Big-bang nucleosynthesis enters the precision era

The last parameter of big-bang nucleosynthesis, the density of ordinary matter (baryons), is being pinned down by measurements of the deuterium abundance in high-redshift hydrogen clouds. When it is, the primeval abundances of the light elements D, ³He, ⁷Li, and ⁴He will be fixed. The first three will then become ``tracers'' in the study of Galactic and stellar chemical evolution. A precision determination of the ⁴He abundance will allow an important consistency test of big-bang nucleosynthesis and will sharpen nucleosynthesis as a probe of fundamental physics, e.g., the bound to the number of light neutrino species. An independent consistency test is on the horizon: a high-precision determination of the baryon density from measurements of the fluctuations of the cosmic background radiation temperature.