What is the big-bang-nucleosynthesis prediction for the baryon density and how reliable is it?

Together, the standard theory of big-bang nucleosynthesis (BBN) and the primeval deuterium abundance now very precisely peg the baryon density. Based upon our analysis of the deuterium data and the theoretical uncertainties associated with the BBN predictions, we determine ΩBh²=0.020±0.002 (95% C.L.), with the uncertainty from the measured deuterium abundance about twice that from the predicted abundance. We discuss critically the reliability of the BBN baryon density, and in light of possible systematic uncertainties also derive a very conservative range. We conclude that within the standard cosmology and standard theory of BBN a baryon density ΩBh²=0.032 (the central value implied by recent CMB anisotropy measurements) simply cannot be accommodated.