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, Ω_B. Recent high-redshift D/H measurements are highly discrepant, although this may reflect observational uncertainties,. The larger primordial D/H values imply a low Ω_B (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 athigh redshift imply an Ω_B greater than that derived from ⁷ Li and ⁴ He abundance measurements, and may require a deuterium-abundance evolution that is too low to easily explain. Here wereport 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) × 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.