5.4

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


115. The passage is primarily concerned with
(A) defending a controversial approach
(B) criticizing an accepted view
(C) summarizing research findings
(D) contrasting competing theories
(E) describing an innovative technique

116. The authors' study indicates that, in comparison with
the outermost regions of a typical spiral galaxy, the
region just outside the nucleus can be characterized
as having
(A) higher rotational velocity and higher luminosity
(B) lower rotational velocity and higher luminosity
(C) lower rotational velocity and lower luminosity
(D) similar rotational velocity and higher luminosity
(E) similar rotational velocity and similar luminosity

117. 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 34–37) 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.

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.


119. The authors propose all of the following as possibly
contributing to the "missing matter" in spiral galaxies
EXCEPT
(A) massive black holes
(B) small black holes
(C) small, dim stars
(D) massive stars
(E) large planets