BIG-BANG NUCLEOSYNTHESIS AND THE BARYON DENSITY OF THE UNIVERSE

For almost 30 years, the predictions of big-bang nucleosynthesis have been used to test the big-bang model to within a fraction of a second of the bang. The agreement between the predicted and observed abundanc es of deuterium, helium-3, helium-4, and lithium-7 confirms the standa rd cosmology model and allows accurate determination of the baryon den sity, between 1.7 x 10(-31) and 4.1 x 10(-31) grams per cubic centimet er (corresponding to about 1 to 15 percent of the critical density). T his measurement of the density of ordinary matter is pivotal to the es tablishment of two dark-matter problems: (i) most of the baryons are d ark, and (ii) if the total mass density is greater than about 15 perce nt of the critical density, as many determinations indicate, the bulk of the dark matter must be ''nonbaryonic,'' composed of elementary par ticles left from the earliest moments.