Massive neutrinos in cosmology and galactic astronomy

This article gives a brief qualitative discussion of the consequences for cosmology and galactic astronomy of a neutrino type possessing a rest-mass of order tens of electron volts. Emphasis is laid on the possible detectability of ultra-violet photons which may be emitted by massive neutrinos dominating both the universe and individual galaxies including the Milky Way. One speculative possibility is that ionization observed both in the intergalactic medium and in the halo of our Galaxy may be produced by such photons. This would require a neutrino mass about 100 ev and a radiative lifetime about 10 to the 27th seconds, values which are compatible with both cosmological constraints and considerations from elementary particle physics.