The Essential Cosmic Perspective, 8e (Bennett et al.)
Chapter 18 Dark Matter, Dark Energy, and the Fate of the Universe
18.1 Multiple Choice Questions
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 radiation of any wavelength.
D) It blocks out the light of stars in a galaxy.
2) 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
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 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.
3) Dark matter in our galaxy is thought to exist based on the observed orbital motions of stars
and gas in the outer part of the galaxy. What is the best alternative explanation for the dark
matter in the Milky Way?
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.
4) 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
5) The distribution of the dark matter in a spiral galaxy is
A) approximately spherical and about the same size as the galaxy’s visible halo.
B) approximately spherical and much larger than the galaxy’s visible 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.
6) 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 Doppler shift of spectral 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.
7) Why can we not measure the mass of elliptical galaxies using the 21-cm line of hydrogen gas?
A) All elliptical galaxies are too far away for us to detect 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) Elliptical galaxies have little gas, so they do not have strong 21-cm line emission.
D) Supermassive black holes in the center of elliptical galaxies disturb measurements of the 21-
cm line.
8) Which of the following items is not evidence for dark matter in clusters of galaxies?
A) the way the cluster bends the light of background galaxies
B) the high orbital speeds of cluster galaxies
C) the high temperatures of the hot gas in between the galaxies
D) the huge luminosity in visible light coming from cluster galaxies
9) 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.
10) 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 gravitational lensing
D) none of the above
11) 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 the gravitational effects of
dark matter. More dark matter leads to greater heating, and hence stronger X-ray emission.
B) Dark matter absorbs X-rays. Therefore, more dark matter leads to weaker X-ray emission
from galaxy clusters.
C) 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.
12) 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.
13) The gravitational lens effect has been verified by
A) observations of the bending of starlight by the Sun during a 1919 eclipse.
B) observations of multiple images of the same background galaxy seen towards a massive
galaxy cluster.
C) single images of highly distorted, high-redshift galaxies seen towards massive galaxy clusters.
D) all of the above
E) None of the above; this effect is purely hypothetical.
14) 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
15)
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
16) Which of the following is the most likely candidate for what makes up the majority of dark
matter?
A) brown dwarfs
B) Jupiter-size objects
C) WIMPs
D) faint red stars
E) black holes
17) 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) less than 1 percent
B) 10 percent
C) 25 percent
D) 50 percent
18) 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) about 1 percent
B) about 10 percent
C) about 30 percent
D) about 50 percent
E) 100 percent
19) Measuring the amount of deuterium in the universe allows us to set a limit on
A) the temperature of the universe at the end of the era of nuclei.
B) the total amount of mass in the universe.
C) the density of ordinary (baryonic) matter in the universe.
D) the expansion rate of the universe.
E) the current age of the universe.
20) Based on the observed amount of deuterium in the universe, we can conclude that
A) ordinary (baryonic) matter makes 75 percent of the mass of the universe.
B) neutrons outnumber protons 7 to 1 in the universe.
C) most of the deuterium that was created during the era of nucleosynthesis has since been
destroyed.
D) the density of ordinary (baryonic) matter is about 5 percent of the critical density.
E) we live in a critical-density universe.
21) 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.
22) 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.
23) 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, therefore 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.
24) 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.
25) 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
26) 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
27) 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.
28) Why do astronomers think the expansion rate of the universe is accelerating, rather than
decelerating (as previously thought)?
A) The average distance between galaxies is greater than expected when looking at very distant
galaxies.
B) The average distance between galaxies is less than expected when looking at very distant
galaxies.
C) The average distance between galaxies is greater than expected when looking at nearby
galaxies.
D) The average distance between galaxies is less than expected when looking at nearby galaxies.
29) 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.
30) What might be causing the universe to accelerate?
A) WIMPs
B) brown dwarfs
C) white-dwarf supernovae
D) gravitation
E) dark energy
31) Which observation supports 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 the form of a repulsive force) to explain the
observed acceleration in the expansion rate of the universe.
B) This is just the right amount of energy that is predicted to have been released during the
period of inflation in the early universe.
C) This is just the right amount of energy to cause the temperature of the universe to be 3 K, as
predicted by the Big Bang theory.
D) all of the above
32) The more baryons there are in the universe (a higher density of baryonic matter), the lower
the ratio of deuterium to hydrogen. Therefore, if Harry measures a higher ratio of deuterium to
hydrogen than Sally, Harry infers
A) the same density of baryons as Sally.
B) a lower density of baryons than Sally.
C) a higher density of baryons than Sally.
