Big Bang Nucleosynthesis
The forging of the light elements in the first few minutes of the universe.
Definition
Big Bang nucleosynthesis (BBN) is the production of the light atomic nuclei — primarily helium-4, with traces of deuterium, helium-3, and lithium-7 — during the first few minutes of cosmic history, when the universe was hot and dense enough to drive nuclear fusion. Its signature prediction is that about a quarter of all ordinary (baryonic) matter, by mass, should be helium-4, a figure confirmed by observations of the most pristine, least-processed gas in the universe.
The helium yield follows from a chain of physics. Around one second after the Big Bang, at a temperature near 0.7 MeV, the weak interactions could no longer interconvert neutrons and protons, freezing the neutron-to-proton ratio at roughly 1:6. A bottleneck then delayed fusion: the universe held about a billion photons per baryon, energetic enough to dissociate any deuterium that formed. Only near three minutes, when the temperature fell below about 0.1 MeV, could deuterium survive, after which nearly every remaining neutron was swept into a helium-4 nucleus. With the ratio reduced to about 1:7 by neutron decay during the wait, the predicted helium mass fraction is Y_p ≈ 0.25.
BBN is also a precision probe of cosmic composition. The yields of deuterium and lithium depend sharply on the baryon-to-photon ratio η, making the measured deuterium abundance a sensitive 'baryometer' that weighs all the ordinary matter in the universe. The value it returns agrees with the entirely independent measurement from the CMB anisotropies — a concordance, spanning physics separated by 380,000 years, that is a cornerstone of hot Big Bang cosmology and implies that baryons make up only about 5% of the cosmic energy budget.
History
The theory was developed in 1948 by Ralph Alpher, Hans Bethe, and George Gamow (the 'alpha-beta-gamma' paper) and refined over subsequent decades. Modern BBN, computed with detailed nuclear reaction networks and tested against abundances in metal-poor systems, is one of the most quantitatively successful predictions of cosmology.