  After cosmological recombination, the primordial hydrogen gas decoupled from
the cosmic microwave background (CMB) and fell into the gravitational potential
wells of the dark matter. The neutral hydrogen imprinted acoustic oscillations
on the pattern of brightness fluctuations due to its redshifted 21cm absorption
of the CMB. Unlike CMB temperature fluctuations which probe the power spectrum
at cosmic recombination, we show that observations of the 21cm fluctuations at
z ~ 20-200 can measure four separate fluctuation modes (with a fifth mode
requiring very high precision), thus providing a unique probe of the geometry
and composition of the universe.
