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1) Why do we call dark matter "dark"?
A) It emits no visible light.
B) We cannot detect the type of radiation that it emits.
C) It emits no or very little radiation of any wavelength.
D) It blocks out the light of stars in a galaxy.
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C
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2) What is meant by "dark energy"?
A) the energy associated with dark matter through E=mc2
B) any unknown force that opposes gravity
C) the agent causing the universal expansion to accelerate
D) highly energetic particles that are believed to constitute dark matter
E) the total energy in the Universe after the Big Bang but before the first stars
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C
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3) Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?
A) The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
B) Although dark matter emits no visible light, it can be seen with radio wavelengths, and such observations confirm that the halo is full of this material.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk, suggesting that the halo is full of dark matter.
D) Our view of distant galaxies is sometimes obscured by dark blotches in the sky, and we believe these blotches are dark matter located in the halo.
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A
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5) What evidence suggests that the Milky Way contains dark matter?
A) We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
B) We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way.
C) We see many dark voids between the stars in the halo of the Milky Way.
D) When we observe in different wavelengths, such as infrared or radio, we see objects that don’t appear in visible-light observations.
E) When we look at the galactic center, we are able to observe a large black hole that is composed of dark matter.
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A
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6) If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?
A) We are not measuring the orbital velocities of atomic clouds and stars properly.
B) We are not measuring the distances to atomic clouds and stars properly.
C) We are not attributing enough mass to the visible or "bright" matter.
D) We are not observing all the visible or "bright" matter in the galaxy.
E) Our understanding of gravity is not correct for galaxy-size scales.
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E
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10) When we see that a spectral line of a galaxy is broadened, that is, spanning a range of wavelengths, we conclude that
A) we do not have very good resolution of a star’s orbital velocity.
B) there are many stars traveling at extremely high orbital velocities.
C) there are different Doppler shifts among the individual stars in the galaxy.
D) we are actually measuring the orbital velocity of a cloud of atomic gas.
E) we are actually measuring the orbital velocity of dark matter.
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C
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11) A large mass-to-light ratio for a galaxy indicates that
A) the galaxy is very massive.
B) the galaxy is not very massive.
C) on average, each solar mass of matter in the galaxy emits less light than our Sun.
D) on average, each solar mass of matter in the galaxy emits more light than our Sun.
E) most stars in the galaxy are more massive than our Sun.
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C
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12) What is the mass-to-light ratio for the inner region of the Milky Way Galaxy, in units of solar masses per solar luminosity?
A) 1,000
B) 600
C) 100
D) 6
E) 0.1
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D
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15) Which of the following methods used to determine the mass of a cluster does not depend on Newton’s laws of gravity?
A) measuring the orbital velocities of galaxies in a cluster
B) measuring the temperature of X-ray gas in the intracluster medium
C) measuring the amount of distortion caused by a gravitational lens
D) none of the above
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C
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16) Why wasn’t the intracluster medium in galaxy clusters discovered until the 1960s?
A) We did not know how much dark matter existed before then.
B) We didn’t have the resolution to observe galaxy clusters until then.
C) The Milky Way was blocking our view of distant galaxy clusters.
D) The medium emits X rays, which are blocked by the Earth’s atmosphere and require X-ray satellites in space in order to be observed.
E) Radiation emitted by the medium was so dim that we couldn’t detect it until we built much larger telescopes.
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D
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18) Gravitational lensing occurs when
A) massive objects bend light beams that are passing nearby.
B) massive objects cause more distant objects to appear much larger than they should and we can observe the distant objects with better resolution.
C) dark matter builds up in a particular region of space, leading to a very dense region and an extremely high mass-to-light ratio.
D) telescope lenses are distorted by gravity.
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A
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19) Which of the following is not evidence for dark matter?
