Big Bang Nucleosynthesis and the observed abundances of light elements

The standard model of the early universe is extraordinarily simple: it just assumes a globally isotropic and uniform universe. In the simplest version there is no structure of any kind on scales larger than individual elementary particles; indeed the contents are determined by “just physics”— global expansion governed by relativity, particle interactions governed by the Standard Model, and distributions governed by statistical mechanics. Since the gravity is itself dominated by the relavistic plasma early on, parameters that become prominent at late times (global curvature, dark matter density) make no difference, so the model has only one parameter: the ratio η of the number of baryons to the number of photons, often expressed in units of η10 ≡ η/10 . Even this parameter has almost no effect on the early evolution of the universe, but it does affect the principal observational relic of the early expansion, the relative abundances of light nuclei. Since the number of photons in the universe is known today from the microwave background temperature (it is 411±2cm), η specifies the present day baryon density: Ωbh 2 = 3.65× 10η10. The model would be beautiful even if simplicity were its only virtue, but remarkably, as observations improve they verify it with increasing precision as a good description of the real world. The most precise verification of the basic framework now comes from observations of the isotropy and spectrum of the microwave background, which verify the precise uniformity of the universe on large scales the primordial origin of the radiation. Here I discuss the other principal observational relic of the early universe, the cosmic abundances of light nuclei, which probe homogeneity over a much wider range of scales than the microwave background, and which record processes starting with redshifts of the order of 10. In particular, I discuss observations of abundances as a precise test of the model and a precise test of the parameter η. Unfortunately the later evolution of the universe is anything but simple. The uniform gas of the Big Bang long ago converted into a complex universe of