Deck 18: Dark Matter, Dark Energy, and the Fate of the Universe

Question 1

Based on current evidence, a supercluster is most likely to have formed in regions of space where .

A)supermassive black holes were present in the very early universe
B)the acceleration of the expansion was proceeding faster than elsewhere
C)there was an excess concentration of hydrogen gas when the universe was very young
D)the density of dark matter was slightly higher than average when the universe was very young

Answer 1

D

Question 2

Based on the observed amount of deuterium in the universe, we can conclude that

A)the density of ordinary (baryonic)matter is about 5 percent of the critical density.
B)neutrons outnumber protons 7 to 1 in the universe.
C)we live in a critical- density universe.
D)ordinary (baryonic)matter makes 75 percent of the mass of the universe.
E)most of the deuterium that was created during the era of nucleosynthesis has since been destroyed.

Answer 2

A

Question 3

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
It is bright everywhere. You have been able to travel around, and it's clear that you are not inside a star. Yet your surroundings seem as bright as looking directly at the Sun. As you travel around, you notice that you cannot find a single neutral atom anywhere, nor can you find any nucleus besides those of hydrogen and helium. While it is hot (a few thousand degrees Kelvin), it is nowhere near the temperature needed for nuclear fusion. Where are you?

A)You are in the universe more than 10100 years in the future.
B)You are in an accretion disk around a supermassive black hole.
C)You are in the universe during its first 380,000 years.
D)You are in the planetary nebula that the Sun will form about 5 billion years from now.
E)You are in the central regions of a quasar.

Answer 3

C

Question 4

What evidence suggests that the Milky Way contains dark matter?

A)When we observe in different wavelengths, such as infrared or radio, we see objects that don't appear in visible- light observations.
B)We see many dark voids between the stars in the halo of the Milky Way.
C)When we look at the galactic center, we are able to observe a large black hole that is composed of dark matter.
D)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.
E)We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way.

Question 5

Which of the following best describes how galaxies are distributed on large scales in the universe?

A)Galaxies are uniformly distributed.
B)Galaxies are distributed in a great shell expanding outward from the center of the universe.
C)Galaxies are distributed in a hierarchy of clusters, superclusters, and hyperclusters.
D)Galaxies are randomly distributed.
E)Galaxies appear to be distributed in chains and sheets that surround great voids.

Question 6

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?

A)Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
B)Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.
C)Dark matter is present between galaxies in clusters, but not within individual galaxies.
D)Dark matter is the dominant form of mass in both clusters and in individual galaxies.

Question 7

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

A)Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.
B)Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
C)Yes, they tell us that dark matter is concentrated near the center of the galaxy.
D)No, we cannot determine anything about the location of dark matter from the rotation curve.

Question 8

When we measure the broadening of absorption lines in the spectrum of an elliptical galaxy, we can infer

A)the amount of gas and dust in the galaxy.
B)the mass of the black hole at the galaxy's center.
C)how fast the stars in the galaxy are moving relative to one another.
D)the galaxy's rotation curve.
E)how quickly the galaxy is forming new stars.

Question 9

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
You are in a hot, dense place. You are surrounded by nuclear reactions that are rapidly fusing hydrogen into helium. You notice that your surroundings are cooling and rapidly dropping in density. Within about 5 minutes, the fusion reactions stop. Where are you?

A)You are inside a nuclear power plant on Earth.
B)You are in the center of a massive star near the end of its life.
C)You are in the early universe during the era of nucleosynthesis.
D)You are in the center of a star much smaller than the Sun.
E)You are in the center of a star very much like our Sun.

Question 10

The more baryons there are in the universe (a higher density of baryonic matter), the lower the ratio of deuterium to hydrogen is. Therefore, if Harry measures a higher ratio of deuterium to hydrogen than Sally, Harry infers

A)a lower density of baryons than Sally.
B)a higher density of baryons than Sally.
C)the same density of baryons as Sally.

Question 11

The distribution of the dark matter in a spiral galaxy is

A)approximately spherical and about the same size as the galaxy halo.
B)approximately spherical and about ten times the size of the galaxy halo.
C)flattened in a disk but about ten times larger than the stellar disk.
D)predominantly concentrated in the spiral arms.
E)flattened in a disk and about the same size as the stellar disk.

