OG12 第21篇

jacjoaquin

第15行的 by a factor of roughly 70.是什么意思？

弱智问题……求解……

jacjoaquin

贴上原文，红色字体部分就是想问的地方：
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.

ericafdl

70应当指的是density

jiaoyu

70 means 70 times of the density of the luminous matter