Hydrogen 21-cm Intensity Mapping at redshift 0.8
  Observations of 21-cm radio emission by neutral hydrogen at redshifts z ~ 0.5
to ~ 2.5 are expected to provide a sensitive probe of cosmic dark energy. This
is particularly true around the onset of acceleration at z ~ 1, where
traditional optical cosmology becomes very difficult because of the infrared
opacity of the atmosphere. Hitherto, 21-cm emission has been detected 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 HI emission from the volumes
surrounding about ten thousand galaxies (from the DEEP2 optical galaxy redshift
survey. We detect the aggregate 21-cm glow at a significance of ~ 4 sigma.
