High-resolution imaging of the cosmic mass distribution from gravitational lensing of pre-galactic H i

Low-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionisation will provide ~ 1000 quasi-independent source planes, each of precisely known redshift, which can be used to reconstruct the projected mass distribution of foreground material. Structure in these source planes is linear and gaussian at high redshift (30 10 MHz should be able to make mass maps of high quality.