Primordial Deuterium and Big Bang Nucleosynthesis: A Tale of Two Abundances

Recent confrontations of the predictions of standard big bang nucleosynthesis (SBBN) with the primordial abundances of the light nuclides inferred from observational data reveal a conflict. Simply put, compared to theoretical expectations the inferred primordial abundances of either deuterium or helium-4 (or both) are too small". Here I outline the tension" between D and ${}^4$He in the context of SBBN. The incipient crisis for SBBN may be resolved by observations of deuterium in nearly pristine environments such as the high-redshift, low-metallicity QSO absorbers. At present the big bang abundances of deuterium inferred from such data fall into two, apparently mutually exclusive, groups. I describe the deuterium dichotomy and its implications for SBBN as well as for cosmology in general.