Big Bang Nucleosynthesis & Recombination: Physical Mechanisms

During nucleosynthesis, protons and neutrons assembled into deuterium, helium-3, helium-4, and small amounts of lithium. During recombination, the temperature fell enough for neutral atoms to persist and photons to decouple, producing the cosmic microwave background.

Nuclear reaction rates compete with rapid expansion and cooling, limiting early element production. Later, abundant neutral atoms sharply reduce free-electron scattering of photons, allowing radiation to travel long distances.

Observed primordial deuterium and helium abundances agree with nuclear-reaction calculations for the baryon density inferred from the CMB. The CMB blackbody spectrum and anisotropies record conditions near recombination.

Rapid expansion limits how long nuclear reactions can operate, while later neutralization greatly reduces Thomson scattering and makes the universe transparent to radiation.