Deck 16: Dark Matter, Dark Energy, and the Fate of the Universe

Question 1

What are peculiar velocities?

A) velocities perpendicular to our line of sight
B) velocities directly along our line of sight measured by the Doppler effect
C) velocities that we cannot explain by the force of gravity alone
D) velocities caused by the expansion of the universe
E) velocities of distant galaxies that are not caused by the expansion of the universe

Answer 1

E

Question 2

Which of the following particles are baryons?

A) electrons
B) neutrinos
C) protons
D) WIMPs
E) photons

Answer 2

C

Question 3

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

A) by extrapolation of the measured rotation curve
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 measuring the broadening of the galaxy's absorption lines

Answer 3

B

Question 4

A gravitational lens occurs when

A) a massive object bends light beams that are passing nearby.
B) a massive object causes 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) a telescope lens is distorted by gravity.

Question 5

Which of the following is an example of baryonic matter?

A) you
B) the particles produced by physicists in particle accelerators
C) electrons and positrons produced by pair production
D) WIMPs
E) neutrinos

Question 6

A mass-to-light ratio for a galaxy of much greater than one 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 much less light than our Sun.
D) on average, each solar mass of matter in the galaxy emits much more light than our Sun.
E) most stars in the galaxy are more massive than our Sun.

Question 7

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

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

Question 8

Which of the following are candidates for galactic dark matter?

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

Question 9

Which of the following is not evidence for dark matter?

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

Question 10

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.

Question 11

How do we determine the amount of dark matter in elliptical galaxies?

A) We measure the orbital velocities of star-forming gas clouds in the outer portion of the galaxy.
B) We measure the broadening of the galaxy's absorption lines to infer the speeds of stars at different distances from the galactic center.
C) We count the number of stars in the galaxy. Combining this with the galaxy's volume, we can calculate the galaxy's density.
D) We search for dark lanes of dust and black holes within the galaxy.
E) We measure how fast the galaxy rotates as a whole.

Question 12

Some potential MACHOs (massive compact halo objects) emit radiation. Why are they nevertheless considered dark matter?

A) They are not as bright as a normal star and hence are not visible across great distances of space.
B) They do not emit light in the visible wavelength range.
C) They emit radiation only in the form of particles of dark matter.
D) They are made of particles of dark matter that release some radiation.

Question 13

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.

Question 14

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

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

Question 15

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 orbital velocities of galaxies in the 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

Question 16

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

Question 17

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 have zero mass like the photon.
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.

Question 18

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

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

Question 19

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

A) Hubble's law of expansion applies only to the space between galaxies.
B) As we are inside our solar system and the Milky Way, we cannot observe their expansion.
C) The universe is not old enough for the solar system or Milky Way to have begun 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 20

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 and about the same size as the stellar disk.
D) flattened in a disk but about ten times larger than the stellar disk.
E) predominantly concentrated in the spiral arms.

Question 21

If the universe is accelerating, it will expand forever.

Question 22

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) 10 percent
C) 25 percent
D) 100 percent
E) 200 percent

Question 23

Gravitational Lensing: Summarize the observational evidence that a gravitational field can deflect the path of light. In figures such as 16.8 on page 451, how would you prove to a skeptic that the distorted images indicated by the arrows are indeed multiple images of a single gravitationally-lensed galaxy. Muster as much observational evidence as you can.

Question 24

A galaxy with a lot of dark matter would have a high mass-to-light ratio compared to the Sun.

Question 25

Some galaxy clusters have not finished forming and are still attracting new members today.

Question 26

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

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

Question 27

If we were to learn that the universe is a re-collapsing universe, then we must have misinterpreted Hubble's Law because the universe must be contracting, rather than expanding as generally believed.

Question 28

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

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

Question 29

An Accelerating Universe? Current evidence that the expansion of the universe is accelerating is based entirely upon observations of distant white dwarf supernova. List all of the assumptions that you can think of that are made in using white dwarf supernova as distance indicators. Is it conceivable that any of these assumptions may be incorrect?

Question 30

How do we know that there can only be insignificant amounts of dark matter in our solar system?

Question 31

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

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

Question 32

MACHOs are expected to be found only in the disk of the Milky Way, whereas WIMPs would be found only in the halo.

Question 33

One possible ingredient of dark matter is known as WIMPs, or weakly interacting massive particles. Our best guess is that WIMPs probably are made of protons and neutrons.

Question 34

What might be causing the universe to accelerate?

A) WIMPs
B) MACHOs
C) White-dwarf supernovae
D) Gravitation
E) Dark energy

Question 35

The visible parts of galaxies contribute about 10 percent of the critical density of the universe.

Question 36

If all the "dark matter" in our universe were to be instantaneously removed, which of the following would not happen?

A) The Solar System would fly apart.
B) The Milky Way would fly apart.
C) Clusters of galaxies would fly apart.
D) The Universe would expand forever.
E) all of the above

Question 37

"Beauty" in Astronomy: In the absence of observational evidence, scientists often rely on other considerations to make decisions. One such consideration that we have met before (Chapter 3) is Occam's razor: the simplest explanation is to be preferred. Another consideration that is often used (and sometimes championed) by astronomers is the perceived "beauty" of the ideas themselves. Imagine you lived at a time before observational tests could be brought to bear on which of the ultimate fates (re-collapsing, critical, coasting, or accelerating) applies to our Universe. Which fate you would support based on your own personal idea of "beauty"? Give your arguments, even if you think they are not scientific. Did you choose the observationally supported theory (you may need to skip ahead to Chapter 17)? What do you think of the role such considerations play in science? Are they sometimes necessary?

