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RC OG 13(After the evidence obtained)

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gocoder Master | Next Rank: 500 Posts Default Avatar
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Target GMAT Score:
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RC OG 13(After the evidence obtained)

Post Thu Nov 24, 2016 9:51 am
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.

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.

I was stuck between A and D. Please help

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Post Fri Nov 25, 2016 8:04 am
gocoder wrote:
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.

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.

I was stuck between A and D. Please help
Much of this passage is about the concentration of luminous v nonluminous (dark) matter in the universe.

Paragraph 2, boiled way down, tell us this: researchers have concluded that there is a great deal of nonluminous (dark) matter in spiral galaxies. This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous
mass.

So spiral galaxies have lots of nonluminous matter. The claim in lines 34-37 is that as much as 90% of the universe's mass is nonluminous. This makes sense if the composition of spiral galaxies is representative of the composition of all galaxies in the universe.

Now look at answer choice A: Spiral galaxies are less common than types of
galaxies that contain little nonluminous matter.

Well, if there are other, more common, galaxies that have little nonluminous matter, then the conclusion that the universe is mostly nonluminous matter is a good deal weaker. Answer is A

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