  We reconsider the Alcock-Paczynski effect on 21cm fluctuations from high
redshift, focusing on the 21cm power spectrum. We show that at each accessible
redshift both the angular diameter distance and the Hubble constant can be
determined from the power spectrum. Furthermore, this is possible using
anisotropies that depend only on linear density perturbations and not on
astrophysical sources of 21cm fluctuations. We show that measuring these
quantities at high redshift would not just confirm results from the cosmic
microwave background but provide appreciable additional sensitivity to
cosmological parameters and dark energy.
