Big Bang Nucleosynthesis (BBN) began approximately 1 second into the Big Bang and lasts for about 3 minutes. BBN is the only window into the conditions of the early universe before the CMB. During this epoch, the temperature is optimal for the formation of light nuclei, resulting in the synthesis of D,3He,4He, and 7Li. The story begins with all of the energy in the universe condensed into a single point. Suddenly, it exploded outwards and the universe was born. As it expanded, it cooled and began to form matter.
After the quark-hadron transition (when quarks and hadrons condensed to form particles like protons and neutrons), the neutron-to-proton ratio (n/p) was held in equilibrium through weak interactions with electrons, neutrinos, and their antiparticles. However, as the temperature continued to drop due to expansion, neutrinos decoupled from the background and the n/p ratio could no longer remain in equilibrium. In other words, the top left equation in the above diagram started to shift to one side. Roughly 1 second into the Big Bang, the neutron/proton froze at ≈1/6. Subsequently, almost all neutrons were incorporated into helium nuclei. The details of this are dependent upon the nuclear reaction rates, the baryon densities, and the cosmic expansion as the nuclear reactions freeze out.
In recent years, many updates have been made to these nuclear reaction rates. Working with Grant Mathews and Nishanth Sasankan (Notre Dame), I incorported some of these changes (NACRE and REACLIB in particular) into a Big Bang Nucleosynthesis code developed by Kawano to determine what the updated uncertainties on nuclear abundances should be (Foley et al 2017). However, these updates do not present a solution to the long-standing lithium problem in which theory predicts a primordial lithium abundance that is 3-4 times higher than that which is observed in very metal poor stars.