EXTRAGALACTIC ABUNDANCES OF HYDROGEN, DEUTERIUM AND HELIUM New Steps, Missteps and Next Steps

Estimates of the deuterium abundance in quasar absorbers are reviewed, including a brief account of incorrect claims published by the author and a brief review of the problem of hydrogen contamination. It is concluded that the primordial abundance may be universal with a value (D/H)P ≈ 10 , within about a factor of two, corresponding to ΩBh 2 0.7 ≈ 0.02 or η10 ≈ 2.7 in the Standard Big Bang. This agrees with current limits on primordial helium, YP ≤ 0.243, which are shown to be surprisingly insensitive to models of stellar enrichment. It also agrees with a tabulated sum of the total density of baryons in observed components. Much lower primordial deuterium (≈ 2 × 10) is also possible but disagrees with currently estimated helium abundances; the larger baryon density in this case fits better with current models of the Lyman-α forest but requires the bulk of the baryons to be in some currently uncounted form.