  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 $\sim
3$, 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.
