gmatt1476 wrote:
⠀⠀⠀⠀⠀⠀⠀⠀This passage is excerpted from material published
in 1997.
⠀⠀⠀⠀⠀⠀⠀⠀Is there a massive black hole at the center of our
⠀⠀⠀ galaxy, the Milky Way? The evidence is inconclusive.
(5) Just as the Sun's mass can be determined, given
⠀⠀⠀ knowledge of other variables, by the velocity at
⠀⠀⠀ which its planets orbit, the mass at the center of the
⠀⠀⠀ Milky Way can be revealed by the velocities of stars
⠀⠀⠀ and gas orbiting the galactic center. This dynamical
(10) evidence, based on recently confirmed assumptions
⠀⠀⠀ about the stars' velocities, argues for an extremely
⠀⠀⠀ compact object with a mass two to three million
⠀⠀⠀ times the mass of our Sun. Although according to
⠀⠀⠀ current theory this makes the mass at the center
(15) of the galaxy too dense to be anything but a black
⠀⠀⠀ hole, the relative lack of energy radiating from the
⠀⠀⠀ galactic center presents a serious problem. A black
⠀⠀⠀ hole's gravity attracts surrounding matter, which
⠀⠀⠀ swirls around the black hole, emitting some energy
(20) as it is engulfed. Scientists believe that the amount of
⠀⠀⠀ energy that escapes the black hole should be about
⠀⠀⠀ 10 percent of the matter's rest energy (the energy
⠀⠀⠀ equivalent of its mass according to the equation
⠀⠀⠀ E=mc^2). But when the energy coming from the
(25) galactic center is compared to widely held predictions
⠀⠀⠀ based on how much matter should be falling into a
⠀⠀⠀ theoretical central black hole, there is a discrepancy
⠀⠀⠀ by a factor of a few thousand.
1. The primary purpose of the passage is to
A. present several theories that could account for a particular phenomenon
B. argue that a certain question needs to be reframed in light of new evidence
C. resolve an apparent inconsistency between two lines of evidence
D. explain why a certain issue remains unresolved
E. present evidence that calls into question certain assumptions of a current theory
RC60500.01-10
2. According to the passage, the dynamical evidence referred to in lines 9–10 supports which of the following?
A. Recent assumptions about the velocities of stars
B. Widely held predictions about the amount of matter a black hole will engulf
C. The existence of an extremely dense object at the center of the Milky Way
D. The contention that too much energy is coming from the mass at the Milky Way's galactic center for that mass to be a black hole
E. The conclusion that a compact object of two to three million times the mass of our Sun is too dense to be anything but a black hole
RC60500.01-20
3. The “serious problem” referred to in line 17 could be solved if which of the following were true?
A. Current assumptions about how much matter a black hole would engulf proved to be several thousand times too high.
B. Current assumptions about how much matter a black hole would engulf proved to be a few thousand times too low.
C. The object at the center of the Milky Way turned out to be far more dense than it is currently estimated to be.
D. The object at the center of the Milky Way turned out to be far more massive than it is currently estimated to be.
E. Matter being engulfed by a black hole radiated far more energy than is currently assumed.
RC60500.01-30
4. The “widely held predictions” mentioned in line 25 are predictions about the
A. compactness of objects whose mass is millions of times the mass of our Sun
B. velocities of stars orbiting the galactic center
C. amount of matter swirling around the object at the center of the Milky Way
D. amount of matter falling into a theoretical central black hole
E. amount of energy that should be coming from a black hole at the center of the Milky Way
RC60500.01-40
RC60500.01-30
3. The “serious problem” referred to in line 17 could be solved if which of the following were true?A. Current assumptions about how much matter a black hole would engulf proved to be several thousand times too high.
B. Current assumptions about how much matter a black hole would engulf proved to be a few thousand times too low.
C. The object at the center of the Milky Way turned out to be far more dense than it is currently estimated to be.
D. The object at the center of the Milky Way turned out to be far more massive than it is currently estimated to be.
E. Matter being engulfed by a black hole radiated far more energy than is currently assumed.
ApplicationWhich one of the five options would, if true, indicate a possible solution to the serious problem referred to? The serious problem is said to arise from the relative lack of energy radiating from the galactic center compared with the expected value if a matter-engulfing black hole were truly at the center of the Milky Way.
According to the passage, the radiated energy turned out to be a few thousand times less than had been expected, on theoretical grounds, to radiate from mass engulfed by the hypothetical black hole. However, if it were discovered that the matter engulfed were several thousand times less than previously estimated, the relatively low level of radiated energy observed would no longer seem at odds with the existence of the hypothesized black hole.
A. Correct. Reviewing the assumptions underlying the widely held predictions could lead to revised predictions that harmonize with the observational evidence regarding radiated energy from the galaxy's center.
B. The passage states that the assumed quantity of engulfed matter already seems to be radiating too little; this answer choice would actually make the “serious problem” in the passage worse. That is, assuming that there is actually more engulfed matter with such a small amount of radiation would simply worsen the problem.
C. The passage does not specify what impact a greater-than-estimated density would have on the quantity of energy that is radiated. That is, even if greater mass would change predictions, greater density may not do so.
D. If the object were more massive, then presumably the matter engulfed by this hypothetical more massive black hole would radiate a quantity of energy even greater than the observed quantity. This again would simply make the “serious problem” worse.
E. Given that not enough radiated energy is currently being observed, finding that the matter being engulfed radiates even more energy than expected would make the “serious problem” significantly worse. That is, the relatively low quantity of radiated energy observed would fall even farther short of the quantity expected.
The correct answer is A. of a factor of few thousands in the light emitted by the center of the galaxy.
According to official explanation, the discrepancy has been interpreted as a shortage of energy.
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28 Oct 2019, 07:23
