[問題]OG11-Q115

[dibert8]

Line After evidence was obtained in the 1920's 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
(40) 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,
(45) 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.

115. 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.


Ans: (C)

根據文章,既然 (line 7~9) DOM = 3 atoms/m^3 宇宙就停止擴張, DOM << 3 atoms/m^3 宇宙老早就已經停止擴張. 3 atoms/m^3 是臨界密度, DOM 在 3 atoms/m^3 以下顯然'不可能'.

OG 的解釋不合邏輯,看不懂也!
This question refers to the first paragraph, where the authors explain that the critical density of matter needed to brake the expansion and "close" the universe is equivalent to three hydrogen atoms per cubic meter (lines 7-9). If the density is significantly less, then the universe will not "close" but continue to expand indefinitely.

這一題以正常邏輯的觀點切入,像是雞同鴨講.不知從哪個角度看比較看得出在問什麼? 又怎麼讓答案合理化?

[kotokolin]

老大, 您是累了嗎? 怎麼會有一下子說DOM=3 又說DOM<<3 , 真把我搞糊塗了~

7-9行說的只有DOM=3的時候, 宇宙就不會繼續擴張下去
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.

所以題目問我們<3會發生啥事, 就像OG說的, 不用想太多, 爽快的選C
(C) The universe would continue to expand indefinitely. 宇宙會繼續無限擴張

[dibert8]

其它選項其實都不對,題目讓人不得不選 (C)

我想邏輯是這樣:在宇宙到達臨界密度時並沒有停止擴張(所以臨界密度停止擴張的假說不成立), DOM 持續下降到遠低於臨界密度.

所以做 GMAT 題目不能太'孤意',不然會被自己打敗.

[rurulu]

有人可以幫忙解釋這一題嗎? 看不懂OG的解釋。

114. 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 39-42) 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.

答案是：A

[larry.tw]

(A)Spiral galaxies不能代表所有galaxies(作者假設所有galaxies都一樣)
(B)與composition 無關
(C)..................................(小弟不會解釋)
(D)與density無關
(E)與Rotation無關

哪位大大來幫忙解C一下喔~~Thanks

[Philosophia]

114.
(C) 在 paragraph 2 的最後一句話中 This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous mass. 表示 luminous mass 與 nonluminous mass 呈反比關係．但這題主要是在問 nonluminous mass，所以應是無關吧～