Extragalactic Abundances of Hydrogen, Deuterium and Helium

Abstract

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 ≈ 10⁻⁴, within about a factor of two, corresponding to Ω_B h 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 × 10⁻⁵) 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.