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

Abstract

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.

To appear in the Proceedings of the IAGUSP Workshop on Young Galaxies and QSO Absorbers (Santos, Brazil; April 8-11, 1996), eds. S. M. Viegas, R. Gruenwald & R. R. de Carvalho. Latex with six embedded postscript figures