Neutrino viscosity and isotropization of anisotropic-curvature cosmological models

We investigate the evolution of Bianchi spaces of type ${\mathrm{VII}}_h$, VIII, and IX during the lepton era, and find that a large entropy amplification occurs due to neutrino viscosity for large initial anisotropies. The curvature has a weak influence on the dissipative phenomena. However, following the adiabatic evolution in subsequent epochs we find that models with large anisotropic curvature and high dissipation are not consistent with the isotropy of the Hubble expansion inferred from the observations of the cosmic background radiation. We conclude that our universe is either remarkably flat or nondissipative, the only exception being the particular case of isotropically curved open spaces.