At this stage, about 1 second has elapsed of cosmic time since the Big Bang. There is approximately one neutron remaining for every ten protons. The lifetime of a free neutron to decay is about ten minutes. However most neutrons do not have time to decay. After only about three minutes have elapsed, something else occurs.
Neutrons interact with protons to form nuclei of deuterium, or heavy hydrogen.
The deuterium soon gains another neutron to form tritium, which in turn rapidly absorbs a proton to form a helium nucleus of mass 4, consisting of two protons and two neutrons. One finds that practically every neutron ends up in a helium nucleus. The Big Bang therefore predicts that there should be one helium nucleus for every ten protons, created in the first three minutes of the expansion. This abundance is in fact just what we observe when we look at material which we believe to be close to primordial. If the baryon density is low, however, one makes an amount of deuterium that is comparable to what is observed by astronomers.