BULK VISCOSITY AND COSMOLOGICAL EVOLUTION

In a recent interesting paper, Pimentel and Diaz-Rivera (Nuovo Cimento B, 109 (1994) 1317) have derived several solutions with bulk viscosit y in homogeneous and isotropic cosmological models. They also discusse d the properties of these solutions. In this paper we relate their sol utions by simple transformations to previous solutions published in th e literature, showing that all their solutions can be derived from the known existing ones. Drawbacks to these approaches of studying bulk v iscosity are pointed out, and better approaches indicated.