再问OG12 关于宇宙膨胀的文章

加州女孩

OG12117
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.

117. The authors’ suggestion 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” (lines 34–37) would be most weakened if
which of the following were discovered to be true?
(A) Spiral galaxies are less common than types of
galaxies that contain little nonluminous matter.
(B) Luminous and nonluminous matter are
composed of the same basic elements.
(C) The bright nucleus of a typical spiral galaxy also
contains some nonluminous matter.
(D) The density of the observable universe is
greater than most previous estimates have
suggested.
(E) Some galaxies do not rotate or rotate too slowly
for their rotational velocity to be measured
首先，请问这篇文章的思路到底是怎样的啊？每段之间到底什么关系，怎么感觉段落之间讲的东西都没什么关系啊？
其次，117题真是百思不得其解，，，，请大牛啊！！！

shirryzhan

OG答案里说的conclusion based on their study of rotational velocity 是与解题没有太大关系，只是赘述了下 ‘90%暗物质论’的研究由来（因为spiral galaxies的rotational velocity不随着其与内核nucleus的距离而减少，说明在外围有大量看不见的暗物质维持着外围rotational velocity ）。  这只是理论推理的逻辑链条，和解此题完全无关。
着眼点应该在于‘a typical spiral galaxy’, 因为如果spiral 星系不典型，（宇宙中spiral星系数量少），就不会有那么多暗物质了。（因为暗物质组成spiral星系）

chillstone

自己有点想明白了。
问题问的是什么条件成立那么“世界上百分之90的物质是暗物质”就不成立了。
作者是通过观察spiral galaxy来推出结论的，那么如果spiral galaxy只是一个特例，那么这样的推论就不能成立了。

真是笨，想明白了才看懂ls各位的解释

chillstone

我也是这道题有问题。OG12的解释是说
The authors' conclusion about nonluminous matter is based on the study of the rotational velocity of spiral galaxies.....
但是整个题干里面只提到这个东西看不见，没法被探测到，没有说关于速度的阿。。。
搞不懂。

twoflying

今天做了这道题。虽说做错了，但多少也明白这类weakened题型的解法了。不知道对不对，至少能说服自己：）

我的逻辑是：文章在第20行提到“typical spiral galaxy",就是说search的对象具有普遍性，即spiral galaxy是galaxies的普遍或典型代表，
spiral galaxy具有的特征也就具有普遍性（common),如果spiral galaxy 不具有普遍性（less common)，是个特例，那么文章得出任何结论的基础就会被削弱（weakened)或者不存在了。

bsbandjay2

很明显是选A啦 宇宙有无数个galaxy组成 如果由较多暗物质组成spiral galaxy是个别现象的话 即宇宙中大部分galaxy都只含有少量的暗物质的话 那么题干中的“组成宇宙的物质中 有90%以上为暗物质的说法”就明显不可能成立了 也可以理解为被削弱了

zhangxinrui3

如果按照特殊spiral galaxy不能推广到所有universe, 答案A的说法应该是：galaxies that contain (little) nonluminous matter. 文章一直指出是有nonluminous matter去平衡luninou matter, 为什么答案是little nonluminous matter, 即和没有nonluminous matter的galaxies去类比呢？

Michelle_yu

额，不好意思，我还是没明白116题，D选项中的higher luminosity是怎么看出来的呀？文中不是一直再说luminosity falls off么

太笨了

加州女孩

pterosaur 的逻辑很清晰，看的很明白哦，谢谢咯~

pterosaur

第一段 提出宇宙是继续膨胀呢 还是停止膨胀了，这取决于是否有 invisible matter的存在！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.
第二段 invisible matter 存在吗？存在，This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous mass.
第三段 invisible matter是什么东西？最后宇宙是膨胀还是停止，作者并没有给出答案。只是说converging on the critical value.