COSMOLOGICAL BARYON DENSITY DERIVED FROM THE DEUTERIUM ABUNDANCE AT REDSHIFT Z=3.57

THE primordial ratio of deuterium to hydrogen nuclei (D/H), created as a result of the Big Bang, provides the most sensitive measure of the cosmological density of baryons(1-5). Measurements of the D/H ratio in the interstellar medium of our Galaxy place a strict lower limit on t he primordial ratio(6), because processing of gas by stars reduces the abundance of deuterium relative to hydrogen. Absorption of radiation from distant quasars by intervening clouds of gas offers a means of pr obing D/H ratios at large redshifts, where the effects of stellar proc essing should be negligible. Measurements(7,8) on one absorption syste m have indicated an extremely high primordial abundance ratio of 24 x 10(-5). Here we report a measurement of the D/H ratio in another high- redshift absorption system, and obtain a value that is an order of mag nitude lower than that reported previously(7,8). The measured ratio of 2.3 x 10(-5) is consistent with that in the interstellar medium (afte r allowing for Galactic chemical evolution(9,10)), and indicates that the absorption spectra on which the earlier estimates are based may ha ve been subject to strong contamination. We calculate a baryon density that is 5% of the critical density required to close the Universe.