Cosmic Deuterium or a Hydrogen Interloper?

Two groups have independently reported the possible detection of extragalactic deu-terium in the absorption spectrum of the same high redshift, low metalicity QSO. Although the high value for the inferred deuterium abundance poses no problems for cosmology (i.e., big bang nucleosynthesis), it is in apparent conflict with solar system observations of deu-terium and helium-3. This latter inconsistency is explained and made quantitative and it is shown that, unless the inferred D/H ratio is too high by a factor of three, these data challenge our understanding of the stellar and galactic evolution of helium-3. This conflict is resolved if the observed absorption feature is, in fact, due to a hydrogen interloper rather than to high z, low Z deuterium. The observed expansion of the Universe and the presence of the cosmic background radiation ensure that, during its early evolution, the Universe passed through a hot, dense epoch when nuclear reactions synthesized the light nuclides (deuterium, helium-3, helium-4 and lithium-7). In the context of standard Big Bang Nucleosynthesis (BBN), deuterium (D) and helium-3 (3 He) are rapidly burned to helium-4 (4 He) and, the higher the nucleon density (measured by η, the present ratio of nucleons to photons; η 10 ≡ 10 10 η), the more 1