BIG-BANG NUCLEOSYNTHESIS ENTERS THE PRECISION ERA

The last parameter of big-bang nucleosynthesis, the density of ordinar y matter (baryons), is being pinned down by measurements of the deuter ium abundance in high-redshift hydrogen clouds. When it is,the primeva l abundances of the light elements D, He-3, Li-7, and He-4 will be fix ed. The first three will then become ''tracers'' in the study of Galac tic and stellar chemical evolution. A precision determination of the H e-4 abundance will allow an important consistency test of big-bang nuc leosynthesis and will sharpen nucleosynthesis as a probe of fundamenta l physics, e.g., the bound to the number of light neutrino species. An independent consistency test is on the horizon: a high-precision dete rmination of the baryon density from measurements of the fluctuations of the cosmic background radiation temperature.