Question 12

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
A moment before, you were enveloped in a bright plasma. You could not see very far as free electrons zipped around your head, scattering photons. But now, the universe has suddenly become transparent and you can see clearly. There seem to be no planets or stars, only a gas filled with mostly neutral hydrogen atoms. Where are you?

A)You are in intergalactic space within a rich cluster of galaxies.
B)You are in a closed universe just as it begins to re- collapse.
C)You are in the universe about 380,000 year after the Big Bang during the formation of the cosmic background radiation.
D)You are in the universe when it was about 500 million years old, just before galaxies began to form.
E)You are in the universe about 5 minutes after the Big Bang just as the nucleosynthesis era is ending.

Question 13

The gravitational lens effect has been verified by

A)observations of multiple images of the same background galaxy seen towards a massive galaxy cluster.
B)single images of highly distorted, high- redshift galaxies seen towards massive galaxy clusters.
C)observations of the bending of starlight by the Sun during a 1919 eclipse.
D)all of the above
E)None of the above; this effect is purely hypothetical.

Question 14

The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. What is this prediction?

A)The universe should be geometrically "flat" (in the four dimensions of spacetime).
B)Inflation predicts that the entire universe must be far larger than the observable universe.
C)Inflation predicts that the temperature of the cosmic microwave background should be almost (but not exactly)the same everywhere.
D)Inflation predicts that the early universe should have regions of enhanced density that could have acted as "seeds" for the formation of galaxies and large structures.

Question 15

Which of the following would not be considered evidence for dark matter in clusters of galaxies?

A)the total amount of stars
B)the curvature of gravitationally- lensed galaxies
C)the temperature of the intergalactic gas
D)the velocities and radii of galaxy orbits around the center of mass in the cluster

Question 16

Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into the galaxy's disk?

A)WIMPs annihilate when they come into contact with ordinary matter, such as stars.
B)WIMPS cannot produce photons, therefore they rarely interact and exchange energy with other particles.
C)Shock waves from generations of supernovae have blown the WIMPs out into the halo.
D)WIMPs are light enough that they have expanded out into the halo.
E)WIMPs were produced in the early stages of galaxy evolution, and objects in the halo, such as globular clusters, were formed first.

Question 17

This figure shows the X- ray emission from hot gas in two colliding galaxy clusters (red)and the distribution of mass (blue). Which of these are reasonable conclusions that can be drawn from this image?
I)The mass found by gravitational lensing in these colliding galaxy clusters is in a separate physical location from the hot gas that resulted from the collision.
II)Dark matter does not physically interact with regular matter, other than through gravity.
III)Hot intracluster gas makes up the majority of the mass of galactic clusters.

A)only I
B)I and II
C)I and III
D)I, II, and III

Question 18

Which model of the universe gives the youngest age for its present size and expansion rate?

A)a coasting universe
B)an accelerating universe
C)a critical universe
D)a re- collapsing universe
E)All models give the same age.

Question 19

Which statement correctly describes the role of dark matter in galaxy formation?

A)Where baryons collect, dark matter follows.
B)Dark matter plays very little role in galaxy formation.
C)Where dark matter collects, the baryons follow.

Question 20

Compared to the critical density of matter, about how much matter and dark energy is there?

A)99% matter, 1% dark energy
B)30% matter, 70% dark energy
C)1% matter, 99% dark energy
D)100% matter, 0% dark energy

Question 21

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

A)It bends or distorts the light coming from galaxies located behind it.
B)The overall shape of the cluster is that of a lens.
C)It magnifies the effects of gravity that we see in the cluster.
D)It is an unusually large cluster that has a lot of gravity.

Question 22

A mass- to- light ratio for a galaxy of much greater than one indicates that

A)most stars in the galaxy are more massive than our Sun.
B)the galaxy is very massive.
C)on average, each solar mass of matter in the galaxy emits much more light than our Sun.
D)the galaxy is not very massive.
E)on average, each solar mass of matter in the galaxy emits much less light than our Sun.