33) The graph above shows 4 models for how the average distance between galaxies could
change with time, from the past (left) to now (middle) to the future (right hand side). The graph
also shows real data, based on studies of supernovae. Each black dot on the graph is for one
supernova explosion. The data are plotted with dots and black lines that indicate the range of
uncertainty of each individual measurement. Use this graph to answer the following questions
about cosmological models for the expansion of the universe. (Note that the models are in the
same order, from top to bottom, whether on the right hand side of the graph or the left hand side.
For example the accelerating model is the top line on both sides of the graph.)
Which model(s) predict(s) that galaxies are getting farther apart NOW?
A) accelerating
B) coasting
C) critical
D) recollapsing
E) all of them
F) none of them
34) Considering the graph of expansion models, which model(s) predict(s) that galaxies will
eventually get closer together?
A) accelerating
B) coasting
C) critical
D) recollapsing
E) all of them
F) none of them
35) Considering the graph of expansion models, which model predicts that galaxies had the
largest separations in the past?
A) accelerating
B) coasting
C) critical
D) recollapsing
36) Considering the graph of expansion models along with the data points, which model is most
strongly supported by the data?
A) critical
B) accelerating
C) recollapsing
D) coasting
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.
37) 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 center of a young star.
B) You are in the early universe before the Planck time.
C) You are floating somewhere in the universe near its end, 10100 years from now.
D) You are inside the nucleus of an atom.
E) You are in the universe shortly after inflation.
38) 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 star very much like our Sun.
B) You are in the early universe during the era of nucleosynthesis.
C) You are inside a nuclear power plant on Earth.
D) You are in the center of a massive star near the end of its life.
E) You are in the center of a white dwarf.
39) 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 the Earth.
B) You are on the surface of a planet that is somewhat more massive than the Earth.
C) You are on the surface of a white dwarf.
D) You are “on” an accretion disk around a black hole.
E) You are on the surface of a neutron star.
40) 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 during its first 380,000 years.
B) You are in the universe more than 10100 years in the future.
C) You are in an accretion disk around a supermassive black hole.
D) You are in the central regions of a quasar.
E) You are in the planetary nebula that the Sun will form about 5 billion years from now.
41) 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 in the center of a star very much like our Sun.
B) You are in the early universe during the era of nucleosynthesis.
C) You are inside a nuclear power plant on Earth.
D) You are in the center of a star much smaller than the Sun.
E) You are in the center of a massive star near the end of its life.
42) As far as you look, there seems to be nothing at all. Even the nearest electron is light-years
away. No matter how far you travel, you can find no solid matter, not even a single proton. It’s as
if all of the matter in the universe has disintegrated. You do, however, detect a few strong
gravitational fields—probably due to supermassive black holes—at enormous distances from
you. Where are you?
A) You are in the Planck Era of the early universe.
B) You are in the universe when it is 1040 years old.
C) You are in the universe when it is 10100 years old.
D) You are in the central regions of a quasar.
E) You are in the GUT era of the early universe during the period of inflation.
43) 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 center of the Milky Way Galaxy, looking outward into the Local Group.
C) You are in the universe about 380,000 years after the Big Bang when the cosmic microwave
background radiation was formed.
D) You are in intergalactic space within a rich cluster of thousands of galaxies.
E) You are in the outskirts of a galaxy whose nucleus is a powerful quasar.
44) 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 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 year after the Big Bang during the formation of the
cosmic background radiation.
C) You are in the universe about 5 minutes after the Big Bang just as the nucleosynthesis era is
ending.
D) You are in a closed universe just as it begins to re-collapse.
E) You are in intergalactic space within a rich cluster of galaxies.
18.2 True/False Questions
1) Dark matter in galaxies and clusters of galaxies is purely hypothetical because we have no
way of detecting its presence.
2) A galaxy with a lot of dark matter would have a high mass-to-light ratio compared to the Sun.
3) If the universe is accelerating, it will likely expand forever.
4) If we were to learn that the fate of the universe is to re-collapse, then we must have
misinterpreted Hubble’s law because the universe must be contracting, rather than expanding as
generally believed.
5) 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.
6) Dark matter particles were definitively detected in 2013.
7) Some galaxy clusters have not finished forming and are still attracting new members today.
8) The visible parts of galaxies make up about 10 percent of the critical density of the universe.
9) GUT theories predict that protons will eventually decay, causing all solid objects in the
universe to fall apart if the universe keeps expanding forever.
10) Recent measurements of temperature fluctuations in the cosmic microwave background
radiation indicate the geometry of the universe is flat. This means that about 70 percent of the
mass-energy in the universe must be in the form of dark energy.
18.3 Process of Science Questions
1) 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?