Measurement of intergalactic deuterium abundances

In the last few years, new instruments have allowed astronomers to measure the ratio of deuterium to hydrogen in intergalactic gas clouds at high redshift. The abundances of heavy elements in these clouds show directly that little processing of gas through stars has occurred. Therefore, the measurements are thought to represent a direct sampling of the deuterium abundance produced in the Big Bang. The numbers are not consistent with expectations, in that the values are not constant across different sightlines and in that the mean is not much higher than the Galactic [D/H] values. The latter were expected to be lower than intergalactic values of [D/H] by at least a factor of two because the primordial gas has been turned into stars, producing heavy elements but burning deuterium. Here, I explore the sources of systematic errors that might be underestimated in the analysis.