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RC: OG-12: very specific doubt

This topic has 2 expert replies and 3 member replies
buoyant Master | Next Rank: 500 Posts Default Avatar
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RC: OG-12: very specific doubt

Post Sun Sep 01, 2013 11:08 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    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?

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    Java_85 Master | Next Rank: 500 Posts
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    Target GMAT Score:
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    Post 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

    Post 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?

    Can someone please explain?
    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 Smile

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    Thanked by: buoyant
    buoyant Master | Next Rank: 500 Posts Default Avatar
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    Post Sat Sep 07, 2013 6:59 am
    Yes. That makes sense.
    Thank you for the nice explanation,Mike !

    ela4490 Newbie | Next Rank: 10 Posts Default Avatar
    Joined
    13 Sep 2016
    Posted:
    1 messages
    Target GMAT Score:
    750
    Post 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?

    Can someone please explain?[/quote]
    [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 Smile[/quote]

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

    Doesn't this means that luminosity decreases??

    GMAT/MBA Expert

    Post 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. Smile

    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 Smile

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