  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 cannot be
accommodated.
