  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.
