The present universe is expanding and is filled with radiation (the 2.7 K Cosmic Microwave Background - CMB) as well as "ordinary" matter (baryons), "dark" matter and, "dark energy". Extrapolating back to the past, the early universe was hot and dense, with the overall energy density dominated by relativistic particles ("radiation dominated"). During its early evolution the universe hurtled through an all too brief epoch when it served as a primordial nuclear reactor, leading to the synthesis of the lightest nuclides: D, 3He, 4He, and 7Li. These relics from the distant past provide a unique window on the early universe, probing our standard models of cosmology and particle physics. By comparing the predicted primordial abundances with those inferred from observational data we may test the standard models and, perhaps, uncover clues to modifications or extensions beyond them.
These notes summarize the lectures delivered at the XIII Canary Islands Winter School of Astrophysics: "Cosmochemistry: The Melting Pot of Elements". The goal of the lectures was to provide both theorists and observers with an overview of the evolution of the universe from its earliest epochs to the present, concentrating on the production, evolution, and observations of the light nuclides. Standard Big Bang Nucleosynthesis (SBBN) depends on only one free parameter, the universal density of baryons; fixing the primordial abundances fixes the baryon density at the time of BBN. But, since baryons are conserved (at least for these epochs), fixing the baryon density at a redshift ~ 1010, fixes the present-universe baryon density. Comparing this prediction with other, independent probes of the baryon density in the present and recent universe offers the opportunity to test the consistency of our standard, hot big bang cosmological model.
Since these lectures are intended for a student audience, they begin with an overview of the physics of the early evolution of the Universe in the form of a "quick and dirty" mini-course on Cosmology (Section 1). The experts may wish to skip this material. With the necessary background in place, the second lecture (Section 2) discusses the physics of primordial nucleosynthesis and outlines the abundances predicted by the standard model. The third lecture considers the evolution of the abundances of the relic nuclides from BBN to the present and reviews the observational status of the primordial abundances (Section 3). As is to be expected in such a vibrant and active field of research, this latter is a moving target; the results presented here represent the status in November 2001. Armed with the predictions and the observations, the fourth lecture (Section 4) is devoted to the confrontation between them. As is by now well known, this confrontation is a stunning success for SBBN. However, given the precision of the predictions and of the observational data, it is inappropriate to ignore some of the potential discrepancies. In the end, these may be traceable to overly optimistic error budgets, to unidentified systematic errors in the abundance determinations, to incomplete knowledge of the evolution from the big bang to the present or, to new physics beyond the standard models. In the last lecture I present a selected overview of BBN in some non-standard models of Cosmology and Particle Physics (Section 5). Although I have attempted to provide a representative set of references, I am aware they are incomplete and I apologize in advance for any omissions.