Precision cosmology and the density of baryons in the universe

Big-bang nucleosynthesis (BBN) and cosmic microwave background (CMB) anisot ropy measurements give independent, accurate measurements of the baryon den sity and can test the framework of the standard cosmology. Early CMB data a re consistent with the long-standing conclusion from BBN that baryons const itute a small fraction of matter in the Universe, but may indicate a slight ly higher value for the baryon density. We clarify precisely what the two m ethods determine and point out that differing values for the baryon density can indicate either an inconsistency or physics beyond the standard models of cosmology and particle physics. We discuss other signatures of the new physics in CMB anisotropy.