title: A high deuterium abundance at redshift z = 0.7.
abstract: Of the light elements, the primordial abundance of deuterium relative to hydrogen, (D/H)p, provides the most sensitive diagnostic for the cosmological mass density parameter, omegaB. Recent high-redshift D/H measurements are highly discrepant, although this may reflect observational uncertainties. The larger primordial D/H values imply a low omegaB (requiring the Universe to be dominated by non-baryonic matter), and cause problems for galactic chemical evolution models, which have difficulty in reproducing the steep decline in D/H to the present-day values. Conversely, the lower D/H values measured at high redshift imply an omegaB greater than that derived from 7Li and 4He abundance measurements, and may require a deuterium-abundance evolution that is too low to easily explain. Here we report the first measurement of D/H at intermediate redshift (z = 0.7010), in a gas cloud selected to minimize observational uncertainties. Our analysis yields a value of D/H ((2.0 +/- 0.5) x 10[-4]) which is at the upper end of the range of values measured at high redshifts. This finding, together with other independent observations, suggests that there may be inhomogeneity in (D/H)p of at least a factor of ten.
authors.affiliation: School of Physics, University of New South Wales, Sydney, Australia.
authors.forename: J K
authors.initials: JK
authors.lastname: Webb
authors.forename: R F
authors.initials: RF
authors.lastname: Carswell
authors.forename: K M
authors.initials: KM
authors.lastname: Lanzetta
authors.forename: R
authors.initials: R
authors.lastname: Ferlet
authors.forename: M
authors.initials: M
authors.lastname: Lemoine
authors.forename: A
authors.initials: A
authors.lastname: Vidal-Madjar
authors.forename: D V
authors.initials: DV
authors.lastname: Bowen
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D003903:Deuterium / Q000032:analysis*; D005118:Extraterrestrial Environment; D013057:Spectrum Analysis
chemical_list: D003903:Deuterium