Precision Cosmology and the Density of Baryons in the Universe

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  • 31 July 2000
Abstract
Big-bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) anisotropy measurements give independent, accurate measurements of the baryon density and can test the framework of the standard cosmology. Early CMB data are consistent with the longstanding conclusion from BBN that baryons constitute a small fraction of matter in the Universe. We clarify precisely what the two methods determine, and point out that differing values for the baryon density need not indicate inconsistency if the entropy has changed since BBN. Such an entropy change has a clear signature in the CMB anisotropy.

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