Zero-curvature Friedmann-Robertson-Walker models as exact viscous magnetohydrodynamic cosmologies

It is shown that FRW cosmologies, in particular the zero-curvature models,do not necessarily represent perfect fluid solutions but also can be exact solutions of the field equations for a viscous fluid, with or without an electromagnetic field, and that these solutions can be physically acceptable. The viscous, magnetohydrodynamic model with metric identical to that of the perfect fluid model with equation of state p-bar = γrho-bar is investigated and particular attention is given to the Einstein-de Sitter model which corresponds to γ = 0. The behavior of the expansion of the velocity congruence in the viscous interpretation is quite different from that of the standard model, which suggests that the usual treatment of the critical density problem may be questionable