The cosmological evolution of general Bianchi models in the adiabatic regime

It is analysed the evolution of homogeneous anisotropic cosmologies in the adiabatic regime allowing for a cosmic velocity field and taking into account the contribution of the quantum vacuum to the cosmic energy density. It is shown that a constant or slowly varying energy-momentum tensor of vacuum isotropizes the Hubble expansion. It is suggested, therefore, that an imperfect tuning of field parameters in grand unified theories may naturally explain the isotropy of the Universe