Question 23

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
You are in a place that is extremely hot and dense. You can't see far because your surroundings are opaque to light. Around you, nuclear fusion is converting carbon into oxygen and other elements. Where are you?

A)You are in the center of a white dwarf.
B)You are inside a nuclear power plant on Earth.
C)You are in the center of a massive star near the end of its life.
D)You are in the center of a star very much like our Sun.
E)You are in the early universe during the era of nucleosynthesis.

Question 24

Why can't the dark matter in galaxies be made of neutrinos?

A)Neutrinos have zero mass like the photon.
B)Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.
C)There are not enough neutrinos to make up all the dark matter.
D)We know that dark massive objects, such as planets and neutron stars, are not made of neutrinos.

Question 25

Which of the following methods used to determine the mass of a cluster of galaxies does not depend on Newton's law of gravity?

A)measuring the amount of distortion caused by a gravitational lens
B)measuring the temperature of X- ray gas in the intracluster medium
C)measuring the orbital velocities of galaxies in the cluster
D)none of the above

Question 26

Visible, luminous matter (such as the stars within galaxies)amounts to what percentage of the critical density (the density of mass- energy needed to make the geometry of the universe flat)?

A)10 percent
B)50 percent
C)less than 1 percent
D)25 percent

Question 27

Measuring the amount of deuterium in the universe allows us to set a limit on

A)the expansion rate of the universe.
B)the density of ordinary (baryonic)matter in the universe.
C)the temperature of the universe at the end of the era of nuclei.
D)the current age of the universe.
E)the total amount of mass in the universe.

Question 28

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

A)Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.
B)The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.
C)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.
D)The question is nonsense-gas temperature cannot possibly tell us anything about mass.

Question 29

The quantity called the "enclosed mass" M(

A)concentrated in the center
B)distributed in the shape of a doughnut, with a hole in the center
C)spread out over a large volume
D)impossible to say

Question 30

A gravitational lens occurs when

A)a massive object bends light beams that are passing nearby.
B)a telescope lens is distorted by gravity.
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)a massive object causes more distant objects to appear much larger than they should, and we can observe the distant objects with better resolution.

Question 31

How do we know the expansion of the universe is accelerating?

A)We're really not that sure the expansion is accelerating; it's all just a theory.
B)White dwarf supernova in distant galaxies are fainter than they would be if the expansion were not accelerating.
C)We see more of the universe every night, as the observable universe grows in size.
D)The rotation curves of galaxies indicate the universe is speeding up.
E)Redshifts of distant galaxies are increasing with time, so when we repeat the measurements, the redshifts are larger.

Question 32

Why isn't the space within our solar system or the Milky Way expanding according to Hubble's Law?

A)The universe is not old enough for the solar system or Milky Way to have begun their expansion.
B)Hubble's law of expansion applies only to the space between galaxies.
C)As we are inside our solar system and the Milky Way, we cannot observe their expansion.
D)The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe.

Question 33

Which of the following is not evidence for dark matter?

A)gravitational lensing around galaxy clusters
B)the broad absorption lines found in the spectra of elliptical galaxies
C)the expansion of the universe
D)the flat rotation curves of spiral galaxies
E)X- ray observations of hot gas in galaxy clusters

Question 34

Why is the cause of the acceleration of the universe called "dark energy"?

A)We're pretty sure that whatever is causing the acceleration is pure evil.
B)Dark energy and dark matter are just two forms of the same thing.
C)The term dark energy was the one that caught on with other astronomers.

Question 35

What is a rotation curve?

A)a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis
B)a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
C)a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
D)a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy

Question 36

What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)?

A)It emits a great deal of light.
B)It can attract other matter through the force of gravity.
C)It consists of atoms or ions with nuclei made from protons and neutrons.
D)It is made of subatomic particles that scientists call WIMPs.

Question 37

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 tend to orbit at large distances from the galactic center.
B)They are subatomic particles.
C)They travel at speeds close to the speed of light.
D)They can neither emit nor absorb light.

Question 38

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)There is no longer any doubt that dark matter is made mostly of WIMPs.
C)Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.
D)About 90% of dark matter consists of ordinary matter in the halo of the galaxy, and of the other 10% of WIMPs.

Question 39

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?

