 Big Bang nucleosynthesis

Big Bang nucleosynthesis forms light atomic nuclei during the first minutes of the expanding, cooling universe. Weak interactions initially maintain neutron-proton chemical equilibrium. After cosmological weak freeze-out, free-neutron decay after freeze-out reduces the available neutron supply.

The deuterium bottleneck delays efficient assembly: energetic photons break up newly formed deuterium until sufficient cooling occurs. Most surviving neutrons then enter helium-4, with smaller residual abundances of other light nuclei. If $r=n_n/n_p$ at assembly and nearly all neutrons enter helium, counting two neutrons per helium nucleus gives the approximate helium mass fractionThis links nuclear reactions and particle decay to an astronomical abundance measurement. Comparing primordial deuterium and helium with the independently inferred cosmological baryon density tests the thermal history; the cosmological lithium problem remains an important qualification to the comparison.^[ref]

Big Bang nucleosynthesis (BBN) refers to the process that took place during the first few minutes of the universe's existence, leading to the formation of light atomic nuclei from fundamental particles. According to the Big Bang model, the universe began in an extremely hot and dense state and expanded rapidly. As the universe expanded, it cooled down, allowing for the formation of protons, neutrons, and eventually light elements.