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19. 宇宙扩张
【v1】还有一篇关于宇宙扩张的……各种没读懂……3道猜了两道
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 passage is primarily concerned with
A.defending a controversial approach
B.criticizing an accepted view
C.summarizing research findings
D.contrasting competing theories
E.describing an innovative technique
C
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
正确答案D
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" (highlight text) 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.
正确答案A
提交答案收藏题目分享题目报告题目错误
It can be inferred from information presented in the passage that if the density of the universe were equivalent to significantly less than three hydrogen atoms per cubic meter, which of the following would be true as a consequence?
A.Luminosity would be a true indicator of mass.
B.Different regions in spiral galaxies would rotate at the same velocity.
C.The universe would continue to expand indefinitely.
D.The density of the invisible matter in the universe would have to be more than 70 times the density of the luminous matter.
E.More of the invisible matter in spiral galaxies would have to be located in their nuclei than in their outer regions.
正确答案C
The authors propose all of the following as possibly contributing to the “missing matter” in spiral galaxies EXCEPT
A.massive black holes
B.small black holes
C.small, dim stars
D.massive stars
E.large planets
正确答案D
In the 1980's, astronomer Bohdan Paczynski proposed a way of determining whether the enormous dark halo constituting the outermost part of the Milky Way galaxy is composed of MACHO's (massive compact halo objects), which are astronomical objects too dim to be visible. Paczynski reasoned that if MACHO's make up this halo, a MACHO would occasionally drift in front of a star in the Large Magellanic Cloud, a bright galaxy near the Milky Way. The gravity of a MACHO that had so drifted, astronomers agree, would cause the star's light rays, which would otherwise diverge, to bend together so that, as observed from Earth, the star would temporarily appear to brighten, a process known as microlensing. Because many individual stars are of intrinsically variable brightness, some astronomers have contended that the brightening of intrinsically variable stars can be mistaken for microlensing. However, whereas the different colors of light emitted by an intrinsically variable star are affected differently when the star brightens, all of a star's colors are equally affected by microlensing. Thus, if a MACHO magnifies a star's red light tenfold, it will do the same to the star's blue light and yellow light. Moreover, it is highly unlikely that a star in the Large Magellanic Cloud will undergo microlensing more than once, because the chance that a second MACHO would pass in front of exactly the same star is minuscule.
The passage is primarily concerned with
A.outlining reasons why a particular theory is no longer credited by some astronomers
B. presenting data collected by a researcher in response to some astronomers' criticism of a particular line of reasoning
C.explaining why a researcher proposed a particular theory and illustrating how influential that theory has been
D.showing how a researcher's theory has been used to settle a dispute between the researcher and some astronomers
E.describing a line of reasoning put forth by a researcher and addressing a contention concerning that line of reasoning
正确答案E
According to the passage, Paczynski's theory presumes that if MACHO's constituted the Milky Way's dark halo, occasionally a MACHO would
A.drift so as to lie in a direct line between two stars in the outer Milky Way
B.affect the light rays of a star in the Large Magellanic Cloud with the result that the star would seem for a time to brighten
C.become obscured as a result of the microlensing of a star in the Large Magellanic Cloud
D.temporarily increase the apparent brightness of a star in the Large Magellanic Cloud by increasing the gravity of the star
E.magnify each color in the spectrum of a star in the Large Magellanic Cloud by a different amount
正确答案B
It can be inferred from the passage that which of the following would constitute the strongest evidence of the microlensing of a star in the Large Magellanic Cloud?
A.The brightness of such a star is observed to vary at irregular intervals.
B.The brightening of such a star is observed to be of shorter duration than the brightening of neighboring stars.
C. The red light of such a star is observed to be brighter than its yellow light and its blue light.
D.The red light, yellow light, and blue light of such a star are observed to be magnified temporarily by the same factor.
E.The red light of such a star is observed to have increased tenfold.
正确答案D
12. Galaxy测量
【v1】一篇讲galaxy的测量什么的，看得我云里雾里的
In addition to conventional galaxies, the universe contains very dim galaxies
that untilrecently went unnoticed by astronomers. Possibly as numerous as conventional
galaxies, these galaxies have the same general shape and even the same approximate number
of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because
these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than
do conventional galaxies. Apparently these low-surface brightness galaxies, as they are called, take much longer than conventional galaxies to condense their
primordial gas and convert it to stars—that is, they evolve much more slowly.
