ABSTRACT
A first step to understanding primordial nucleosynthesis the consequences of nuclear statistical equilibrium (NSE) among the light nuclear species is considered. The ratio of neutrons to protons is of particular importance to the outcome of primordial nucleosynthesis, as all the neutrons in the Universe become incorporated into helium. The idea of primordial nucleosynthesis dates back to Gamow in 1946. The abundance of D has been measured in solar system studies, in UV absorption studies of the local interstellar medium (ISM), and in studies of deuterated molecules in the ISM. Primordial nucleosynthesis is both the earliest and most stringent test of the standard cosmology, and an important probe of cosmology and particle physics. The lithium abundance is proving to be a very powerful probe of the early Universe. The success of the very simple, standard scenario of nucleosynthesis is all that more impressive in the light of the apparent inability of scenarios with adjustable parameters to achieve the same success.
