Possibility of Precise Measurement of the Cosmological Power Spectrum   With a Dedicated 21cm Survey After Reionization
  Measurements of the 21cm line emission by residual cosmic hydrogen after reionization can be used to trace the power spectrum of density perturbations through a significant fraction of the observable volume of the Universe. We show that a dedicated 21cm observatory coule probe a number of independent modes that is two orders of magnitude larger than currently available, and enable a cosmic-variance limited detection of the signature of a neutrino mass ~0.05eV. The evolution of the linear growth factor with redshift could also constrain exotic theories of gravity or dark energy to an unprecedented precision. 
Abraham Loeb (Harvard), Stuart Wyithe (Melbourne)