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.
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.
正确答案是C
我不太理解density和expansion的关系，我认为越expand，density应该会越小才对吧？expand是膨胀的意思吧？体积变大了密度当然会小了，相应的只有密度大于标准密度（three hydrogen atoms per cubic meter）时才会膨胀，为什么题目中密度小反而更膨胀呢？不理解。。。、
谢谢作者: goodfish 时间: 2010-4-7 07:00
定位回原文:
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.
原文说,只要不达到three hydrogen atoms per cubic meter, 就会持续的expansion.
定位到以下原文：
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.
能够抑制宇宙扩张的物质的必须具备以下要素：density is equivalent to three hydrogen atoms per cubic meter.
按照题目所说，density is substantially less than three hydrogen atoms per cubic meter，宇宙的扩张就得不到抑制。
