Can the reionization epoch be detected as a global signature in the cosmic background?

The reionization of the Universe is expected to have left a signal in the form of a sharp step in the spectrum of the sky. If reionization took place at 5 ≲ z_ion ≲ 20, a feature should be present in the radio sky at 70 ≲ v ≲, 240 MHz due to redshifted HI 21-cm line emission, accompanied by another feature in the optical/near-IR at 0.7 ≲ λ ≲ 2.6 μm due to hydrogen recombination radiation. The expected amplitude is well above fundamental detection limits, and the sharpness of the feature may make it distinguishable from variations due to galactic and extragalactic foregrounds. Because this is essentially a continuum measurement of a signal which occurs over the whole sky, relatively small telescopes may suffice for detection in the radio. In the optical/near-IR, a space telescope is needed with the lowest possible background conditions, since the experiment will be severely background-limited.