Probing the epoch of early baryonic infall through 21-cm fluctuations

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 21-cm absorption of the CMB. Unlike CMB temperature fluctuations which probe the power spectrum at cosmic recombination, we show that observations of the 21-cm fluctuations at z similar to 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.