answer 662.ans.006 Louis J. Sheehan, Esquire

Big Bang nucleosynthesis begins about three minutes after the Big Bang, when the universe has cooled down sufficiently to form stable protons and neutrons, after baryogenesis.[2] The relative abundances of these particles follow from simple thermodynamical arguments, combined with the way that the mean temperature of the universe changes over time (if the reactions needed to reach the thermodynamically favoured equilibrium values are too slow compared to the temperature change brought about by the expansion, abundances will remain at some specific non-equilibrium value). Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at this point. The answer is that there are about seven protons for every neutron at the beginning of nucleogenesis, a ratio that would remain stable even after nucleogenesis is over. This fraction is in favour of protons initially primarily because lower mass of the proton favors their production. Free neutrons also decay to protons with a half-life of about 15 minutes, and this time-scale is too short to affect the number of neutrons over the period in which BBN took place, primarily because most of the free neutrons had already been absorbed in the first 3 minutes of nucleogenesis-- a time too short for a significant fraction of them to decay to protons.

One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies are very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant.