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# OG 12 RC : Q117

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OG 12 RC : Q117 [#permalink]

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12 Nov 2011, 08:12
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.

Q1
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.

OA: A

I don't understand why D is incorrect. If the density of the observable universe is twice previously estimated, then the suggestion of author would be that - "as much as 45 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth"

Another issue with the option A is that it attacks the premise of the argument whose conclusion is -"as much as 90 percent ....Earth". In GMAT CR we are supposed to assume that GMAT writers don't make errors in premises. Why it has been assumed that authors didnot consider the fact of the frequency of distribution of the spiral galaxies.
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Manager
Joined: 15 Mar 2009
Posts: 147
Concentration: Finance, Entrepreneurship
Schools: UCLA (Anderson) - Class of 2014
GMAT 1: 710 Q49 V35
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Kudos [?]: 12 [0], given: 51

Re: OG 12 RC : Q117 [#permalink]

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07 May 2012, 00:57
I think that the question is very tough and complicated.

This is the context of the question extracted from the reading passage:"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."

I think that D is wrong because it addresses "previous estimates", which is out of scope of the question. The question specifically addresses "our findings", which are the findings concluded from research carried out by the group of researchers who were the authors of the passage.

So to deny the result of the findings concluded by the researchers 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”, we should point out that there is something wrong about the very findings carried out these researchers, not any previous, unrelated sources of estimates.

Hope that helps.
Manager
Joined: 15 Mar 2009
Posts: 147
Concentration: Finance, Entrepreneurship
Schools: UCLA (Anderson) - Class of 2014
GMAT 1: 710 Q49 V35
Followers: 2

Kudos [?]: 12 [1] , given: 51

Re: OG 12 RC : Q117 [#permalink]

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07 May 2012, 02:06
1
KUDOS
To be honest, this is a very tough question. The line of reasoning is hard to establish. The conclusion's wording is hard to understanding. I don't think that I would be able to get a right answer to such a question within 1 minute on a GMAT exam. During my revision, it took me about 30 minutes to decode the reading passage and available choices and to pick the right choice.

This is the answer from OG12: The authors’ conclusion about non-luminous matter is based on their study of the rotational velocity of spiral galaxies. If spiral galaxies were found to be atypical of galaxies, then it would be possible that, in those other galaxies, nonluminous matter does not increase as luminous matter decreases. If this were the case, the authors’ conclusion would be based on a sample of galaxies not representative of the whole, and their argument would be seriously weakened.

This question is like an CR question. To weaken a conclusion, basically, you need to weaken its assumptions, its hypothesis, and evidence that support a conclusion.

The conclusion deals with 3 issues: - Spiral galaxies. - Non-luminous matters. - Spiral velocity of non-luminous matters.

Conclusion: You can paraphrase the conclusion addressed by the question: Non-luminous matters are discovered and their mass is calculated by their spiral velocity. Their mass accounts for 90% of the total mass of the universe.

Line of reasoning: study spiral galaxies --> calculate mass of non-luminous matters through their spiral velocity --> non-luminous matters account for 90% of total mass of spiral galaxies --> non-luminous matters account for 90% of total mass of the universe.

Assumption: Spiral galaxies are assumed to be representative of the whole universe, because the authors draw a conclusion of the universe based on spiral galaxies.

A directly attacks the assumption. So A is the most correct answer.

B is obviously out of scope. C and E seem to relate but C doesn't weaken the conclusion and E doesn't directly and strongly attack the line of reasoning as A does.

C doesn't weaken the conclusion, it just adds additional hypothesis. Even if non-luminous matters exist alongside with luminous matters, their spiral velocity could still be calculated. These non-luminous matters may already be counted in the 90% mass already. Non-luminous mass increases as luminous mass decreases: This finding does not rule out that the nucleus contains some non-luminous mass; there is a chance that the argument is not affected.

E could be a right answer. It could be argued that the researchers haven't taken into account the non-luminous matters whose velocity could not be calculated, thus, underestimate the mass of non-luminous matters. However, E doesn't directly attack the line of reasoning of the researchers as A does. Therefore, E is not as strong as A in weakening the researchers.
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After evidence was obtained in the 1920s that the universe  [#permalink]

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21 Jun 2013, 03:42
Complete discussion is given at the below mentioned link
after-evidence-was-obtained-in-the-1920s-that-the-universe-83242.html#p624026

AbhiJ 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.

Q1
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.

OA: A

I don't understand why D is incorrect. If the density of the observable universe is twice previously estimated, then the suggestion of author would be that - "as much as 45 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth"

Another issue with the option A is that it attacks the premise of the argument whose conclusion is -"as much as 90 percent ....Earth". In GMAT CR we are supposed to assume that GMAT writers don't make errors in premises. Why it has been assumed that authors didnot consider the fact of the frequency of distribution of the spiral galaxies.

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After evidence was obtained in the 1920s that the universe   [#permalink] 21 Jun 2013, 03:42
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