Gravitational Lensing of Pregalactic 21 cm Radiation
  Low-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionization will provide hundreds of quasi-independent source planes, each of precisely known redshift, if a resolution of ~ 1 arcminutes or better can be attained. These planes can be used to reconstruct the projected mass distribution of foreground material. A wide-area survey of 21 cm lensing would provide very sensitive constraints on cosmological parameters, in particular on dark energy. These are up to 20 times tighter than the constraints obtainable from comparably sized, very deep surveys of galaxy lensing although the best constraints come from combining data of the two types. Any radio telescope capable of mapping the 21cm brightness temperature with good frequency resolution (~ 0.05 MHz) over a band of width ~> 10 MHz should be able to make mass maps of high quality. If the reionization epoch is at z ~ 9 very large amounts of cosmological information will be accessible. The planned Square Kilometer Array (SKA) should be capable of mapping the mass with a resolution of a few arcminutes depending on the reionization history of the universe and how successfully foreground sources can be subtracted. The Low-Frequency Array (LOFAR) will be able to measure an accurate matter power spectrum if the same conditions are met. 
R. Benton Metcalf