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(1) for t he cosmological mass density parameter, Omega(B). Recent high-redshift D/H measurements are highly discrepant(2-6), although this may reflec t observational uncertainties(7,8). The larger primordial D/H values i mply a low Omega(B) (requiring the Universe to be. dominated by non-ba ryonic matter), and cause problems for galactic chemical evolution mod els, which have difficulty in reproducing the steep decline in D/H to the present-day values, Conversely, the lower D/H values measured at h igh redshift imply an Omega(B) greater than that derived from Li-7 and He-4 abundance measurements, and may require a deuterium-abundance ev olution that is too low to easily explain. Here we report the first me asurement 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 inhom ogeneity in (D/H)(p) of at least a factor of ten.