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

This OG RC passage has the first two questions on which I don't agree with the OA.

116.
Passage says: Velocity either remains constant with increasing distance from the center or increases slightly.
Question: compare "outermost regions" vs "region just outside the nucleus".
=> "region just outside the nucleus" could have either similar or lower velocity.
=> Both B and D could be right; a case for D has strong foundations given that we are comparing the "region just outside the nucleus" with the "outermost region" -> greatest possible distance withing the galaxy.

117.
OA is A. I believe A is wrong. Explanation:
"Galaxies that contain little nonluminous matter" = dark matter (in the form of either dim stars, black holes or large planets). Thus, spiral galaxies being less common than "obscure" galaxies does not weaken the 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".
D is the best answer. If the density of the observable universe (small fraction) is increased, that 90% would be lower; increase the fraction significantly, and you can have an observable reduction in that 90%.

I still do not understand Q 117
mikemcgarry please clear my doubt here.

In option A) Spiral galaxies are less common than types of galaxies that contain little nonluminous matter.
It could be very possible that the researchers already took this into account. The passage states that the researchers studied typical spiral galaxies and through their studies concluded that 90% percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth.

Researchers line of reasoning - spiral galaxies are less common --- spiral galaxies research suggest that there's x amount of non-luminous mass --- considering everything, this accounts to 90%.
The passage never mentions that the researchers assumed that spiral galaxies were very common. How can we assume that they wrongly estimated the proportion of spiral galaxies.

Notice that the author's assertion is not very precise: "as much as 90 percent of the mass . . . " So if we find that the observable universe is denser than we thought, it wouldn't really undermine this vague assertion. We would just adjust the maximum down by some unknown amount. Furthermore, the author's team is not basing their assertion on current estimates of the density of the observable universe, but on the observation of galaxies. This is why (A) presents a problem for this view. If it turned out that spiral galaxies like ours, which are rich in nonluminous matter, were uncommon, this would undermine the entire basis of the author's idea.

Hello friend, it is a combination of this two parts:



and...

@emmafoaster, check out this sentence in paragraph 2: "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."

This implies that the luminosity is highest in the nucleus, a bit lower outside the nucleus, and a whole lot lower as you get toward the edge.

Hi sayantanc2k,
Hope you're well brother. I've stuck with question# 118. The passage says that if the density is equivalent to 3 hydrogen atoms per cubic meter, then the expansion of the universe will be stop. That means: if the density is LESS than 3 hydrogen atoms per cubic meter, then expansion will be be continued, WHICH is answer option C (the correct choice). But, my question is: if the density is MORE THAN 3 hydrogen atoms per cubic meter, then the expansion of the universe will STILL be continued?
Thanks...

iMyself, there's no support for the conclusion that expansion would continue if the density were higher than the needed amount. If I said that you needed $10,000 to open an investment account or that you needed a 3.0 GPA to apply to a program, you wouldn't assume that you were out of luck if you had $50,000 and a 4.0, right?

If the intended meaning were that the density needed to be exactly three hydrogen atoms per cubic meter, the passage would need to say that.

That's a bit too long, but it depends in part on how your time was allocated. Keep in mind that the GMAT will only give you 3-4 questions per passage, so doing 5 as a timed set is not realistic. Aim to read the passage & do 3-4 questions in 6-8 minutes. Then time the other questions separately, aiming to average 1 minute per question.

Hi Experts / GMATNinjaTwo

Q116:
As per passage:
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
which implies: as distance increases form center / nucleus, luminosity decreases.

Then why OA is D, I am clear about rotational velocity

Yes, as distance from the center increases, luminosity decreases. This is consistent with choice (D): In comparison with the outermost regions of a typical spiral galaxy, the region just outside the nucleus can be characterized as having higher luminosity. In other words, the luminosity is lower in the outermost regions than the luminosity just outside the nucleus.

We also know "that the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly." This is also consistent with choice (D): In comparison with the outermost regions of a typical spiral galaxy, the region just outside the nucleus can be characterized as having similar rotational velocity.

