The formation of galaxies from massive neutrinos

Scenarios are described that, by including an unstable tau-neutrino, facilitated galaxy formation. Although the unstable particle is chosen to be the tau-neutrino, it is noted that another particle (perhaps not a neutrino at all) with similar mass, lifetime, and decoupling time would serve as well. Without the massive, unstable particle, however, the lighter neutrinos by themselves seem to make galaxy formation on scales less than or equal to 10 to the 12th solar masses almost impossible.