ABSTRACT
Nuclear abundances are certainly synthesised in stars. In particular heavy nuclei can be well explained within a conventional stellar nucleosynthesis. However, light elements, such as deuterium and 4He, are much more abundant than expected from stellar nucleosynthesis. The theory of Big Bang nucleosynthesis, based on the Hot Big Bang model, provides an elegant explanation for this observed primordial nuclear composition of the universe. The discussion proceeds along similar lines for the case of recombination. The difference is that in the case of nucleosynthesis there is a competition among different nuclear abundances. The correct predictions have to be calculated solving a set of coupled kinetic equations describing the evolution of the nuclear abundances with time and taking into account the value of the rates of nuclear reactions and of course of the expansion rate. However, before neutrons have fully decayed, the formation of nuclei from fusion processes, nucleosynthesis, occurs.
