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What makes Mileura and the other projects so powerful is that by tuning the receivers to different radio frequencies, they will be able to pick up signals broadcast by hydrogen atoms at different periods in the Dark Ages. When you map cosmic hydrogen at, say, 50 million years after the Big Bangbefore the first stars had a chance to formthen at 100 million, 200 million or half a billion years later, you get a series of snapshots. Combine them, says Loeb, and "you'll be able to make a 3-D picture of hydrogen gas as the universe evolves. At some point, you'll start to see holes, like Swiss cheese," as the gas clouds become ionized and transparent. Precisely how the holes grow and merge over time will help determine whether the clearing out is being done by small galaxies, big black holes or something entirely different--and depending on which it is, some theorists could be vindicated and others refuted. But astronomers will at last have an answer to the mystery they have puzzled over for a decade and a half.
In the meantime, observers have been chipping away at this mystery as best they can with the tools they have. One important clue comes from the observation of the most distant quasars--objects believed to be giant black holes swallowing huge volumes of gas at the cores of young galaxies. The Sloan Digital Sky Survey, a comprehensive scan of the heavens, has turned up several of these from about a billion years after the Big Bang. By watching how the light of quasars is altered by surrounding gas, astronomers have concluded that there was still some atomic hydrogen around then, although not much.
But the WMAP satellite, launched to look for light left from the Big Bang using a broadly analogous technique, determined the clearing out of hydrogen between the stars was well under way much earlier, just half a billion years post--Big Bang. "Theorists have been telling us that it should have happened fairly quickly once it began," says Michael Strauss, a Princeton University astronomer and deputy project scientist for the Sloan survey. "But the observations may be telling us otherwise."
Indeed, observations often take theorists by surprise. Last fall a focus on one tiny region of the universe by the Hubble, the Spitzer space telescope (which operates in infrared wavelengths) and the European Space Organization's ground-based Very Large Telescope in Chile revealed the existence of a galaxy dating to about 1 billion years after the Big Bang that was far larger and more mature looking than the primordial dwarf galaxies everyone assumed they would see. "It was unexpected," admits Mark Dickinson of the National Optical Astronomy Observatory, in Tucson, Ariz., who worked on the project. "But maybe it shouldn't have been." The theorists might have things all wrong. But it could also simply be that any population will have a few individuals that are way outside the average--humans who stand over 7 ft. tall, for example. They're very noticeable but not at all typical.