  The expansion of the universe appears to be accelerating, and the mysterious
anti-gravity agent of this acceleration has been called ``dark energy''. To
measure the dynamics of dark energy, Baryon Acoustic Oscillations (BAO) can be
used. Previous discussions of the BAO dark energy test have focused on direct
measurements of redshifts of as many as $10^9$ individual galaxies, by
observing the 21cm line or by detecting optical emission. Here we show how the
study of acoustic oscillation in the 21 cm brightness can be accomplished by
economical three dimensional intensity mapping. If our estimates gain
acceptance they may be the starting point for a new class of dark energy
experiments dedicated to large angular scale mapping of the radio sky, shedding
light on dark energy.
