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Register now and save up to $200 Available with Beat the GMAT members only code • Free Trial & Practice Exam BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code ## RC: OG-12: very specific doubt This topic has 2 expert replies and 3 member replies buoyant Master | Next Rank: 500 Posts Joined 02 Mar 2013 Posted: 106 messages Upvotes: 4 #### RC: OG-12: very specific doubt Sun Sep 01, 2013 11:08 am 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. 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 I chose the correct answer . however, I am still having doubt on answer choice B. The portion of the passage says that "Instead we have found that the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly." Doesn't it also mean that the rotational velocity near the nucleus may also be lower than that in outermost region? Can someone please explain? ### GMAT/MBA Expert Mike@Magoosh GMAT Instructor Joined 28 Dec 2011 Posted: 768 messages Followed by: 137 members Upvotes: 387 Wed Oct 26, 2016 2:33 pm ela4490 wrote: luminosity falls off rapidly with distance from the center Doesn't this means that luminosity decreases?? Dear ela4490, I'm happy to respond. The short answer is "yes," although this idiom means something more specific. To decrease simply means to go down, to decrease in value. The idiom "to fall off" or "to drop off" means to decrease rapidly. If the amount in my bank account simply decreases, I may have$500 the first week, then $400 the second week, then$300, etc. That's a nice moderate decrease.

If the amount in my bank account falls off, maybe I have $500 on Monday of the first week, but say by Wednesday of the first week it is down to$37. In other words, there was a huge decrease right at the beginning. That is an extreme example of "falling off."

To say "luminosity falls off rapidly with distance from the center" is to say that up close, there is high luminosity, then a short distance away it's much lower, then it continues to get even lower as we move farther away.

My friend, here is a set of GMAT Idiom Flashcards. It doesn't include this idiom, but at least the basic ones.

The best way to learn all these subtle idioms is to read. See this blog:
How to Improve Your GMAT Verbal Score

Does all this make sense?
Mike

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ela4490 Newbie | Next Rank: 10 Posts
Joined
13 Sep 2016
Posted:
1 messages
Target GMAT Score:
750
Wed Oct 26, 2016 10:03 am
[quote="buoyant"][color=blue]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[/color]

I chose the correct answer . however, I am still having doubt on answer choice B.

The portion of the passage says that "Instead we have found that the rotational velocity
in spiral galaxies either remains constant with increasing distance from the center or increases
slightly." Doesn't it also mean that the rotational velocity near the nucleus may also be lower than that in outermost region?

[quote="Java_85"]Isn't E the OA?[/quote]
[quote="Mike@Magoosh"]First of all, [b]Java_85[/b], the passage very clearly states that "[color=darkblue]luminosity falls off rapidly with distance from the center[/color]." Thus, the "[color=darkblue]region just outside the nucleus[/color]" would have a high luminosity, higher than anything out toward the edge. Thus, we know we need an answer with "[color=darkblue]higher luminosity[/color]", [b](A)[/b], [b](B)[/b], or [b](D)[/b]. As it happens, the OA = [spoiler][b](D)[/b][/spoiler].

Now, [b]bouyant[/b], what you ask is an excellent question. The text says: "[color=darkblue]the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly[/color]." So, a region toward the center would have a rotational velocity that is about the same or slightly lower than that toward the outer reaches. Two possibilities: (1) about the same, or (2) slightly lower.

Now, think about [b](B)[/b] vs. [b](D)[/b]
[color=darkblue](B) lower rotational velocity
(D) similar rotational velocity [/color]
Which one of those includes both possibilities? Choice [b](B)[/b] definitely includes (2) but doesn't include (1). By contrast, choice [b](D)[/b] includes both (1) and (2), because both "[color=blue]slightly higher[/color]" and "[color=blue]slightly lower[/color]" are included in the idea of "[color=blue]similar[/color]".
That's why the OA is a better answer.
Mike [/quote]

[color=red][b]luminosity falls off rapidly with distance from the center[/b][/color]

Doesn't this means that luminosity decreases??

Java_85 Master | Next Rank: 500 Posts
Joined
16 Jul 2013
Posted:
234 messages
9
Target GMAT Score:
700+
Sun Sep 01, 2013 1:44 pm
In my opinion the answer is E,
passage says "rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly." it means that the rotational velocity either increases or remain constant. therefore it's A D E, but luminosity cannot increase as the distance is increasing ==> Just E can be the answer.
Isn't E the OA?

### GMAT/MBA Expert

Mike@Magoosh GMAT Instructor
Joined
28 Dec 2011
Posted:
768 messages
Followed by:
137 members
387
Fri Sep 06, 2013 3:46 pm
buoyant wrote:
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

I chose the correct answer . however, I am still having doubt on answer choice B.

The portion of the passage says that "Instead we have found that the rotational velocity
in spiral galaxies either remains constant with increasing distance from the center or increases
slightly." Doesn't it also mean that the rotational velocity near the nucleus may also be lower than that in outermost region?

Java_85 wrote:
Isn't E the OA?
First of all, Java_85, the passage very clearly states that "luminosity falls off rapidly with distance from the center." Thus, the "region just outside the nucleus" would have a high luminosity, higher than anything out toward the edge. Thus, we know we need an answer with "higher luminosity", (A), (B), or (D). As it happens, the OA = (D).

Now, bouyant, what you ask is an excellent question. The text says: "the rotational velocity in spiral galaxies either remains constant with increasing distance from the center or increases slightly." So, a region toward the center would have a rotational velocity that is about the same or slightly lower than that toward the outer reaches. Two possibilities: (1) about the same, or (2) slightly lower.

Now, think about (B) vs. (D)
(B) lower rotational velocity
(D) similar rotational velocity

Which one of those includes both possibilities? Choice (B) definitely includes (2) but doesn't include (1). By contrast, choice (D) includes both (1) and (2), because both "slightly higher" and "slightly lower" are included in the idea of "similar".
That's why the OA is a better answer.
Mike

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buoyant Master | Next Rank: 500 Posts
Joined
02 Mar 2013
Posted:
106 messages
4
Sat Sep 07, 2013 6:59 am
Yes. That makes sense.
Thank you for the nice explanation,Mike !

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