【天文学】宇宙是否会一直膨胀？暗物质“Missing Matter ...

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原文：
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(物质临界密度） 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（原子核） 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.
译文：
在20世纪20年代获得宇宙膨胀的证据后，我们有理由问:宇宙会继续无限膨胀吗?或者宇宙中是否有足够的质量，使其组成部分相互吸引，使这种膨胀停止?可以计算出，阻止膨胀和“关闭”宇宙所需的临界物质密度(物质临界密度)相当于每立方米3个氢原子。但可观测到的宇宙的密度——星系形式的发光物质——只占其中的一小部分。如果要停止宇宙的膨胀，宇宙中必须有足够多的不可见物质，其密度大约是发光物质的70倍。
我们对寻找这种“失踪物质”的贡献是研究星系在距离其旋转中心不同距离处的旋转速度。一段时间以来，人们已经知道，在一个典型螺旋星系的明亮核(原子核)之外，亮度随着距离中心的距离而迅速下降。如果光度是质量的真实指标，那么大部分质量将向中心集中。在原子核外，旋转速度随离中心的距离呈几何级数减小，符合开普勒定律。相反，我们发现螺旋星系的旋转速度要么与中心距离的增加保持恒定，要么略有增加。这个出乎意料的结果表明，发光质量随中心距离的衰减被非发光质量的增加所平衡。
我们的发现表明，宇宙中多达90%的质量没有以任何波长的强度辐射到地球上。这种暗物质可能以极暗的低质量恒星、木星这样的大行星或黑洞的形式存在，或小或大。虽然还没有确定这个质量是否足以使宇宙关闭，但一些物理学家认为，对这个临界值的估计正在趋同，这是很重要的。
课代表圈重点：

• 20世纪20年代获得宇宙膨胀的证据之后，开始研究——宇宙会无限膨胀嘛？宇宙中是否有足够的质量，可以使这种膨胀停止？组织膨胀和“关闭”宇宙所需的临界物质密度相当于每立方米3个氢原子。但是现在可观测到的宇宙密度还只占到这个临界密度的一小部分，还有足够多的不可见物质。
  
• 发现标明，宇宙中多达90%质量没有以任何强度的波长辐射到地球上。这种暗物质可能以极暗的低质量恒星、木星这样的大行星或黑洞的形式存在，或小或大。虽然还没有确定这个质量是否足以使宇宙逐渐关闭，但物理学家认为，这些“暗物质”在逐渐趋近于那个临界值。