og11黄皮-第20篇问题求助113-114

期待蔚蓝

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

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.

113     The passage suggests that the results of the authors' study have changed their ideas about which of the following characteristics of spiral galaxies?

1

I.      II.

The relative luminosity of different regions

2The relative rotational velocity of different regions

3The relative distribution of matter in different regions

A  I    only

B  II  only

C III
                                only

D II and III only

E I,
                            II, and III

答案选D，我想知道3在文中如何看出change，在那里定位distribution of matter?

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.

答案选A，解释这样写到：

The authors' conclusion about nonluminous matter is based on their study of the rotational velocity of spiral galaxies. If spiral galaxies were found to be atypical of galaxies, then it would be possible that, in those other galaxies, nonluminous matter does not increase as luminous matter decreases. If this were the case, the authors' conclusion would be based on a sample of galaxies not representative of the whole, and their argument would be seriously weakened.

A       Correct. The authors' conclusion assumes that spiral galaxies are typical of all galaxies; information calling that assumption into question weakens the argument.

不太明白黄色部分的结论在文中如何提到的，而又怎样推出了A答案？

请大家指点，谢谢！

leepee

以下是引用期待蔚蓝在2006-10-12 8:59:00的发言：

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.

青色部分可以解决113题，

114题一定要注意3个单词：such dark matter.意思既然spiral galaxy都能探测到，more commom nonluminous can  be detected，黄色部分好象没什么关系

[此贴子已经被作者于2006-10-12 22:42:31编辑过]

tootoo5478

如何看出来?

有人解说一下吗?标颜色一样还是看不懂…..

可以的话请解释一下吧…..

这篇文章连看答案都看不懂!

josuellen

我也做错了这两道。大概说说我的理解吧，希望能对你有点点帮助：

113

关键是找对以前的看法change。III是说由于luminosity falloff作者按照Kepler's Law推导出velocity would decrease.然而，事实上作者发现the rotational velocity either remains contant or increasees slightly.就像文中接下来提到的,这个发现是 unexpected result，作者由此推出 luminous mass falloff是balanced by nonluminous mass.

我的理解是：本来发光mass 越外圈越暗，如果mass中仅仅有发光mass这一种成份,则速度应该越外圈越快；但事实上速度保持变化不大，那么发光mass越外圈越慢的原因是什么？作者说应该是nonluminous的balance作用。因此mass中增加了不发光的mass成分。——〉ditribution changed

114

我觉得是范围问题。即，本文作者只分析了spiral galaxy这一种东西，但推出的却是“as much as 90 percent of the mass of the universe is ”整个universe.如果 spiral galaxies占了universe的大部分，则spiral galaxy作为universe的代表可以说得通，但如果像A所说，spiral galaxies在universe中根本不普遍，那他就不能代表universe，因此也就不能用它推出任何关于整个universe的任何结论或推论。所以就weaken了。

自己不太成熟的理解。望牛牛们指正。

danniepop

115题，我不明白，原文到底是想说比3个氢原子多以后宇宙会膨胀还是比3个氢院子少时宇宙会膨胀！

请大虾指点一下，原文哪个地方交代了多会膨胀还是少会膨胀，我只看出来equavalent to 3个氢原子了就不膨胀了。

yanzie

115:

原文‘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. ’也就是密度必须达到three hydrogen atoms per cubic meter的时候，宇宙才会停止膨胀。

115题中说‘significantly less than three hydrogen atoms per cubic meter’所以宇宙会继续膨胀。

关于114：

leepee说要关注such dark matter。。。不理解哦！

josuellen说的意思难道是Spiral galaxies 和types of galaxies that contain little nonluminous matter 是对立比较的2种宇宙类型/形态？可是原文中没有提到galaxies that contain little nonluminous matter 呢。

期待大家一起讨论！

lichabrend

up

charlie6989

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.

想请问instead 是被哪个取代 , 为何前面没说明？？

aeoluseros

instead做副词...其后面的东西（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.

）就是取代前面观点的东西。

charlie6989

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.

所以说（we have found that）是指之前所提到的观点也就是作者观点被取代

是这样看吗？？

纳闷中