请问OG11黄皮第20篇第114题

north7896

    

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.

    

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没有问题 我想知道D错在哪里了 OG上的解释没有看懂 希望各位帮忙看下

    
    

north7896

有人做过嘛

aeoluseros

D说的是密度（一个数值），而原文结论说的是比例（90%）。

north7896

我的理解是density of the observable universe is greater 所以不可见的部分mess减少了 所以就不足90%啦 不知道这样理解为什么不行
请指点

aeoluseros

visible matter和invisible matter的密度共同组成了宇宙的物质密度。

这段背景或许对你有帮助：

大爆炸后的膨胀过程是一种引力和斥力之争，爆炸产生的动力是一种斥力，它使宇宙中的天体不断远离；天体间又存在万有引力，它会阻止天体远离，甚至力图使其互相靠近。引力的大小与天体的质量有关，因而大爆炸后宇宙的最终归宿是不断膨胀，还是最终会停止膨胀并反过来收缩变小，这完全取决于宇宙中物质密度的大小。

理论上存在某种临界密度。如果宇宙中物质的平均密度小于临界密度，宇宙就会一直膨胀下去，称为开宇宙；要是物质的平均密度大于临界密度，膨胀过程迟早会停下来，并随之出现收缩，称为闭宇宙。

问题似乎变得很简单，但实则不然。理论计算得出的临界密度为5×10－30克/厘米3。但要测定宇宙中物质平均密度就不那么容易了。星系间存在广袤的星系间空间，如果把目前所观测到的全部发光物质的质量平摊到整个宇宙空间，那么，平均密度就只有2×10－31克/厘米3，远远低于上述临界密度。

然而，种种证据表明，宇宙中还存在着尚未观测到的所谓的暗物质，其数量可能远超过可见物质，这给平均密度的测定带来了很大的不确定因素。因此，宇宙的平均密度是否真的小于临界密度仍是一个有争议的问题。不过，就目前来看，开宇宙的可能性大一些。

north7896

3kx a lot