  Low-frequency observatories are currently being constructed with the goal of
detecting redshifted 21cm emission from the epoch of reionization. These
observatories will also be able to detect intensity fluctuations in the
cumulative 21cm emission after reionization, from hydrogen in unresolved damped
Ly-alpha absorbers (such as gas rich galaxies) down to a redshift z~3.5. The
inferred power spectrum of 21cm fluctuations at all redshifts will show
acoustic oscillations, whose co-moving scale can be used as a standard ruler to
infer the evolution of the equation of state for the dark energy. We find that
the first generation of low-frequency experiments (such as MWA or LOFAR) will
be able to constrain the acoustic scale to within a few percent in a redshift
window just prior to the end of the reionization era, provided that foregrounds
can be removed over frequency band-passes of >8MHz. This sensitivity to the
acoustic scale is comparable to the best current measurements from galaxy
redshift surveys, but at much higher redshifts. Future extensions of the first
generation experiments (involving an order of magnitude increase in the
antennae number of the MWA) could reach sensitivities below one percent in
several redshift windows and could be used to study the dark energy in the
unexplored redshift regime of 3.5<z<12. Moreover, new experiments with antennae
designed to operate at higher frequencies would allow precision measurements
(<1%) of the acoustic peak to be made at more moderate redshifts (1.5<z<3.5),
where they would be competitive with ambitious spectroscopic galaxy surveys
covering more than 1000 square degrees. Together with other data sets,
observations of 21cm fluctuations will allow full coverage of the acoustic
scale from the present time out to z~12.