A)observing how the cluster bends light from galaxies located behind it
B)studying X- ray emission from hot gas inside the cluster
C)measuring the temperatures of stars in the halos of the galaxies
D)measuring the speeds of galaxies orbiting the cluster's center

Question 40

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
You find yourself in a place that is unimaginably hot and dense. A rapidly changing gravitational field randomly warps space and time. Gripped by these huge fluctuations, you notice that there is but a single, unified force governing the universe. Where are you?

A)You are in the early universe before the Planck time.
B)You are inside the nucleus of an atom.
C)You are in the universe shortly after inflation.
D)You are floating somewhere in the universe near its end, 10100 years from now.
E)You are in the center of a young star.

Question 41

The actual matter density of the universe, accounting for all of the luminous matter and all of the dark matter known to exist in galaxies and clusters, is what fraction of the critical density?

A)1 percent
B)25 percent
C)10 percent
D)100 percent
E)200 percent

Question 42

How have astronomers measured the acceleration of the universe?

A)by comparing lookback times for white- dwarf supernovae (by measuring their apparent brightness)with the average distance between galaxies (based on their cosmological redshifts)
B)by measuring changes in galaxies' redshifts from year to year
C)They have not. It is a purely hypothetical idea.

Question 43

Based on observations of the cosmic microwave background, the overall composition of the universe is approximately _ .

A)4% ordinary (baryonic)matter, 23% non- ordinary (nonbaryonic)dark matter, 73% dark energy
B)15% ordinary (baryonic)matter, 85% non- ordinary (nonbaryonic)dark matter
C)100% ordinary (baryonic)matter
D)1% ordinary (baryonic)matter, 99% non- ordinary (nonbaryonic)dark matter

Question 44

Which of the following statements about large- scale structure is probably not true?

A)Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe.
B)Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
C)Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members.
D)Galaxies and clusters have grown around tiny density enhancements that were present in the early universe.

Question 45

Comparing the following 4 models, which model predicts that galaxies had the largest separations in the past? (Use the graph of expansion models as a guide.)

A)accelerating
B)coasting
C)critical
D)recollapsing

Question 46

Which hypothetical universe will eventually contract, if there were no dark energy?

A)the universe with 0.5 times the critical density of matter
B)the universe with 1.0 times the critical density of matter
C)the universe with 1.5 times the critical density of matter

Question 47

When we speak of the large- scale structure of the universe, we mean .

A)the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
B)the structure of any large galaxy
C)the structure of any individual cluster of galaxies
D)the overall shape of the observable universe

Question 48

How do astronomers create three- dimensional maps of the universe?

A)by interpreting the peculiar velocities of each galaxy
B)by using a galaxy's position on the sky and its redshift to determine its distance along the line of sight
C)through the comparison of computer models of galaxy formation with observations
D)by carefully measuring the parallax of each galaxy
E)by using a galaxy's position on the sky and its brightness as a measure of distance along the line of sight

Question 49

What is the best alternative explanation for the dark matter in the Milky Way based on the observed orbital motions of stars and gas?

A)We are not observing all the visible matter in the Milky Way.
B)We are not measuring the distances to gas clouds and stars properly.
C)We are not measuring the orbital velocities of gas clouds and stars properly.
D)We are not attributing enough mass to the visible matter in the Milky Way.
E)There is something wrong with our understanding of how gravity works.

Question 50

Why do we call dark matter "dark"?

A)It blocks out the light of stars in a galaxy.
B)It contains large amounts of dark- colored dust.
C)It emits no radiation that we have been able to detect.
D)It is dark brown or dark red in color.

Question 51

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 detect this gas with X- ray telescopes.
B)We infer its existence by observing its gravitational effects on the galaxy motions.
C)The hot gas shows up as bright pink in visible- light photos of galaxy clusters.
D)We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.

Question 52

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

A)The orbital speeds would rise upward with increasing distance from the galactic center, rather than remaining approximately constant.
B)The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve of a merry- go- round.
C)The orbital speeds would fall off sharply with increasing distance from the galactic center.
D)The rotation curve would look the same with or without the presence of dark matter.

