BBN (t = 1s to ~3 min): At T~1 MeV, weak interactions freeze, locking the n/p ratio. As the Universe cools through T~0.1 MeV, deuterium forms (the "deuterium bottleneck"), enabling fusion to ⁴He. The predicted abundances — 75% H, 25% ⁴He, trace D and ⁷Li — match observations and confirm the hot Big Bang model. The baryon density η sets all yields; this is the key BBN observable.