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After evidence was obtained in the 1920s that the universe is expandin

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Re: After evidence was obtained in the 1920s that the universe is expandin [#permalink]

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New post 06 Oct 2017, 07:05
nitya34 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.



Passage: Expanding Universe

Question: Weaken

The Simple Story


The passage first proposes a question: will the universe keep expanding forever, or will it eventually stop? In order for the universe to stop expanding, there would have to be a large amount of invisible matter in the universe. The passage then describes a specific scientific investigation into the presence of invisible matter. The investigation has shown that this “dark matter” is, in fact, present in large quantities, but it is not clear whether there is enough to stop the expansion of the universe.

Sample Passage Map

Here is one way to map this passage. (Note: abbreviate as desired!)

univ. keeps expanding? ← yes, if lots of invis. matter

exper. shows: invis. matter in spiral galaxies

but: enough to stop expansion??

Step 1: Identify the Question

The phrase would be most weakened is unusual in a Reading Comprehension question stem. In the context of a Critical Reasoning problem, it would indicate a Weaken the Argument question. Use that same process to answer this question.

Step 2: Find the Support

In order to answer this question, you’ll need to understand why the authors believe that up to 90% of the mass of the universe is dark matter. Their argument is given in the second paragraph.

Step 3: Predict an Answer

The authors’ argument is based on the rotational velocity and luminosity of spiral galaxies. If a part of a galaxy is rotating more quickly, it has more mass. However, parts of these galaxies that don’t appear to have much mass are also rotating quickly. The authors conclude that there is invisible mass in these parts of the spiral galaxies—it can’t be seen, but it is still there, affecting rotation speeds.

In order to weaken this claim, consider the assumptions that the authors are making. They’ve investigated a specific set of galaxies and found evidence that, in those galaxies, the rotational velocity is higher than expected. In concluding that up to 90% of the universe is dark matter, the authors are assuming several things. They’re assuming that there isn’t some other cause for the unexpectedly high rotational velocity. They’re also assuming that their observations were accurate. Finally, they’re assuming that these spiral galaxies are representative of the universe as a whole. The right answer could attack any of these assumptions.

Step 4: Eliminate and Find a Match

(A) CORRECT. This answer attacks the assumption that spiral galaxies are an accurate representation of the universe as a whole. If there are only a few spiral galaxies in the universe, it doesn’t matter that they contain a lot of dark matter. There would still be a relatively small amount of dark matter in the universe overall.

(B) The authors don’t make any claims about the composition of luminous and nonluminous matter, and there’s nothing in the passage that would link the composition of these types of matter to their prevalence in the universe.

(C) This would actually support the authors’ claim. If this were true, there would be even more nonluminous matter than the authors anticipated.

(D) If the density of the observable universe were greater, this would imply that there was more luminous matter in the universe. However, the authors’ research doesn’t draw conclusions about the overall amount of dark matter in the universe. Instead, they conclude, based on their observations of spiral galaxies, that dark matter represents a certain percentage of the universe. Even if there were much more luminous matter, there might simply be much more dark matter, as well, so that the percentages observed remained consistent—and the authors’ claim would still be valid.

(E) Unless these slow-rotating galaxies would disprove the authors’ claim by containing less dark matter than expected, this doesn’t affect the argument. This answer choice says nothing about the amount of dark matter in the hypothetical slow-rotating galaxies, so it is equally likely to strengthen the argument, weaken it, or have no effect.
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Re: After evidence was obtained in the 1920s that the universe is expandin [#permalink]

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New post 24 Nov 2017, 22:08
Not a 700 level i feel. Took 8 mins 41 secs and got 2 wrong. 1 was a silly mistake. Marked it in a hurry. The other one was due to unclear understanding of the last paragraph.

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Re: After evidence was obtained in the 1920s that the universe is expandin   [#permalink] 24 Nov 2017, 22:08

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