These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe. Baryons—subatomic particles that are generally protons or neutrons—are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based
on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy,
suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate.
Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic
space or as some large population of galaxies that are difficult to detect.
The author mentions the fact that baryons are the source of stars' luminosity primarily in order to explain
A.how astronomers determine that some galaxies contain fewer stars per unit volume than do others
B.how astronomers are able to calculate the total luminosity of a galaxy
C.why astronomers can use galactic luminosity to estimate baryonic mass
D.why astronomers' estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
E.how astronomers know bright galaxies contain more baryons than do dim galaxies
正确答案C
The primary purpose of the passage is to
A.describe a phenomenon and consider its scientific significance
B.contrast two phenomena and discuss a puzzling difference between them
C.identify a newly discovered phenomenon and explain its origins
D.compare two classes of objects and discuss the physical properties of each
E.discuss a discovery and point out its inconsistency with existing theory
A
t can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?
A.They are large spiral galaxies containing fewer stars than do conventional galaxies.
B.They are compact but very dim spiral galaxies.
C.They are diffuse spiral galaxies that occupy a large volume of space.
D.They are small, young spiral galaxies that contain a high proportion of primordial gas.
E.They are large, dense spirals with low luminosity.
C
还有个神马暗物质的
In the 1980's, astronomer Bohdan Paczynski proposed a way of determining whether
the enormous dark halo constituting the outermost part of the Milky Way galaxy is composed
of MACHO's (massive compact halo objects), which are astronomical objects too dim to be visible.
Paczynski reasoned that if MACHO's make up this halo, a MACHO would occasionally drift in front
of a star in the Large Magellanic Cloud, a bright galaxy near the Milky Way. The gravity of a MACHO
that had so drifted, astronomers agree, would cause the star's light rays, which would otherwise diverge,
to bend together so that, as observed from Earth, the star would temporarily appear to brighten, a process
known as microlensing. Because many individual stars are of intrinsically variable brightness, some astronomers
have contended that the brightening of intrinsically variable stars can be mistaken for microlensing. However,
whereas the different colors of light emitted by an intrinsically variable star are affected differently when the
star brightens, all of a star's colors are equally affected by microlensing. Thus, if a MACHO magnifies a star's red
light tenfold, it will do the same to the star's blue light and yellow light. Moreover, it is highly unlikely that a star
in the Large Magellanic Cloud will undergo microlensing more than once, because the chance that a second MACHO
would pass in front of exactly the same star is minuscule.
The passage is primarily concerned with
A.outlining reasons why a particular theory is no longer credited by some astronomers
B. presenting data collected by a researcher in response to some astronomers' criticism of a particular line of reasoning
C.explaining why a researcher proposed a particular theory and illustrating how influential that theory has been
D.showing how a researcher's theory has been used to settle a dispute between the researcher and some astronomers
E.describing a line of reasoning put forth by a researcher and addressing a contention concerning that line of reasoning
正确答案E
According to the passage, Paczynski's theory presumes that if MACHO's constituted the Milky Way's dark halo, occasionally a MACHO would
A.drift so as to lie in a direct line between two stars in the outer Milky Way
B.affect the light rays of a star in the Large Magellanic Cloud with the result that the star would seem for a time to brighten
C.become obscured as a result of the microlensing of a star in the Large Magellanic Cloud
D.temporarily increase the apparent brightness of a star in the Large Magellanic Cloud by increasing the gravity of the star
E.magnify each color in the spectrum of a star in the Large Magellanic Cloud by a different amount
正确答案B
It can be inferred from the passage that which of the following would constitute the strongest evidence of the microlensing of a star in the Large Magellanic Cloud?
A.The brightness of such a star is observed to vary at irregular intervals.
B.The brightening of such a star is observed to be of shorter duration than the brightening of neighboring stars.
C. The red light of such a star is observed to be brighter than its yellow light and its blue light.
D.The red light, yellow light, and blue light of such a star are observed to be magnified temporarily by the same factor.
E.The red light of such a star is observed to have increased tenfold.
正确答案D
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发表于 2012-9-12 15:56:27
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