Figure caption: The ratio of deuterium to hydrogen (D/H) in 6 clouds towards 5 different quasars plotted against the metallicity. Our new result is shown as the filled circle. The name of the quasar is shown next to each point. The shaded region is the predicted D/H value based on the CBR baryon density of 4%. The metallicity is defined as: {log[N(Si)/N(H)] in the gas cloud} - {log[N(Si)/N(H)] in the solar system}. Here N(X) is the column density, with units of absorbing atoms per square centimetre, of element X. The D/H errors are one sigma errors. The errors in metallicity are not shown, but do not exceed 0.5.

How much of the universe is made up of baryonic matter? How well do we understand what happened to create the elements in the universe 3 minutes after the big bang? These are the questions we and our collaborators in France and Britain have been trying to answer by analysing the light from bright quasars which has passed through intergalactic gas clouds.

There are several ways of measuring the baryon density of the universe. One is to use measurements of the cosmic background radiation (CBR), which was generated about 300,000 years after the big bang. Another is to measure the relative abundance of elements that were produced in the nucleosynthesis 3 minutes after the big bang. Comparing these estimates of the baryon density gives us an important test of our understanding of the big bang.

Recent measurements of the CBR have put the baryon density at 4% of the total density. The rest is believed to be made up of dark matter and dark energy, both of which we know very little about. Several groups have tried to measure the ratio of deuterium to hydrogen (D/H), two of the elements produced during big bang nucleosynthesisis, in distant gas clouds by fitting profiles to the elements' absorption lines seen in the spectra of distant quasars. The baryon density calculated from these D/H values is broadly consistent with the CBR density, but there is unexplained scatter in the D/H values. We have measured D/H in a new gas cloud and find D/H to be smaller than the values found by other groups. This may be due to the higher abundance of heavy elements in our gas cloud compared to the clouds used in other measurements, since the presence of heavy elements implies some deuterium has been destroyed through star formation. We plan to make further measurements to find the source of the scatter in D/H values.

Neil Crighton and John Webb