New constraints on neutrino masses from cosmology

We raise the lower bounds on both heavy unstable and stable meutrino masses, exploiting both the phenomena of neutrino mass mixing and possible Majorana mass terms. Stable heavy neutrinos with Majorana masses (the generic case) have an annihilation rate with is p-wave suppressed. Thus in order to have sufficient cosmic annihilation we find the previous lower bound of m ≥ 2 GeV must be raised to m_ν ≳ 5 GeV assuming estimates of the present mass density remain unchanged. Unstable neutrinos are constrained by incorporating both new data on mass mixing from experiment and the annihilation suppression described above with standard nucleosynthesis cosmology. In this case we find m ≥ 23 MeV, increasing the former bound by more than an order of magnitude.