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  3. It is hardly surprising that something called dark energy is hard to study. It is important, though. If it exists at all, it mak

It is hardly surprising that something called dark energy is hard to study. It is important, though. If it exists at all, it mak

admin2010-10-28  26
问题      It is hardly surprising that something called dark energy is hard to study. It is important, though. If it exists at all, it makes up about three-quarters of the stuff in the universe. And if it does not exist, then existing theories of physics will have to be useless.
     The latest evidence that dark energy really does exist was produced on December 16th by Alexey Vikhlinin, of the Smithsonian Observatory in Cambridge, Massachusetts, and his colleagues. They used an orbiting X-ray telescope called Chandra to study the way that groups of galaxies grow. They discovered that this growth is restrained in exactly the way that dark-energy theory predicts.
     The idea of dark energy was dreamed up ten years ago, to explain why the expansion of the universe that began with the Big Bang seems to be accelerating, rather than slowing down. That unexpected finding was the result of studies of supernovas (超新星) whose apparent brightness (and therefore distance) did not match the previous theory. Dark energy causes the acceleration by pushing space itself apart, altering the distances to the supernovas as it does so.
     Dark energy is also a convenient explanation for another curiosity. Geometrically, space is flat. For it to be held that way, rather than getting more and more curved over time, the amount of matter and energy it contains must be at a particular, critical density. Without the dark energy needed to explain the acceleration, the universe would have only a quarter of the necessary density. It is therefore a relief that Dr Vikhlinin’s results agree with the theory: the pushing apart of space that dark energy causes makes it harder than it otherwise would be for galactic clusters(银河星团) to grow.
     This result does not bring physicists much closer to understanding what dark energy actually is. The favored explanation is the so-called universal constant -- an as-yet-unobserved consequence of the general theory of relativity. But that theory predicts a force far more powerful than the one actually seen. So the truth is that physicists are still in the dark.
As for dark energy, what is favored by the physicists to explain it?
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答案particular，critical density
解析 由定位句可知，物理学家们倾向于认为“黑色能量”就是所谓的宇宙恒量，是广义相对论尚未观察到的结果，可知物理学家们喜欢将黑色能量解释成The so-called universal constant。
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