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OG RC 117 ......Line After evidence was obtained in the 1920

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conquistador Master | Next Rank: 500 Posts
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OG RC 117 ......Line After evidence was obtained in the 1920

Post Wed Apr 08, 2015 3:16 am
Line 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) velocitywould decrease geometricallywith 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.
Ourfindings 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.

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.

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Post Mon May 11, 2015 11:08 am
You bring up a great point - there is a good deal of overlap between RC and CR. Broadly speaking, RC is testing your comprehension of subject matter whereas CR is testing your ability to follow lines of reasoning. However, there are several major overlaps, starting with the one you mentioned:

- Some RC questions will ask you to strengthen or weaken a contention or theory in the passage, as this one did. You should treat these in much the same way that you treat CR: find the conclusion/contention/theory, find the evidence that supports it, and see what's missing. This question type is not very common, though, based on evidence from OG13/2015 (only 4 questions out of 139). Here are those 4:
#43
#85
#113
#117

- inference questions on RC and draw conclusion / inference / "what must be true" questions on CR both ask you to do the same thing: draw logical connections about something unstated based upon what is stated. These two question types are often very similar, and are both very common.

- there is a very rare question type often described as "mimic the argument" that can show up in either RC or CR: "which of the following would be most analogous to the way in which..." All our research suggests that these questions are probably being phased out, though: http://www.manhattanprep.com/gmat/forums/mimic-the-argument-question-type-t13893.html

Hope this helps!

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andymal Junior | Next Rank: 30 Posts Default Avatar
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Post Mon May 11, 2015 9:15 am
Thanks for the breakdown Ceilidh. Correct me if I'm wrong, but this question seems oddly like a critical reasoning question? We need to find the assumption, then pick the option that undercuts it. How often are CR like questions in RC?

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Post Mon Apr 20, 2015 7:53 am
Whenever you're asked about a specific set of lines in RC, make sure to look several lines before and several lines after the quoted portion.

The entire paragraph before the quoted portion is telling us that scientists have been "the rotational velocity of galaxies," and specifically, what we know to be true about "a typical spiral galaxy." The observations of these spiral galaxies indicate that they're not just made of luminous material: it's "balanced by an increase in nonluminous mass."

The author moves from that statement to the assertion in the next paragraph that "our finding suggest that as much as 90 percent of the mass of the universe is [nonluminous]." This is a very common structure for an RC science passage:
paragraph 1: A scientific question to be answered
paragraph 2: Research findings / data
paragraph 3: Conclusions / implications drawn from those findings

"Our findings suggest" means that we're discussing the conclusions/deductions drawn from the findings. If the author has made a conclusion about the whole universe based on data about spiral galaxies, then we can infer that the findings about the spiral galaxies give us representative information about the whole universe.

If we want to weaken this conclusion, we have to disrupt the connection from findings --> conclusion. What if information about spiral galaxies was not typical? Then we couldn't draw a conclusion about the entire universe.

(A) Spiral galaxies are less common than types of galaxies that contain little nonluminous matter.
Bingo! If this were true, then data about spiral galaxies would not allow us to draw conclusions about the universe as a whole.

(B) Luminous and nonluminous matter are composed of the same basic elements.
We don't care what they're composed of. We only want to know if our findings lead to the conclusion stated.

(C) The bright nucleus of a typical spiral galaxy also contains some nonluminous matter.
"Some" nonluminous matter at the nucleus doesn't matter. This wouldn't disrupt our conclusion about nonluminous matter outside of the center.

(D) The density of the observable universe is greater than most previous estimates have suggested.
This has no bearing on whether 90 percent is non-observable.

(E) Some galaxies do not rotate or rotate too slowly for their rotational velocity to be measured.
The word "some" is too nebulous here. It could mean just a handful out of billions, and thus have no bearing on the general deduction made.

The correct answer is A.

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