Big Bang Nucleosynthesis

Big Bang nucleosynthesis is the process that produced the light elements — hydrogen, helium, and traces of lithium — in the first few minutes after the Big Bang. It depends on the strong interaction binding protons and neutrons into nuclei, and its predicted abundances are compared with observations to constrain the density of ordinary matter in the universe. The precise agreement between cosmic microwave background measurements and Big Bang nucleosynthesis predictions for the baryon density is one of the great triumphs of the Standard Model of cosmology, though the lithium abundance remains a persistent puzzle.

Big Bang nucleosynthesis is often celebrated as a triumph of precision cosmology, a place where theory and observation agree to several decimal places. But this agreement is fragile. It depends on assuming that the strong interaction had the same strength in the first three minutes of the universe as it does today — an assumption that is reasonable but not guaranteed. If the coupling constants of nature are dynamical, as some theories propose, then Big Bang nucleosynthesis is not a confirmation of our cosmological model but a probe of whether the laws of physics themselves are stable across cosmic time. The lithium abundance problem is not a measurement error. It is a whisper that something fundamental may have been different.