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RC OG 13(After the evidence obtained)

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gocoder Master | Next Rank: 500 Posts Default Avatar
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RC OG 13(After the evidence obtained)

Post Thu Nov 24, 2016 9:51 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.

    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.

    I was stuck between A and D. Please help

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    Post Fri Nov 25, 2016 8:04 am
    gocoder 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.

    I was stuck between A and D. Please help
    Much of this passage is about the concentration of luminous v nonluminous (dark) matter in the universe.

    Paragraph 2, boiled way down, tell us this: researchers have concluded that there is a great deal of nonluminous (dark) matter in spiral galaxies. This unexpected result indicates that the falloff in luminous mass with distance from the center is balanced by an increase in nonluminous
    mass.

    So spiral galaxies have lots of nonluminous matter. The claim in lines 34-37 is that as much as 90% of the universe's mass is nonluminous. This makes sense if the composition of spiral galaxies is representative of the composition of all galaxies in the universe.

    Now look at answer choice A: Spiral galaxies are less common than types of
    galaxies that contain little nonluminous matter.

    Well, if there are other, more common, galaxies that have little nonluminous matter, then the conclusion that the universe is mostly nonluminous matter is a good deal weaker. Answer is A

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