Big Bang nucleosynthesis

Neutrons froze out at n/p near 1/6, decayed to about 1/7 by the time deuterium survived, and nearly all went into helium, Y = 2 (n/p) / (1 + n/p) near 0.25; leftover deuterium pins ordinary matter at about 5 percent of critical, far below all matter.

About 30 minutes8 activities Paper packet