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 diagnostic1 for the cosmological mass density parameter, ΩB. Recent high-redshift D/H measurements are highly discrepant2,3,4,5,6, although this may reflect observational uncertainties7,8. The larger primordial D/H values imply a low ΩB (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 ΩB 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.