First Detection of Cosmic Structure in the 21-cm Intensity Field
  We present the first statistically significant detection of cosmic structure using broadly distributed hydrogen radio emission. This is accomplished using a cross correlation with optical galaxies. Statistical noise levels of $20 \mu $K are achieved, unprecedented in this frequency band. This lends support to the idea that large volumes of the universe can be rapidly mapped without the need to resolve individual faint galaxies, enabling precise constraints to dark energy models. We discuss strategies for improved intensity mapping. 
Ue-Li Pen, Lister Staveley-Smith, Jeffrey Peterson, Tzu-Ching Chang