The Big-Bang Nucleosynthesis Limit to the Number of Neutrino Species

Concern about systematic uncertainty in the $^4$He abundance as well as the chemical evolution of $^3$He leads us to re-examine this important limit. It is shown that with conservative assumptions no more than the equivalent of 4 massless neutrino species are allowed. Even with the most extreme estimates of the astrophysical uncertainties a meaningful limit still exists, less than 5 massless neutrino species, and illustrates the robustness of this argument. A definitive measurement of the deuterium abundance in high-redshift hydrogen clouds should soon sharpen the limit.