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 \Omega _ { B } h ^ { 2 } = 0.020 \pm 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 \Omega _ { B } h ^ { 2 } = 0.032 ( the central value implied by recent CMB anisotropy measurements ) simply can not be accommodated .