If we cannot find discrete objects at theseultra-high redshifts, then the best hope ofmapping how the primordial gas got heatedand ionized may be to detect the emission oflight with a wavelength of 21 cm from theneutral state of hydrogen. It is a very weaksignal compared with other radio back-grounds, but because hydrogen emits a linenot a continuum, one can do 3D mapping or"tomography" of this substance. Major in-strument arrays, including the Square Kilo-meter Array (SKA), which will be built ineither Australia or southern Africa, will ad-vance this area of research. We also awaittheoretical progress to pin down the physicsof the ultra-early universe in a similar waythat the physics of the nucleosynthesis erawas pinned down 40 years ago.
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