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We consider the evolution of the sky-averaged 21-cm background during the early phases of structure formation. Using simple analytic models, we calculate the thermal and ionization histories, assuming that stellar photons dominate the radiation background. The resulting 21-cm spectra can constrain the properties of the first generations of stars and quasars. If Population II stars dominate, Ly coupling renders the intergalactic medium (IGM) visible before it is heated by X-rays and long before reionization. Thus the 21-cm background has a strong absorption epoch followed by weaker emission that fades during reionization. The harder spectra of very massive Population III stars compress these transitions into a shorter time interval and decreases the signal amplitude. However, the reionization epoch will remain visible except in extreme cases. The global 21-cm signal will be challenging to observe because of astronomical foregrounds, but it offers an exciting opportunity to study the first sources of light. It also fixes the overall amplitude of the fluctuating background whose detection is a major goal of several next-generation low-frequency radio interferometers.
Steven R. Furlanetto (Wed,) studied this question.