BIG-BANG NUCLEOSYNTHESIS LIMIT TO THE NUMBER OF NEUTRINO SPECIES

Concern about the systematic uncertainty in the He-4 abundance as well as the chemical evolution of He-3 leads us to reexamine this importan t limit. It is shown that with conservative assumptions no more than t he equivalent four massless neutrino species are allowed. Even with th e most extreme estimates of the astrophysical uncertainties a meaningf ul limit still exists, less than five massless neutrino species, and i llustrates the robustness of this argument. We show that a definitive measurement of the deuterium abundance in high-redshift hydrogen cloud s can sharpen the limit.