The Process of Big Bang Nuclear Synthesis

The process of Big Bang Nuclear Synthesis works as we now understand it. Neutrons, if they're not bound up in a nuclei, will decay with a lifetime of about 15 minutes. So you have like a window of about 15 Minutes to fuse those neutrons into nuclei or they'll disappear. Also during this period of time, the nuclear fusion reactions are only efficient when the density and temperature are really high.