Big Bang Nucleosynthesis
From t=1-200 sec the
Universe had about the density of water and a temperature of about a billion degrees. Protons and neutrons froze
out of the radiation field (fewer
neutrons because they are a little more massive), and neutrons began to decay. There were 14 protons for 2 neutrons. The
neutrons fused to make deuterium, and
then helium. This left 12 protons and 1 helium (He4), or about 8% Helium by
number and 25% by mass. Just as we see everywhere today! A little bit of deuterium and lithium was also left, and we see
that too. The exact density back then
determines the amount left today
(so we know what fraction of critical it was).