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该题平均耗时:2m32s,平均正确率:54.7% 添加做题笔记

After evidence was obtained in the 1920s that the universe is expanding, it became reasonable to ask: Will the universe continue to expand indefinitely, or is there enough mass in it for the mutual attraction of its constituents to bring this expansion to a halt? It can be calculated that the critical density of matter needed to brake the expansion and “close” the universe is equivalent to three hydrogen atoms per cubic meter. But the density of the observable universe—luminous matter in the form of galaxies—comes to only a fraction of this. If the expansion of the universe is to stop, there must be enough invisible matter in the universe to exceed the luminous matter in density by a factor of roughly 70.

Our contribution to the search for this “missing matter” has been to study the rotational velocity of galaxies at various distances from their center of rotation. It has been known for some time that outside the bright nucleus of a typical spiral galaxy luminosity falls off rapidly with distance from the center. If luminosity were a true indicator of mass, most of the mass would be concentrated toward the center. Outside the nucleus the rotational velocity would decrease geometrically with distance from the center, in conformity with Kepler’s law. Instead we have found that the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly. This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous mass.

Our findings suggest that as much as 90 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth. Such dark matter could be in the form of extremely dim stars of low mass, of large planets like Jupiter, or of black holes, either small or massive. While it has not yet been determined whether this mass is sufficient to close the universe, some physicists consider it significant that estimates are converging on the critical value.

The authors’ study indicates that, in comparison with the outermost regions of a typical spiral galaxy, the region just outside the nucleus can be characterized as having

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网友解析

当前版本由 kisenana 更新于2015-11-02 15:14:42感谢由 kisenana 对此题目的解答所做出的贡献。
定位到 It has been known for some time that outside the bright nucleus of a typical spiral galaxy luminosity falls off rapidly with distance from the center. 。。。Instead we have found that the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly. 
问的是靠内部的亮度和速度问题
越往外面,亮度越低;越往外面,速度不变或者略高,
180度旋转,选一个就可以了,注意题目问的是anthour的观点,所以并不是选假设只有明亮的是质量,开普勒理论下的速度猜测,
选D


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    老再

    直接题读错
    3楼  | 2018-09-01 22:18:41 回复
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    苏利文的欧尼

    题目在问和远离中心区域相比,靠近中心区域的核心有什么特征,原文第二段先说了luminosity的特征是越远越会下降,又说了根据开普特定律的rotation是越远越下降,但是用instead转折,观测到的实际情况是rotation没有太大的变化。
    2楼  | 2017-12-06 04:14:58 回复
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    歌五声平

    选的不是理论情况(c)而是实际情况D
    1楼  | 2017-08-21 00:07:40 回复
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