Word: lightness
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...this radiation--the ultraviolet light from hot, newly formed stars--that many theorists suspect finally reionized the remaining hydrogen, making it transparent again and bringing the Dark Ages to a close. Others suggest that the process may have been powered instead by black holes spewing out X-rays and ultraviolet light. Or it may have been a combination of hot stars and black holes that cleared the hydrogen and put an end to the Dark Ages...
...spot the earliest objects, however, astronomers will have to stop looking for ordinary light. The universe has expanded vastly since its earliest days--but it isn't that galaxies and other objects are flying apart. Rather, it's that space itself has been stretching--a difficult concept even for a physicist to grasp, but which must be true according to the equations of relativity. Cosmologists say you should imagine the universe as a balloon with dots painted on its surface. As the balloon inflates, the dots will get farther apart--not because they're sliding around but because the balloon...
That being the case, a light beam traveling through expanding space is stretched as well, its wavelength getting longer as it goes. Long-wavelength light is red; stretch it out longer and it becomes infrared light and then microwaves and, finally, long-wavelength radio waves. The flash that came from the Big Bang started out as visible light; by now, 13.7 billion years later, it's still streaming through space, but it has been stretched so much that astronomers have to use microwave antennas to detect it. The earliest galaxies came after the Big Bang, so their light...
...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...
...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...