A) the flat rotation curves of spiral galaxies
B) the broadening of absorption lines in an elliptical galaxy’s spectrum
C) X-ray observations of hot gas in galaxy clusters
D) gravitational lensing around galaxy clusters
E) the expansion of the universe
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E
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24) What do we mean when we say that a particle is a weakly interacting particle?
A) It interacts only through the weak force.
B) It interacts only through the weak force and the force of gravity.
C) It is so small that it doesn’t affect objects in the universe.
D) It doesn’t interact with any type of baryonic matter.
E) It is the only type of particle that interacts through the weak force.
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B
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25) Why can’t the dark matter in galaxies be made of neutrinos?
A) There are not enough neutrinos to make up all the dark matter.
B) Neutrinos do not have any mass; they interact only through the weak force.
C) We know that dark massive objects such as planets and neutron stars are not made of neutrinos.
D) Neutrinos travel at extremely high speeds and can escape a galaxy’s gravitational pull.
E) Big Bang nucleosynthesis constrains how many neutrinos there are in the Universe.
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D
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28) Why isn’t space expanding within systems such as our solar system or the Milky Way?
A) Hubble’s law of expansion applies only to the space between galaxies.
B) We are so close to these systems that we don’t observe their expansion.
C) The universe is not old enough yet for these objects to begin their expansion.
D) Their gravity is strong enough to hold them together against the expansion of the universe.
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D
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42) What is not a main source of evidence for the existence of dark matter?
A) massive blue stars
B) rotation curves of disk galaxies
C) stellar motions in elliptical galaxies
D) velocities and positions of galaxies in clusters of galaxies
E) gravitational lensing by clusters of galaxies
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A
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1) Which of the following best summarizes what we mean by dark matter?
A) matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
B) matter that may inhabit dark areas of the cosmos where we see nothing at all
C) matter consisting of black holes
D) matter for which we have theoretical reason to think it exists, but no observational evidence for its existence
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A
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26) Which of the following are candidates for dark matter?
A) brown dwarfs
B) Jupiter-size objects
C) WIMPs
D) faint red stars
E) all of the above
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E
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2) Which of the following best summarizes what we mean by dark energy?
A) It is a name given to whatever is causing the expansion of the universe to accelerate with time.
B) It is the energy contained in dark matter.
C) It is the energy of black holes.
D) It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos.
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A
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3) The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that
A) luminous matter emits white light, much like the light reflected from icebergs.
B) black holes are much more interesting than ordinary stars that give off light.
C) dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way.
D) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter.
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C
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8) In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that
A) the higher amount of mass relative to light (higher mass-to-light ratio), the lower the proportion of dark matter.
B) the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter.
C) the amount of light should be at least one solar luminosity for each solar mass of matter (mass-to-light ratio less than or equal to 1).
D) the higher the amount of mass relative to light (higher mass-to-light ratio), the older the galaxy or cluster.
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B
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9) Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
A) measuring the speeds of galaxies orbiting the cluster’s center
B) studying X-ray emission from hot gas inside the cluster
C) observing how the cluster bends light from galaxies located behind it
D) measuring the temperatures of stars in the halos of the galaxies
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D
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10) When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
A) It magnifies the effects of gravity that we see in the cluster.
B) It is an unusually large cluster that has a lot of gravity.
C) It bends or distorts the light coming from galaxies located behind it.
D) The overall shape of the cluster is that of a lens.
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C
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11) Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
A) Dark matter is the dominant form of mass in both clusters and in individual galaxies.
B) Dark matter is present between galaxies in clusters, but not within individual galaxies.
C) Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
D) Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.
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A
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13) What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
A) The light that they emit is so weak that it is undetectable to our telescopes.
B) They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily.
C) They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
D) They interact with other matter only through the weak force and not through gravity or any other force.
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C
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14) Which of the following best sums up current scientific thinking about the nature of dark matter?
A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
B) Dark matter consists 90% of neutrinos and 10% of WIMPs.
C) There is no longer any doubt that dark matter is made mostly of WIMPs.
D) Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.