Question 53

Weakly interacting particles (WIMPs)interact with other particles using gravity and the weak nuclear force. They cannot emit or absorb light. Therefore, how does the behavior of hydrogen and helium gas differ from that of WIMPs?

A)Hydrogen and helium gas affects particles that we can see.
B)Hydrogen and helium gas interacts with other forms of matter gravitationally.
C)Hydrogen and helium gas can cool by radiating energy.

Question 54

Your friend left your house and is now driving 40 miles per hour, but she has been speeding up the whole time since she left your house, and has gone 10 miles. How long has she been driving?

A)shorter than 15 minutes
B)longer than 15 minutes
C)15 minutes

Question 55

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

A)Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
B)Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers.
C)Elliptical galaxies probably contain far more dark matter than spiral galaxies.
D)Elliptical galaxies probably contain far less dark matter than spiral galaxies.

Question 56

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
You find yourself in a very low- density gas that is extremely hot. In fact, the temperature of the gas is so high that it is emitting lots of X- rays. There are no stars anywhere within about 10,000 light- years of you, but at slightly greater distances, your sky is brightened by hundreds (perhaps thousands)of beautiful, star- filled structures, some with majestic spiral shapes. Where are you?

A)You are in the universe when it was about 500 million years old, just before galaxies began to form.
B)You are in the universe about 380,000 years after the Big Bang when the cosmic microwave background radiation was formed.
C)You are in the center of the Milky Way Galaxy, looking outward into the Local Group.
D)You are in the outskirts of a galaxy whose nucleus is a powerful quasar.
E)You are in intergalactic space within a rich cluster of thousands of galaxies.

Question 57

Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?

A)The acceleration is very important in the cosmos today, but the evidence indicates that it will eventually slow down, allowing the universe to recollapse.
B)The cause of the acceleration is well- understood, and attributed to the particles that make up dark energy.
C)We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
D)The acceleration probably is not real, and what we attribute to acceleration is probably just a misinterpretation of the data.

Question 58

Which of the following items would not be considered "dark matter" by astronomers?

A)people
B)basketballs
C)black holes
D)red giants
E)weakly interacting massive particles

Question 59

How do X- ray measurements help us measure the amount of dark matter in galaxy clusters?

A)X- rays are emitted by hot gas, and the intracluster gas is heated by collisions with dark matter particles. More dark matter leads to greater heating, and hence stronger X- ray emission.
B)X- rays are emitted by hot gas, and the intracluster gas is heated by the gravitational effects of dark matter. More dark matter leads to greater heating, and hence stronger X- ray emission.
C)Dark matter absorbs X- rays. Therefore, more dark matter leads to weaker X- ray emission from galaxy clusters.

Question 60

What is one of the alternatives to the hypothesized existence of dark matter?

A)The observations are incorrect.
B)The theory of gravity is wrong.
C)The current theory of gravity is being used incorrectly.
D)The astronomers are misrepresenting the observations.

Question 61

How does gravitational lensing tell us about the mass of a galaxy cluster?

A)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.
B)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.
C)The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
D)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.

Question 62

The enclosed mass M(

A)distributed like a doughnut, with a hole in the center
B)concentrated in the center
C)spread out over a large range of radii

Question 63

Why can we not measure the mass of elliptical galaxies using the 21- cm line of hydrogen gas?

A)Supermassive black holes in the center of elliptical galaxies disturb measurements of the 21- cm line.
B)The disorderly motion of the material in elliptical galaxies prevents us from measuring different velocities at different sides of the galaxy.
C)All elliptical galaxies are too far away for us to detect the 21- cm line.
D)Elliptical galaxies have little gas, so do not have strong 21- cm line emission.

Question 64

The idea of dark matter arose to explain gravitational effects observed in galaxies and clusters of galaxies. However, studies of the early universe (especially of the cosmic microwave background and of chemical abundances)also tell us something about dark matter. What do they tell us?

A)They do not support the conclusion that dark matter is the dominant form of matter in the universe.
B)They tell us that dark matter was produced during the era of nuclei.
C)They add further support to the idea that dark matter really exists and is made of non- ordinary (nonbaryonic)matter, such as WIMPs.
D)They tell us that dark matter probably exists, but that it must be made of ordinary matter in the form of dim objects in galactic halos.

