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【简答题】

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) 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.
Where did the physicists get the fact that universe expansion is accelerating

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参考答案:
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【多选题】星团可以分哪两种?()

A.
圆状星团
B.
椭圆状星团
C.
球状星团
D.
疏散星团

【单选题】星团的形成过程是()

A.
星际气体云在自身引力作用下塌缩
B.
黑洞吸积周围行星际物质
C.
双星相互碰撞后吸积周围星际物质
D.
红超巨星吸积物质导致超爆

【单选题】蜂巢星团位于( )

A.
金牛座
B.
天蝎座
C.
后发座
D.
巨蟹座
相关题目:
【多选题】星团可以分哪两种?()
A.
圆状星团
B.
椭圆状星团
C.
球状星团
D.
疏散星团
【单选题】星团的形成过程是()
A.
星际气体云在自身引力作用下塌缩
B.
黑洞吸积周围行星际物质
C.
双星相互碰撞后吸积周围星际物质
D.
红超巨星吸积物质导致超爆
【单选题】蜂巢星团位于( )
A.
金牛座
B.
天蝎座
C.
后发座
D.
巨蟹座
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