本人陷入牛角尖之中 求解救~ og 12 rc 118~宇宙暗物质的题

miffyflysky

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

(5)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

(10) 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

(15)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

(20) 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

(25) 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

(30) 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
(35) 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,
(40) 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.

118.It can be inferred from information presented in the
passage that if the density of the universe were
equivalent to significantly less than three hydrogen
atoms per cubic meter, which of the following would
be true as a consequence?

(A)Luminosity would be a true indicator of mass.

(B)Different regions in spiral galaxies would rotate

at the same velocity.
(C)The universe would continue to expand

indefinitely.

(D)The density of the invisible matter in the
universe would have to be more than 70 times
the density of the luminous matter.

(E)More of the invisible matter in spiral galaxies
would have to be located in their nuclei than in
their outer regions.


答案选C
我明白这里停止膨胀的临界点就是three hydrogen atoms per cubic meter

而在临界点的左右有两种状态
但没有说正在膨胀中的状态是多于还是少于three hydrogen atoms per cubic meter

所以何以判断less than是还未达到临界点呢？

我在这里用了点自己的想法在里面，觉得宇宙膨胀到停止时，密度应该最小，所以题中less than的情况应该超过了宇宙膨胀停止的状态

我知道用自己的常识思想判断不对啊  但确实想不通文中有什么证据证明less than是还在膨胀中的情况


求讨论求教育~




118.It can be inferred from information presented in the
passage that if the density of the universe were
equivalent to significantly less than three hydrogen
atoms per cubic meter, which of the following would
be true as a consequence?

(A)Luminosity would be a true indicator of mass.

(B)Different regions in spiral galaxies would rotate

at the same velocity.
(C)The universe would continue to expand

indefinitely.

(D)The density of the invisible matter in the
universe would have to be more than 70 times
the density of the luminous matter.

(E)More of the invisible matter in spiral galaxies
would have to be located in their nuclei than in
their outer regions.


答案选C
我明白这里停止膨胀的临界点就是three hydrogen atoms per cubic meter

而在临界点的左右有两种状态
但没有说正在膨胀中的状态是多于还是少于three hydrogen atoms per cubic meter

所以何以判断less than是还未达到临界点呢？

我在这里用了点自己的想法在里面，觉得宇宙膨胀到停止时，密度应该最小，所以题中less than的情况应该超过了宇宙膨胀停止的状态

我知道用自己的常识思想判断不对啊  但确实想不通文中有什么证据证明less than是还在膨胀中的情况


求讨论求教育~

纪瀞

做的时候完全没有想过这个问题...第一想法就是膨胀停止之前密度应该是越来越大的吧？
坐等回标准答案！

miffyflysky

我的第一想法恰恰跟你相反呢。。。

miffyflysky

我看到有帖子这样说的：
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

(15)by a factor of roughly 70.

这里说了是enough to,所以应该没到停止时是less than 3H per m2

但我觉得这里的enough to说的是invisible 和 luminous matter 的对比，而不是密度enough to  所以这句话也不能说明什么

所以。。。继续坐等nn

hhleaves

文中开头先说，“宇宙正处在膨胀中”；接着说：“如果想要停止膨胀所需的密度是three hydrogen atoms per cubic meter”；再接着说“但是，现在可观察到的宇宙密度只达到了这个值的一小部分，”。因为语气用but转折了，所以可以推断后面所说的“只达到了这个值的一小部分”是不能使膨胀停止的，所以进而推断，膨胀的状态是小于这个值的。

MiaZhang

要有enough density来停止宇宙的扩张，这个density= three hydrogen atoms per cubic meter
enough：感觉上应该是足够多的

取反：少于这个密度要求，宇宙会继续扩张