Big-bang nucleosynthesis limit to the number of neutrino species

Concern about the systematic uncertainty in ${\mathrm{the}}^4$He abundance as well as the chemical evolution ${\mathrm{of}}^3$He leads us to reexamine this important limit. It is shown that with conservative assumptions no more than the equivalent four massless neutrino species are allowed. Even with the most extreme estimates of the astrophysical uncertainties a meaningful limit still exists, less than five massless neutrino species, and illustrates the robustness of this argument. We show that a definitive measurement of the deuterium abundance in high-redshift hydrogen clouds can sharpen the limit.