The topsy-turvy character of the cosmos posited by the so-called concordance model is, by now, widely known: Matter constitutes less than a third of the cosmic matter–energy budget. And only about a sixth of that matter is the ordinary baryonic matter we know anything about. Quoted as an invariant fraction of the total matter–energy density in the expanding universe, the mean cosmic baryon density Ωb is confidently presumed to lie between 4 and 5%.
Less well known is the problem of the missing baryons in the present epoch, a small but embarrassing chink in the edifice of the concordance model. The model gets its name from the reassuring agreement of the cosmological parameters derived from a variety of independent observational regimes. The prediction of Ωb comes primarily from the abundance ratios of the lightest nuclear species, the theory of Big Bang nucleosynthesis, and the small anisotropies seen...
