Big-Bang Nucleosynthesis and Galactic Chemical Evolution

Deuterium is the best indicator of the baryon density; however, only its present abundance is known (and only locally) and its chemical evolution is intertwined with that of $^3$He. Because galactic abundances are spatially heterogeneous, mean chemical-evolution models are not well suited for extrapolating the pre-solar D and $^3$He abundances to their primeval values. We introduce a new approach which explicitly addresses heterogeneity, and show that the decade-old big-bang nucleosynthesis concordance interval $\eta \approx (2 -8)\times 10^{-10}$ based on D and $^3$He is robust.