Question 65

What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?

A)measurements of the rotation curve for the universe
B)measurements of how galaxy speeds away from the Milky Way have increased during the past century
C)observations of the speeds of individual galaxies in clusters
D)observations of white dwarf supernovae

Question 66

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

Question 67

What do we mean when we say that a particle is weakly interacting?

A)It interacts only with other weak particles, such as neutrinos.
B)It interacts with other particles only through the weakest force, gravity.
C)It doesn't interact with any type of baryonic matter.
D)It interacts with other particles only through the weak force and the force of gravity.
E)It interacts with other particles only through the weak force.

Question 68

What is the primary way in which we determine the mass distribution of a spiral galaxy?

A)We calculate its mass- to- light ratio.
B)We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
C)We count the number of stars we can see at different distances from the galaxy's center.
D)We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

Question 69

How are rotation curves of spiral galaxies determined for distances beyond where starlight can be detected?

A)by measuring the broadening of the galaxy's absorption lines
B)by observations of the 21 cm line of atomic hydrogen
C)by observations of spectral lines emitted by dark matter
D)by watching the galaxies rotate over a period of decades
E)by extrapolation of the measured rotation curve

Question 70

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 exists but dark matter does not.
B)Dark energy is the dominant form of energy in the cosmos.
C)Neither dark energy nor dark matter really exist.
D)Dark energy does not exist and there is much more dark matter than we are aware of to date.

Question 71

The critical density of the universe is the _ .

A)density of dark matter in the universe
B)density of water
C)average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
D)actual average density of the universe

Question 72

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?

A)They interact with other matter only through the weak force and not through gravity or any other force.
B)They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily.
C)The light that they emit is so weak that it is undetectable to our telescopes.
D)They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.

Question 73

Which of the following are candidates for galactic dark matter?

A)brown dwarfs
B)faint red stars
C)Jupiter- size objects
D)WIMPs
E)all of the above

Question 74

Which observation(s)support(s)the idea that dark energy accounts for about 70% of the total mass- energy density of the universe?

A)This is just the right amount of energy, in addition to the observed matter and dark matter, to explain the pattern of temperatures in the cosmic microwave background.
B)This is just the right amount of a repulsive force to explain the observed acceleration in the expansion of the universe.
C)It is the right amount of energy to make the universe geometrically flat, which is mathematically easier to work with than a curved geometry.
D)A and B
E)B and C

Question 75

Measuring the amount of deuterium in the universe allows us to set a limit on .

A)the acceleration of the universe
B)the density of ordinary (baryonic)matter in the universe
C)the total amount of mass in the universe
D)the current age of the universe

Question 76

What might be causing the universe to accelerate?

A)brown dwarfs
B)WIMPs
C)gravitation
D)dark energy
E)white- dwarf supernovae

Question 77

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
You are on the surface of an object, and you have a fairly clear view out into space. Unfortunately, you are also very squashed. The light you observe from distant objects is slightly blueshifted. The surface of the object is composed primarily of carbon and oxygen, and the distance to the horizon seems about the same as that on Earth. By observing the sky for a few weeks, you realize that there are several planets orbiting your object. Where are you?

A)You are on the surface of a white dwarf.
B)You are "on" an accretion disk around a black hole.
C)You are on the surface of a neutron star.
D)You are on the surface of a planet that is somewhat more massive than the Earth.
E)You are on the surface of the Earth.

Question 78

It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

A)Elliptical galaxies contain much less dark matter than spiral galaxies, so it's much more difficult to measure.
B)We cannot observe spectral lines for elliptical galaxies.
C)Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
D)Stars in elliptical galaxies are dimmer, making them harder to study.

Question 79

Why do we call dark matter "dark"?

A)It emits no or very little radiation of any wavelength.
B)It emits no visible light.
C)It blocks out the light of stars in a galaxy.
D)We cannot detect the type of radiation that it emits.

Question 80

Which of the following best summarizes what we mean by dark energy?

A)It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos.
B)It is a name given to whatever is causing the expansion of the universe to accelerate with time.
C)It is the energy of black holes.
D)It is the energy contained in dark matter.