In the very early universe the so called "dark age" comes after the
time of the first atoms---a moment when suddenly neutral atoms, mostly
hydrogen, could form, allowing photons to stream freely, photons we
now see as the microwave background---but before the first stars formed.
But maybe this era needn't be so dark. Just as numerous finds of arts
and crafts from the European dark ages have helped to enlighten us
on what the sixth to the eleventh centuries were like, so too some
bits of light from the cosmic dark ages might illuminate that epoch.
Abraham Loeb and Matias Zaldarriaga of Harvard believe that the early,
cold, neutral hydrogen can be made to speak, as it were. These atoms,
in a redshift window of about 30 to 100, would be colder than the background
radiation. The atoms would absorb photons and cause a deficit in the
microwave background at cold hydrogen's characteristic wavelength of
21 centimeters. This absorption wavelength, in turn, would be stretched
out, courtesy of the universal expansion of the universe, to a wavelength
of 6-21 meters or so.
Because the cosmic hydrogen is not uniform, the level of absorption
varies across the sky and the microwave background would show anisotropies
at these long wavelengths. These anisotropies could be sought using
special radio interferometers. (Some efforts are already underway to
see this kind of light: see the LOFAR and SKA websites.)
Just as microwave telescopes mapping the early sky see minute temperature
variations, so the primordial hydrogen could also be mapped. This map
might well show the influence of dark matter through its influence
in shepherding early hydrogen.
Interest in this hydrogen has been expressed before, but the Harvard
proposal is the first to be specific about how to search for information
imprinted in the dark-age atom distribution. (Physical
Review Letters, upcoming article; contact Abraham Loeb, firstname.lastname@example.org