American Institute of Physics Bulletin of Physics News Number 683 April 29, 2004 by Phillip F. Schewe, Ben Stein
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.
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.