A high deuterium abundance at redshift z = 0.7

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 at high redshift imply an Ω(B) 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.