Is a Massive Tau Neutrino Just What Cold Dark Matter Needs?

The cold dark matter scenario for structure formation in the Universe has many successes; however, when its spectrum of density perturbations is normalized to the Cosmic Background Explorer anisotropy measurement the level of inhomogeneity predicted on small scales is too large. This can be remedied by a tau neutrino of mass 1-10 MeV and lifetime 0.1-100 sec whose decay products include electron neutrinos because it allows the total energy density in relativistic particles to be doubled without interfering with nucleosynthesis.