An intensity map of hydrogen 21-cm emission at redshift z ≈ 0.8

Observations of 21-cm radio emission by neutral hydrogen at redshifts z approximate to 0.5 to similar to 2.5 are expected to provide a sensitive probe of cosmic dark energy(1,2). This is particularly true around the onset of acceleration at z approximate to 1, where traditional optical cosmology becomes very difficult because of the infrared opacity of the atmosphere. Hitherto, 21-cm emission has been detected(3) only to z = 0.24. More distant galaxies generally are too faint for individual detections but it is possible to measure the aggregate emission from many unresolved galaxies in the 'cosmic web'. Here we report a three-dimensional 21-cm intensity field at z=0.53 to 1.12. We then co-add neutral-hydrogen (H I) emission from the volumes surrounding about 10,000 galaxies (from the DEEP2 optical galaxy redshift survey(4)). We detect the aggregate 21-cm glow at a significance of 4s.