The Dark Ages: Searching for the Epoch of First Light

Prior to the epoch of full reionization, the intergalactic medium and gravitationally collapsed systems will be detectable in 21-cm radiation. Physical mechanisms that would produce a 21-cm signature are discussed. These include Lya coupling of the hydrogen spin temperature to the kinetic temperature of the gas resulting from the radiation by an early generation of stars, preheating by soft x-rays from collapsing dark matter halos, and preheating by ambient Lya photons. A patchwork of either 21-cm emission, or absorption against the Cosmic Microwave Background, will result. Large radio telescopes like the Giant Metre Wavelength Telescope (GMRT) or a Square Kilometre Array Radio Telescope (SKA) offer the prospect of measuring this signature, and so detecting the transitional epoch from a dark universe to one with light.