Cosmological Models With Variable G an Lambda and Bulk viscosity

Abstract

We have analyzed a nonsingular model with a variable cosmological term following the Carvalho {\it et al}. ansatz. The model was shown to approximate to the model of Freese {\it et al}. in one direction and to the Özer-Taha in the other. We have then included the effect of viscosity in this cosmology, as this effect has not been considered before. The analysis showed that this viscous effect could be important with a present contribution to the cosmic pressure, at most, of order of that of radiation. The model puts a stronger upper bound on the baryonic matter than that required by the standard model. A variable gravitational and cosmological constant were then introduced in a scenario which conserves the energy and momentum in the presence of bulk viscosity. The result of the analysis reveals that various models could be viscous. A noteworthy result is that some nonsingular closed models evolve asymptotically into a singular viscous one. The considered models solve for many of the standard model problems. Though the introduction of bulk viscosity results in the creation of particles, this scenario conserves energy and momentum. As in the standard model the entropy remains constant. We have not explained the generation of bulk viscosity but some workers attributes this to neutrinos. Though the role of viscosity today is minute it could, nevertheless, have had an important contribution at early times. We have shown that these models encompass many of the old and recently proposed models, in particular, Brans-Dicke, Dirac, Freese {\it et al}., Berman, Abdel Rahman and Kalligas {\it et al}. models. Hence we claim that the introduction of bulk viscosity enriches the adopted cosmology.