The Crisis Confronting Standard Big Bang Nucleosynthesis

Primordial nucleosynthesis is inevitable during the early evolution of an expanding universe filled with radiation and matter (baryons). However, the precise abundance yields depend sensitively on the baryon density, the radiation content and the early expansion rate. For ``standard" big bang nucleosynthesis (SBBN) only D, $^3$He, $^4$He and $^7$Li are produced in astrophysically interesting abundances which depend on only one ``free" parameter, $\eta = n_{\rm B}/n_{\gamma}$, the ratio of baryons (nucleons) to photons. With four primordial abundances and one adjustable parameter, SBBN is an eminently testable theory. In the confrontation of theory with data current observations suggest a crisis traceable to the tension between deuterium (favoring a ``high" value of $\eta$) and helium-4 (preferring a ``low" $\eta$ value). In this contribution, the written version of the material I discussed at the Princeton University ``Critical Dialogues in Cosmology" (June 1996), I will present the case for a conflict and show how recent observations of deuterium in nearly primordial QSO absorbers exacerbate the problem. I conclude with a brief description of an alternate, ``dynamical" approach to bounding the nucleon density which may be of some value given the current deuterium dichotomy.