LRS Bianchi type-I models with a time-dependent cosmological "constant"

Plane symmetric homogeneous models of the universe with a perfect fluid distribution of matter and a variable cosmological "constant" representing the energy density of vacuum have been presented. The models are highly anisotropic in the early stages of evolution and ultimately evolve to the standard model at the present epoch. Of the three models obtained in this paper, one has an increasing vacuum energy density with time in the early universe. Though highly negative in the very early universe, the vacuum density soon becomes positive, and meets the observational limits at the present epoch. The negative vacuum in the early universe in this model, which decays numerically, adds up to anisotropy and at the same time contributes to the isotropization process. The second model is found to admit matter creation from the decaying Vacuum energy whereas the third one, in which the vacuum density is always one third of the corresponding radiation density, is devoid of any such creation. [S0556-2821(99)01616-1].