Zaldarriaga's telescope

Matias Zaldarriaga (Harvard) gave a colloquium about directly observing atomic hydrogen between the recombination (should be combination) epoch and reionization, in (redshifted) 21-cm radiation. At redshifts 100 to 30, it is visible in absorption (because the spin temperature is locked to the gas temperature, which is colder than the CMB (assuming no stars or AGN have formed). At redshifts around 10, it is visible in emission, because the spin temperature becomes hotter than the CMB (although this doesn't make it very bright, since both the CMB temperature and the spin temperature exceed the characteristic temperature of the transition, about 0.07 K). Anyway, it appears to be a huge store-house of information for fundamental cosmology (better than the CMB because it is 3-d, not 2-d, and it doesn't suffer from Silk damping at small scales), and it will be measured (if all goes well) this decade.