The standard, hot, big bang cosmological model is simple (assuming isotropy, homogeneity, Newtonian/Einsteinian gravity, standard particle physics, etc.) and, likely, simplistic. In broad brush it offers a remarkably successful framework for understanding observations of the present and recent Universe. It may seem hubris to expect that this model could provide a realistic description of the Universe during the first 20 minutes or so in its evolution when the temperature and density were enormously larger than today. According to the standard model, during these first 20 minutes the entire Universe was a nuclear reactor, turning neutrons and protons into the light nuclides. This prediction presents the opportunity to use observations here and now to test the theory there and then. As described in this article, SBBN predicts observable primordial abundances for just four light nuclides D, 3He, 4He, and 7Li, as a function of only one adjustable parameter, , the nucleon-to-photon ratio, which is a measure of the universal baryon abundance. Even though it is currently difficult to use the extant observational data to bound the primordial abundance of 3He, SBBN is still overconstrained, yielding three predicted abundances for one free parameter. Furthermore, the baryon density inferred from SBBN must be consistent with that derived from observations of the present Universe. Given the many possibilities that SBBN could be falsified by the empirical data, it is a remarkable success of the standard model that there is consistency between theory and observations provided that there are a few billion photons (most of them in the 2.7 K cosmic background radiation) for every neutron or proton (nucleon) in the Universe.
This success establishes primordial nucleosynthesis as one of the main pillars of our standard model of cosmology, providing the only probe of the physical Universe during its very early evolution. Alternate theories of gravity and/or particle physics must now be tested against the impressive success of SBBN.