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A
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15) When we speak of the large-scale structure of the universe, we mean
A) the structure of any large galaxy.
B) the structure of any individual cluster of galaxies.
C) the overall shape of the observable universe.
D) the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe.
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D
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17) What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
A) observations of the speeds of individual galaxies in clusters
B) measurements of the rotation curve for the universe
C) measurements of how galaxy speeds away from the Milky Way have increased during the past century
D) observations of white dwarf supernovae
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D
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6) How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
A) We infer its existence by observing its gravitational effects on the galaxy motions.
B) The hot gas shows up as bright pink in visible-light photos of galaxy clusters.
C) We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.
D) We detect this gas with X-ray telescopes.
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D
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7) Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
A) The temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.
B) The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
C) The temperature of the gas tells us the gas density, so we can use the density to determine the cluster’s mass.
D) The question is nonsense—gas temperature cannot possibly tell us anything about mass.
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B
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8) How does gravitational lensing tell us about the mass of a galaxy cluster?
A) The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.
B) The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
C) Using Einstein’s general theory of relativity, we can calculate the cluster’s mass from the precise way in which it distorts the light of galaxies behind it.
D) Newton’s universal law of gravitation predicts how mass can distort light, so we can apply Newton’s law to determine the mass of the cluster.
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C
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9) If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
A) They travel at speeds close to the speed of light.
B) They are subatomic particles.
C) They can neither emit nor absorb light.
D) They tend to orbit at large distances from the galactic center.
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A
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10) Is space expanding within clusters of galaxies?
A) No, because the universe is not old enough yet for these objects to have begun their expansion.
B) No, because expansion of the universe affects only empty space, not space in which matter is present.
C) Yes, and that is why clusters tend to grow in size with time.
D) No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
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D
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11) Which of the following statements about large-scale structure is probably not true?
A) Galaxies and clusters have grown around tiny density enhancements that were present in the early universe.
B) Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe.
C) Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members.
D) Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
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D
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12) Based on current evidence, a supercluster is most likely to have formed in regions of space where
A) the density of dark matter was slightly higher than average when the universe was very young.
B) there was an excess concentration of hydrogen gas when the universe was very young.
C) supermassive black holes were present in the very early universe.
D) the acceleration of the expansion was proceeding faster than elsewhere.
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A
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14) Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
A) The cause of the acceleration is well-understood, and attributed to the particles that make up dark energy.
B) We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
C) The acceleration is very important in the cosmos today, but the evidence indicates that it will eventually slow down, allowing the universe to recollapse.
D) The acceleration probably is not real, and what we attribute to acceleration is probably just a misinterpretation of the data.
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B
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13) Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
A) If we include dark matter, the actual density equals the critical density.
B) The actual density, even with dark matter included, is less than about a third of the critical density.
C) The actual density of dark matter and luminous matter combined is no more than about 1% of the critical density.
D) The actual density of matter is many times higher than the critical density.
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B
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15) Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
A) Dark energy is the dominant form of energy in the cosmos.
B) Dark energy does not exist and there is much more dark matter than we are aware of to date.
C) Neither dark energy nor dark matter really exist.
D) Dark energy exists but dark matter does not.
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B
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16) Hubble’s constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble’s constant today (such as 24 km/s/Mly), the age of the universe is oldest if what is true?
A) The expansion rate has remained nearly constant with time (a coasting universe).
B) The expansion rate has slowed by the amount expected for a universe with the critical density (a critical universe).
C) The expansion rate has been increasing with time (an accelerating universe).
D) The expansion rate is slowing dramatically with time (a recollapsing universe).
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C
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17) Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter that decays into photons or other forms of energy about 50 billion years from now. Based on current understanding, how would that affect the universe at that time?
A) Stars would cease to exist when the dark matter is gone.
B) Planetary systems would expand and disperse.
C) The galaxies in clusters would begin to fly apart.
D) The universe would cease its expansion.
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C
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