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 LCDM 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 z_ion. Two reionization scenarios are analyzed in details: an `early reionization' case with z_ion~13, consistent with the recent discovery by the WMAP satellite of a large optical depth to Thomson scattering, and a `late reionization' case with z_ion~8. It is a generic prediction of these models that the background of Lyalpha 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 `grey age', i.e. z_ion<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 sigma) on scales below 5 arcmin.