Recent Deuterium Observations and Big Bang Nucleosynthesis: A New Paradigm?

A new observation of D in a primordial gas cloud, made using the high resolution spectrograph at the Keck telescope, indicates an abundance $D/H =(1.9-2.5) \times 10^{-4}$ \cite{SCHR}. Since deuterium is destroyed by stars, and the predicted Big Bang Nucleosynthesis (BBN) abundance falls monotonically with increasing baryon density, deuterium places a reliable upper limit on the baryon density of the universe. Because the new measurement is substationally larger than previous, galactic estimates, it would force a reassessment of BBN predictions--- if it is confirmed. Using a new BBN Monte Carlo code and analysis technique \cite{KK} we derive constraints implied by a lower limit of $D/H =1.9 \times 10^{-4}$. We find $\Omega_B \le .0068h^{-2}$, which is definitively incompatible with baryonic halo dark matter. We also explore implications of combining the D measurement with other light element abundances. $^7Li$ provides a lower bound, $\Omega_B \ge .004h^{-2}$. Also, the initial $^4He$ mass fraction ($Y_p$) would have to be less than $23.5\%$, assuming 3 light neutrino species---in good agreement with present best fits. Finally, observational upper limits of $Y_p \le 24 \%$ and $^7Li/H \le 2.3 \times 10^{-10}$ would allow the number of neutrinos to be as big as 3.9.