Primordial nucleosynthesis predicts the abundances of several light elements, notably 2H, 4He, and 7Li. The principle variable is the baryon density, b h2. One finds approximate concordance for b h2 0.015, and with the consensus value 9 of H0 (h = 0.7 ± 0.1) one concludes that b 0.03. Not all pundits agree on concordance, since the primordial 4He abundance requires a somewhat uncertain extrapolation from the most metal-poor galaxies with He emission lines (Z ~ 0.02 Z) to zero metal abundances. 10 Moreover the 2H abundance is based on intergalactic (and protogalactic) 2H observed in absorption at high redshifts toward two quasars, probing only a very limited region of space. 11 However incorporation of 7Li and allowance for the various uncertainties still leaves relatively impressive agreement with simple model predictions.
Direct measurement of the baryon density at z ~ 3 can be accomplished by using the Lyman alpha forest absorption systems toward high redshift quasars. The neutral gas observed is only a small component of the total gas, but the ionizing radiation from quasars is measured. A reasonably robust conclusion finds that gas ~ 0.04, implying that the bulk of the baryons are observed and in diffuse gas at high redshift. 12
At low redshift, the luminous baryon component is well measured, and amounts to * ~ 0.003 in stars. 13 Gas in rich clusters amounts to a significant fraction of cluster mass and far more than the stellar mass, but these clusters only account for about five percent of the stellar component of the universe. Combining both detected gas and stars implies that at z ~ 0, we observe no more than gas ~ 0.005. Here we have a problem: where are the baryons today?
Most baryons must therefore be relatively dark at present. There are two possibilities, neither one of which is completely satisfactory. The dark baryons could be hot gas at T ~ 106 K, in the intergalactic medium. 14 This gas cannot populate galaxy halos, where it is not observed, nor objects such as the Local Group, and is not present in rich clusters in a globally significant amount. It remains to be detected: if the temperature differed significantly from 106 K the presence of so much gas would already have had observable consequences.
The alternative sink for dark baryons is in the form of compact objects. MACHOs are the obvious candidate, detected by gravitational microlensing by objects in our halo of stars in the LMC, and possibly constituting fifty percent of the dark mass of our halo. However star-star lensing provides a possible alternative explanation of the microlensing events, associated with a previously undetected tidal stream in front of the LMC 15 ,16 and with the known extension of the SMC along the line of sight. In the LMC case, at least one out of approximately 20 events has a known LMC distance, and for the SMC, there are only two events, both of which are associated with the SMC. 17 The statistics are unconvincing, and since until now one requires binary lenses to obtain a measure of the distance, any distance determinations are likely to be biased towards star-star lensing events.