A High Deuterium Abundance in the Early Universe

Abstract

The amount of deuterium relative to hydrogen (D/H) in clouds with close to primordial abundance seen at high redshift in the spectra of distant quasars currently provides the best estimate of the baryonic density of the Universe. The first measurements have yielded discrepant values of D/H both high [around 2 x 10^(-4)] and an order of magnitude lower. The low values of D/H imply a high Omega(Baryon) that is difficult to reconcile with determinations of the primordial abundances of other light elements, notably He^4, and with the known number of light neutrinos. We report an independent measurement of the neutral hydrogen (HI) column density in the cloud toward Q1937-1009 where one of the low D/H values was obtained. Our measurement excludes the reported value and we give a lower limit of D/H > 4 x 10^(-5) in this system, which implies Omega(Baryon) < 0.016 for a Hubble constant of 100 km/sec/Mpc. This new upper limit on Omega(Baryon) relieves the conflict with standard Big Bang nucleosynthesis.

12 pages LaTeX, including 2 postscript figures. Also available at http://www.ifa.hawaii.edu/faculty/acowie/deut_limits.html . To be published in Nature

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