Reionization and decaying dark matter.

Gunn-Peterson tests suggest that the Universe was re-ionized after the _tazldard recombination epoch. We present a systematic treatment of the ionization process by deriving the Boltzmann equations appropriate to this regime. A compact solution for the photon spectrum is found in terms of the ionization ratio. These equations are then solved numerically for the Decaying Dark Matter scenario, wherein neutrinos with mass of order 30 eV radiatively decay producing photons which ionize the intergalactic medium. We find that the neutrino mass and lifetime are severely constrained by Gunn-Peterson tests, observations of the diffuse photon spectrum in the ultraviolet regime, and the Hubble parameter.