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

The passage says it is "constant or increases slightly"
Which of the two - lower or similar - capture both the situations?
'Lower' doesn't because it could be constant. But 'similar' works. It could be constant or 'slightly' more so it is similar (not necessarily same.. could be different by a small margin).

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.

This is slightly trickier.

We are told that we studied spiral galaxies and found that their luminosity is mostly in the centre. So we thought that the mass is mostly in centre (and in outskirts there is little mass). But then the velocity in outskirts should have been low. But actually it is same or even higher. This means there is lots of mass in outskirts too though it is not visible (non luminous).
Based on our calculations, we deduced that 90% mass is non luminous.
What if we found that luminous mass is more? To compensate for increased/constant velocity in outskirts, then non luminous mass would be more too proportionally. Since the figure given to us is a percentage of the total, it stays as it is.
If total mass is 100, 10 is luminous and 90 is non luminous.
If we find that it is not 10 but 20, then non luminous mass must be 180 and total must be 200.
Based on our calculations of visible mass, and required non visible mass to match the velocity, we have arrived at 90%. If visible mass is more, non visible would be more too.
Hence, (D) doesn't work.


But what if these kind of galaxies that have luminous as well as non luminous mass are fewer and others have only luminous mass? Then the 90% figure we arrived at using velocity may not work. The non luminous mass may be much lower than 90% because many galaxies would have only luminous mass.
Hence (A) is correct.

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

I am not satisfied with the options choices (B).
here is my reasoning.
The passage stated that OR the author's team stated: "We have found that the rotational velocity (RV) in spiral galaxies (SG) either remains constant with increasing distance from the center or increases slightly" That means, The RV of SG remains the same as the distance increases from the center of the galaxy".
The second point is: "indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous mass" which means, reducing the luminous mass or Simply reduce " Mass" as the distance increase from the center of the galaxy and vice-versa. and that mass is replaced by nonluminous mass(maybe dark matter, dark energy, we still don't know"). That means the luminous Mass is replaced by the Non-luminous mass. I thought does it increase the Non-luminous mass or dark matter mass? yes of course, but how does the correct option choice say " higher luminosity" that means higher the mass of the luminous matter like galactic mass. but the paragraph clearly says its mass is replaced by dark matter.

DEAR GMATNinja ,Could you share your thoughtful insight on question?

Hi S1ny1s,

The question is:

The question is primarily about the region just outside the nucleus. It's not really asking us about the outermost regions.

This statement tells us that, outside the bright nucleus, luminosity drops off rapidly with distance. This means that a region that's very close ("just outside") the bright nucleus will have higher luminosity than a region that isn't as close to the nucleus (like "the outermost regions").

1. The passage is primarily concerned with (C) summarizing research findings.
The passage presents research findings related to the expansion of the universe, the search for "missing matter," and the authors' contribution to studying the rotational velocity of galaxies. It summarizes these findings and their implications. Therefore, the primary concern of the passage is to summarize research findings.
2. 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 (D) similar rotational velocity and higher luminosity.
The passage states that outside the bright nucleus of a typical spiral galaxy, the luminosity falls off rapidly with distance from the center. However, the authors' study found that the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly. This implies that the region just outside the nucleus has similar rotational velocity and higher luminosity in comparison to the outermost regions. Therefore, the correct answer is similar rotational velocity and higher luminosity.
3. 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 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.
The authors' suggestion of "missing matter" in the form of non-radiating mass relies on the assumption that a significant portion of the mass in the universe is not detected due to its lack of radiating intensity. If spiral galaxies were found to be less common than types of galaxies that contain little nonluminous matter, it would suggest that nonluminous matter is not as prevalent in spiral galaxies as initially believed, weakening the authors' suggestion. Therefore, the correct answer is Spiral galaxies are less common than types of galaxies that contain little nonluminous matter.
4. 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? (C) The universe would continue to expand indefinitely.
The passage states that the critical density of matter needed to halt the expansion and "close" the universe is equivalent to three hydrogen atoms per cubic meter. If the density of the universe were significantly less than this critical density, it would imply that there is not enough mass to stop the expansion, and the universe would continue to expand indefinitely. Therefore, the correct answer is the universe would continue to expand indefinitely.
5. The authors propose all of the following as possibly contributing to the "missing matter" in spiral galaxies EXCEPT (D) massive stars.
The passage suggests that the "missing matter" could be in the form of extremely dim stars, large planets like Jupiter, small black holes, or massive black holes. However, it does not propose massive stars as a possible contributing factor. Therefore, the correct answer is massive stars.

 

Hi GMATNinja @DmitryFarber 

From the passage “velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly.", it is also possible that the outer has higher velocity than the inner. Thus, choice B is possible.

Please elaborate how you eliminate choice B and choose choice D over B

Posted from my mobile device­

­Sure, but don't let the phrasing "can be characterized as having" make you think that we're looking for something that COULD be true. The question is asking for an accurate statement, that's all. Which is a correct characterization? We don't know that the velocity always increases; in fact, we know that it may remain constant. If it does increase, it does so only slightly. So D applies correctly to either case: if there's an increase, the velocity is similar, and if there's a slight increase, the velocity is still similar. B, on the other hand, only applies correctly to one of the two cases, and we don't even know how often the increase happens. So while B is something that can be observed, it's not an accurate representation of what happens in general.