I hope that helps!

Regarding question 117 this is my 10 cents.

(A) Spiral galaxies are less common than types of galaxies that contain little nonluminous matter.

Little nonluminous=No Nonluminous= Luminous

So it says Spiral galaxies are less common than galaxies that contain luminous matter.

As per the passage spiral galaxies contain dark matter and A says those galexies are less in number than those galexies which contain no dark matter. So as per this logic a can be considered as a weakener.



(B) Luminous and nonluminous matter are composed of the same basic elements.- Density does not matter. We are talking about percentage of space occupied by luminous or nonluminous matter. So B is out.

(C) The bright nucleus of a typical spiral galaxy also contains some nonluminous matter. -Does not matter


(D) The density of the observable universe is greater than most previous estimates have suggested. -Again density. Density does not matter.We are talking about percentage of space occupied



(E) Some galaxies do not rotate or rotate too slowly for their rotational velocity to be measured.-Does not matter

Could someone please comment on 117 regarding answer D ?

if the density of the "observable universe" (defined in the passage as "luminous matter in the form of galaxies") com[ing] to a small fraction of the [necessary 3 hydrogen atoms/cubic meter] forms the basis for the assertion that "there must be enough INVISIBLE (nonluminous) matter in the universe to exceed the luminous matter by a density by a factor of roughly 70", then wouldn't this density estimate of the observable universe being understated (as answer D states) imply without room for refute that the assertion that nonluminous matter makes up 90% of the mass in the universe is weakened ???

Thanks for addressing this run on sentence. Seriously, this question (and the OG explanation) are quite off-target IMO - the official answer makes a much less airtight argument.

The author does not argue that nonluminous matter must exceed luminous matter by a density factor of 70 BECAUSE that is what would be necessary to stop the expansion of the universe. Rather, the author merely comments that nonluminous matter would have to exceed luminous matter by a density factor of 70 IN ORDER TO stop the expansion of the universe.

Choice (D) would certainly affect the factor of 70 cited in the first paragraph; if there is more luminous matter than had been previously estimated, that would suggest that it would take less invisible matter to stop the expansion of the universe.

However, the author's suggestion mentioned in question #117 is based on the findings described in the second paragraph: "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."

The author believes that those 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." Choice (D) does not affect the author's reasoning at all. The author is using observations of spiral galaxies to draw conclusions about the universe. Choice (A) suggests that, even if the findings are true, they do not accurately reflect what's going on in most galaxies.

I hope that helps!

Q. 118 - if the density of the universe were equivalent to significantly less than 3 HA/CM
For this I referred 1st paragraph of the passage. If mentions that "is there enough mass in it for the mutual attraction of its constituents to bring this expansion to a halt"? God knows what it means - mutual attraction of.. enough mass..! Whoa!
Okay - so does this mean that in order to halt the universe expansion, we need mutual attraction of (some of) its constituents, and this needs enough mass in the universe?? Who would want what? I am confused
Now rightaway, critical density of the matter needed to brake the expansion and close the universe is equivalent to 3HA/cm. But the density of the observable universe is only a fraction.
Here, first of all, I understand mass = density. Now, is the mass (density) referring to the density of the universe or the constituents (matter)? And why would the author talk about the density of the constituents when he himself talks about the density of the universe in the previous sentence? Or both are same?
Then, as the density of observable universe is only a fraction of 3HA/cm, we need 70 times of that fraction for the density of the invisible (inobservable) mass/matter to be. Observable + inobservable = mass needed to halt expansion.

Now, if the mass of universe is less than 3HA/cm, this means that we would need less of inobservable density, i.e., <70. Thus, why would it keep expanding indefinitely?

I apologize if I have confused my peers or senior members. Grateful if someone can help me here. Is my approach faulty? Or do I need to really work hard on my comprehension skills? Can someone advise me what approach I must take to improve in such circumstances. The mind goes blank and stuck.

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.

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.

116. 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

How is D true? The paragraph clearly states that "This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous mass."