Probing beyond the Epoch of Hydrogen Reionization with 21 Centimeter Radiation

We use numerical simulations of hydrogen reionization by stellar sources in the context of ΛCDM cosmogonies to investigate the 21 (1 + z) cm radio signal expected from the diffuse intergalactic medium (IGM) prior to the epoch of reionization breakthrough at redshift zion. Two reionization scenarios are analyzed in detail: an "early reionization" case with zion ≈ 13, consistent with the recent discovery by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite of a large optical depth to Thomson scattering, and a "late reionization" case with zion ≈ 8. It is a generic prediction of these models that the background of Lyα photons produced by the early generation of stars which ultimately ionize the universe will be sufficiently intense to make intergalactic neutral hydrogen visible against the cosmic microwave background during the "gray age," i.e., zion ≲ z ≲ 20. Depending on the redshift of reionization breakthrough, broad-beam observations at frequencies ≲150 MHz (below 100 MHz for the early reionization scenario) with the next generation of radio telescopes should reveal angular fluctuations in the sky brightness temperature in the range 5-20 mK (1 σ) on scales below 5'.