Question 38

What fraction of the mass needed to halt expansion of the universe exists in the form of visible, luminous matter (such as the stars within galaxies)?

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

Question 39

MACHOs (massive compact halo objects) have been searched for using the technique of gravitational lensing, but, to date, none have been found.

Question 40

Dark matter in galaxies and clusters of galaxies is purely hypothetical because we have no way of detecting its presence.

Question 41

The enclosed mass M(

A) Spread out over a large range of radii
B) Distributed like a doughnut, with a hole in the center
C) Concentrated in the center

Question 42

Explain how observations of white-dwarf supernovae provide information on the expansion of the universe when it was younger.

Question 43

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 can cool by radiating energy.
C) Hydrogen and helium gas interacts with other forms of matter gravitationally.

Question 44

Which statement correctly describes the role of gravity in galaxy formation?

A) Gravity slows down photons so they collect into galaxies.
B) Gravity makes galaxies spin.
C) Gravity makes the dense regions denser.
D) Gravity makes the universe more uniform and smooth than it was at the beginning.

Question 45

What is the difference between WIMPs and MACHOs?

Question 46

Which galaxy rotation curve, based on hydrogen gas speeds and locations, would indicate an unusually large amount of dark matter?

A) Orbital speeds decline, beyond where the stars are.
B) Orbital speeds remain constant, beyond where the stars are.
C) Orbital speeds rise, beyond where the stars are.

Question 47

Your cell phone 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) Longer than 15 minutes
B) 15 minutes
C) Shorter than 15 minutes

Question 48

Which hypothetical universe has an older age, all other things being equal?

A) Dark matter doesn't affect the inferred age of the universe.
B) The one with less dark matter
C) The one with more dark matter

Question 49

The mass enclosed in any orbit can be estimated from the equation M(

A) The mass is concentrated in the center.
B) The mass is spread out over large radius.
C) Cannot be determined from the information given.
D) The mass is distributed like a doughnut, with a hole in the center.

Question 50

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

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

Question 51

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

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

Question 52

The mass enclosed in any orbit can be estimated from the equation M(

A) The mass is spread out over large radius.
B) The mass is concentrated in the center.
C) The mass is distributed like a doughnut, with a hole in the center.
D) Cannot be determined from the information given.

Question 53

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

A) People
B) Basketballs
C) Weakly interacting massive particles
D) Red giants
E) Black holes

Question 54

Which hypothetical universe has a older age, all other things being equal?

A) The one with more dark energy.
B) Dark energy doesn't affect the inferred age of the universe.
C) The one with less dark energy

Question 55

The quantity called the "enclosed mass" M(

A) Distributed in the shape of a doughnut, with a hole in the center
B) Impossible to say
C) Concentrated in the center
D) Spread out over a large volume

Question 56

Briefly describe the four possible expansion patterns (or fates) of the universe.

Question 57

Briefly describe two different ways of measuring the mass of a cluster of galaxies.

Question 58

Your cell phone friend left your house and has been driving 60 miles per hour the whole time since then, and has gone 30 miles. How long has she been driving?

A) 60 minutes
B) 1800 minutes
C) ¹/₂ minute
D) 2 minutes
E) 30 minutes

Question 59

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

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

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 astronomers are misrepresenting the observations.
D) The current theory of gravity is being used incorrectly.

Question 61

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

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

Question 62

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.

Question 63

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

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

Question 64

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

A) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
B) The amount of light emitted by stars at different distances is about the same throughout the galaxy.
C) The disk of a spiral galaxy is quite flat rather than spherical like the halo.
D) All the galaxy's mass is concentrated in its flat, gaseous disk.

Question 65

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) It magnifies the effects of gravity that we see in the cluster.
C) It is an unusually large cluster that has a lot of gravity.
D) The overall shape of the cluster is that of a lens.

Question 66

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) dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
B) luminous matter emits white light, much like the light reflected from icebergs
C) black holes are much more interesting than ordinary stars that give off light
D) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter

Question 67

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) Elliptical galaxies probably contain far less dark matter than spiral galaxies.
C) Elliptical galaxies probably contain far more dark matter than spiral galaxies.
D) Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers.

Question 68

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 69

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

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

Question 70

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) About 90% of dark matter consists of ordinary matter in the halo of the galaxy, and of the other 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.

Question 71

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.

Question 72

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 1.5 times the critical density of matter
C) The universe with 1.0 times the critical density of matter

Question 73

What is a rotation curve?

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

Question 74

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

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

Question 75

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 76

The critical density of the universe is the ________.

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

Question 77

Some dark matter may be ordinary matter that orbits in the halo of the galaxy. Which of the following would not be considered a type of ordinary dark matter?

A) Globular clusters in the halo of the galaxy
B) Brown dwarfs in the halo of the galaxy
C) Planets in the halo of the galaxy
D) You, if you happened to be floating in the halo of the galaxy

Question 78

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

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

Question 79

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

Question 80

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

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