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  3. Astronomers believe that the expanding univers is the result of an enormous and powerful explosion catled the Big Bang. The Big

Astronomers believe that the expanding univers is the result of an enormous and powerful explosion catled the Big Bang. The Big

admin2018-09-25  52
问题     Astronomers believe that the expanding univers is the result of an enormous and powerful explosion catled the Big Bang. The Big Bang theory may explain how the universe formed. The Big Bang theory states that the universe began to expand with the explosion of concentrated matter and energy and has been expanding ever since. According to the theory, all the matter and energy in the universe was once concentrated into a single place. This place, of course, was extremely hot and dense. Then some 15 to 20 billion years ago, an explosion - the Big Bang - shot the concentrated matter and energy in all directions. The fastest moving matter traveled farthest away. Energy, too, began moving away from the area of the Big Bang.
    If the Big Bang theory is correct, the energy left from the Big Bang will be eventy spread out throughout the universe. This energy is known as background radiation. And indeed, scientists have discovered that the background radiation is almost the same throughout the entire universe. The constant background radiation is one observation that supports the Big Bang theory. After the initial Big Bang, the force of gravity began to affect the matter racing outward in every direction. Gravity is a force of attraction between objects. All objects have a gravitational attraction. This force of gravity began to pull matter into clumps.
    At some time, the clumps forned huge clusters of matter. These clumps became the galaxies of the universe. But even as the galaxies were forning, the matter inside the galaxies continued to race away from the area where the Big Bang had occurred. And this is just what astronomers have discovered. All of the galaxies are spending away from the center of the universe.
According to the passage, galaxies were formed because of______.
选项 A、the expansiion of the universe
B、the contraction of the universe
C、background radiation
D、the force of gravity
答案D
解析 推理判断题，由文章第二段第三段可知，银河系的形成是因为重力。故答案选D。
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