Word: hydrogenating
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Dates: during 2000-2009
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...agree, gravity was the only force at work. Regions of higher density drew matter to them, becoming denser still--a pattern preserved to this day in the distribution of galaxies, with huge clusters where there were high-density regions back then and great voids in between. Eventually, clouds of hydrogen became so dense that their cores ignited with the fires of thermonuclear reactions--the sustained hydrogen-bomb explosions, in essence, that we know as stars. But whereas the familiar stars of the Milky Way are mostly similar in mass to the sun, these first stars were, on average, gigantic...
Because they were so hot, the first stars would have poured out not just visible light, but also copious amounts of high-energy ultraviolet radiation. One effect of that radiation would have been to knock apart hydrogen atoms, thus destroying their ability to block light. That process is known as reionization, and those stars, forming perhaps 100 million years into the Dark Ages, or roughly at the era's midpoint, might have rendered the universe transparent on their own if they had lived long enough. But unlike the sun, which has survived 5 billion years so far and should live...
...other fusion reaction, the fires that powered these short-lived stars worked by forcing simple hydrogen and helium atoms to meld into heavier, more complex elements. The stars that died explosively spiked the surrounding gas clouds with elements like oxygen and carbon, which had never existed before. Billions of years later, the elements forged in stars like these would be assembled into planets, organic molecules and, ultimately, human beings. At the time, though, they served simply to change the chemistry of the clouds, allowing them to collapse into far smaller objects than they could before. The second generation of stars...
...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...
Still other telescopes will be trying to take pictures not of the first stars and galaxies, but of the clouds of hydrogen atoms they formed from and that they eventually destroyed. The hydrogen atoms emitted radiation too, in the form of radio waves, and several competing projects in various stages of completion in India, China, the Netherlands and Australia are being designed to see them. The last, known as the Mileura Widefield Array, is considered the most promising because its 500 separate antennas will be located on a remote cattle station in western Australia, far from any interference from earthly...