Nucleosynthesis in the Early Universe

One of the reasons why the standard Big Bang picture is taken so seriously is its remarkable success in accounting for the observed abundances of the light elements by primordial nucleosynthesis which took place during the first 10 minutes of our Universe. The results of this analysis are crucial for a number of aspects of galaxy formation. In particular, primordial nucleosynthesis provides one of the most important constraints upon the density parameter in the form of baryons Ω B and this is a key part of our story. In the process of developing these results, we need to study in somewhat more detail the rôle of neutrinos in the early Universe, how they change the dynamics of the expanding Universe and how they decouple from the electrons and positrons. Y p p The neutrinos provide an example of the decoupling processes which may be important for other unknown types of weakly interacting particle. We will find a qualitatively similar example when we study possible forms of the dark matter.