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  3. Before there were black holes and stars and galaxies, the universe went through a period just after the big bang known as the Da

Before there were black holes and stars and galaxies, the universe went through a period just after the big bang known as the Da

admin2015-02-12  74
问题     Before there were black holes and stars and galaxies, the universe went through a period just after the big bang known as the Dark Age of the Universe that lasted until evolving structure ultimately led to the first stars. Jordi Miralda-Escude, an astronomer at Ohio State University in Columbus, describes the Dark Age as the epoch when the universe had become completely dark as it gradually cooled following the big bang and "all the matter that forms us today was spread around space and uniformly distributed."
    Little differences in the density of matter that had been present since the beginning of the universe gradually grew larger as a result of gravitational attraction. Eventually the density was large enough to cause a gravitational collapse and the first star was born. These first stars, according to Miralda-Escude, were likely quite massive and when they exploded as supernovae they littered the universe with atoms heavier than hydrogen and helium which were required for the formation of planets in the next generation of stars. "In the beginning, the universe contained only hydrogen and helium, but at the present time planets like the Earth are made of heavier atoms," he said. "They all come from stars. So all the atomic nuclei on which life is based—except for hydrogen—were made in stars."
    One of the curiosities Miralda-Escude is trying to pin down is when the first star was born after the Dark Age began. "Imagine you are an observer floating in space and looking through the universe in the Dark Age and around you it is completely dark," he said. "The temperature is decreasing so the cosmic background light has all shifted to infrared wavelengths, and there is nothing in visual light. Then, all of a sudden you see the first star appear in the distance."
    Miralda-Escude estimates that the first visible star probably appeared about 75 million years after the big bang. Today, the universe is nearly 14 billion years old and contains billions of galaxies, each one with billions of stars.
What caused the differences in the density of matter to grow larger?
选项 A、Gravitational attraction.
B、Little differences in the density of matter.
C、The big bang.
D、Massive stars.
答案A
解析 属事实细节题。第二段首句告诉我们，由于引力，物质密度的差别逐渐增大。
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