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The discovery that the universe is expanding was one of the great intellectual revolutions of the twentieth century. With hindsight （回过头看）, it is easy to wonder why no one had thought of it before. Newton, and others,should have realized that a static （静止的） universe would soon start to shrink under the influence of gravity. But suppose instead the universe is expanding. If it was expanding fairly slowly, the force of gravity would cause it eventually to stop expanding and to start shrinking. However, if it was expanding at more than a certain critical rate, gravity would never be strong enough to stop it, and the universe would continue to expand forever.
This is a bit like what happens when one fires a rocket upward from the surface of the earth. If it has a fairly low speed, gravity will eventually stop the rocket and it will start falling back. On the other hand, if the rocket has more than a certain critical speed, gravity will not be strong enough to pull it back, so it will keep going away from the earth forever.
This behavior of the universe could have been predicted from Newton’s theory of gravity at any time in the nineteenth, the eighteenth, or even the late seventeenth centuries. Yet so strong was the belief in a static universe that it persisted into the early twentieth century. Even Einstein, when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static that he modified his theory to make this possible, introducing a so-called cosmological constant （宇宙常数） into his equations. When Hubble’s study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his “greatest mistake”.
42. What does the passage mainly talk about?
A.The discovery of the expanding universe theory.
B. The differences between two universe theories.
C.Scientists’ unwillingness to abandon a widely accepted theory.
D.Scientists’ doubts about the theory that the universe is expanding.
43. Which of the following can be inferred about the expanding universe?
A. It did not agree with a theory widely accepted in the nineteenth century.
B.Most scientists nowadays believe that the idea is no longer acceptable.
C.The existence of gravity makes it impossible for the universe to expand.
D.nstein discovered it when working with the general theory of relativity.
44. Why does the author mentions Newton?
A.To show respect for his great scientific work of gravity.
B. To provide evidence that the universe is not expanding.
C.To give one example of a supporter of the expanding universe theory.
D.To show that the expanding universe theory might have been discovered earlier.
45. What can we know from the last two paragraphs?
A.The author mentions a rocket to prove the theory of a static universe.
B.The force driving the universe to expand is similar to that driving the rockets upward.
C.Einstein used the term“cosmological constant” to challenge Newton’s gravity theory.
D.nstein’s mistake is used to show the author’s doubt whether the universe is expanding.
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