2005ApJ...630L..41R

The deuterium abundance in the primordial gas formed during the big bang is dependent on the baryon-to-photon ratio during nucleosynthesis. We report a measurement of the deuterium-to-hydrogen (D/H) ratio from a 6 σ detection of the hyperfine ground-state transition of deuterium at 327 MHz (92 cm) in emission from the interstellar gas in the region of the Galactic anticenter. We find the maximum signal at l=183°, where the deuterium line opacity is expected to be a maximum as a result of velocity crowding. Using these deuterium and existing hydrogen observations, we model the effects of beam dilution, spin temperature, and absorption of continuum to derive a D/H ratio of 2.3^+1.5_–1.3x10^–5 by number. The errors are ±3 σ plus our estimate of the systematic uncertainty in the conversion of the observed line amplitude to the D/H ratio.