Evolution of the cosmic gas and the relic supernova neutrino background

Using the redshift evolution of the cosmic gas, as inferred from QSO absorption line studies, we calculate the past supernova rate and the relic supernova neutrino background. Using this new technique we find the predicted relic neutrino flux at low energies to be at least an order of magnitude below previous estimates. We argue that the evolution of the cosmic gas is consistent with a large decrease in the number of early-type galaxies at redshifts similar to 3, and that this evolution is the source of the reduction in the predicted neutrino flux. Additional observational constraints from recent redshift surveys lead us to propose a modified model for the evolution of the cosmic gas in which significant infall at low redshift occurs. We discuss the possible relevance of our calculations to the X-ray emitting metal-enriched gas observed in the intergalactic medium. (C) 1997 Elsevier Science B.V.