NEUTRINO VISCOSITY AND ISOTROPIZATION OF BIANCHI TYPE-VIII AND TYPE-IX COSMOLOGICAL MODELS

We investigate the isotropization of Bianchi type-VIII and type-IX spaces due to neutrino viscosity. We find that, owing to a peculiar coupling between curvature and anisotropy, neutrino viscosity does not guarantee the present-day isotropy of the cosmological expansion in such spaces. The observed isotropy of the background radiation imposes stringent limits either on the initial anisotropy or on the curvature of space, in contrast with the principle of chaotic cosmology. On the other hand, the amplification of the radiation entropy depends weakly on curvature and may be as large as 1012. © 1979 Società Italiana di Fisica.