Cosmological limits to the number of massive leptons

If massive leptons exist, their associated neutrinos would have been copiously produced in the early stages of the hot, big bang cosmology. These neutrinos would have contributed to the total energy density and would have had the effect of speeding up the expansion of the universe. The effect of the speed-up on primordial nucleosynthesis is to produce a higher abundance of ⁴He. It is shown that observational limits to the primordial abundance of ⁴He lead to the constraint that the total number of types of heavy lepton must be less than or equal to 5.