Precision rates for nucleon weak interactions in primordial nucleosynthesis and He-4 abundance

We report the results of a detailed calculation of nucleon weak interaction s relevant for the neutron to proton density ratio at the onset of primordi al nucleosynthesis. Radiative electromagnetic corrections, finite nucleon m ass terms, thermal radiative effects on weak processes and on neutrino temp erature are taken into account to reduce the theoretical uncertainty on n < -> p rates to 1%. This translates into a sensitivity in the He-4 mass fract ion Y-p prediction up to 10(-4). We find a positive total correction to the Born prediction delta Y-p similar or equal to 0.004. (C) 1999 Elsevier Sci ence B.V.