Massive-neutrino decays and the cosmological nucleosynthesis

The influence of heavy lepton decays on the cosmological nucleosynthesis is discussed. Some simple analytical calculations are made showing how deuterium output in cosmological nucleosynthesis depends on the mass and lifetime of heavy neutrinos. For model parameters implying a deuterium abundance of (4-6) x 10 to the -5th, it is shown that a helium abundance in excess of the observational upper limit is obtained. The influence of the heavy neutrino energy density in this scenario on the cosmological expansion rate near the neutron-hardening epoch is discussed, showing additional difficulties for the considered scenario, including too late a changeover to the 'dustlike' stage in the expansion of the universe.