and Decaying Vacuum Energy Density

Abstract

A new class of LRS Bianchi type ${\rm VI}_{0}$ cosmological models with free gravitational fields and a variable cosmological term is investigated in presence of perfect fluid as well as bulk viscous fluid. To get the deterministic solution we have imposed the two different conditions over the free gravitational fields. In first case we consider the free gravitational field as magnetic type whereas in second case `gravitational wrench' of unit `pitch" is supposed to be present in free gravitational field. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density. The cosmological constant $\Lambda$ is found to be a decreasing function of time and positive which is corroborated by results from recent supernovae Ia observations. The physical and geometric aspects of the models are discussed.