2) “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 would you 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? What do you think of the role such considerations play in science? Are they
sometimes necessary?
3) Gravitational Lensing: Summarize the observational evidence that a gravitational field can
deflect the path of light. In figures such as 18.7 on page 473, 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.
4) New Perspectives? Chapter 18 ends the discussion of the origin and evolution of the universe.
Galaxy and star formation, as well as the origin of planetary systems, were discussed in earlier
chapters. It is perhaps time to reflect upon what has been learned. There is an oft-quoted line in
T.S. Eliot’s poem Little Gidding (1942): “And the end of all our exploring will be to arrive where
we started and know the place for the first time.” Have you gained any new perspectives on the
position of the Earth (and human beings) within the universe through your study of the science of
astronomy? If so, what are they?
5) Changing minds: From the time the idea of dark matter was first proposed in the 1930s, it
took nearly 50 years for astronomers to universally accept the existence of dark matter. Discuss
the process by which that acceptance came about. What evidence was collected, and how was it
checked?
18.4 Short Answer Questions
1) How do we know that there can only be insignificant amounts of dark matter in our solar
system?
2) Briefly describe two different ways of measuring the mass of a cluster of galaxies.
3) Briefly explain why astronomers think that dark matter must be composed of Weakly
Interacting Massive Particles (WIMPs) rather than difficult to see normal objects like planets,
brown dwarfs, and the faintest low mass main-sequence stars.
4) Explain how observations of white-dwarf supernovae provide information on the expansion of
the universe when it was younger.
5) Briefly describe the four possible expansion patterns (or fates) of the universe.
18.5 Mastering Astronomy Reading Quiz
1) Which of the following best summarizes what we mean by dark matter?
A) matter for which we have theoretical reason to think it exists, but no observational evidence
for its existence
B) matter that may inhabit dark areas of the cosmos where we see nothing at all
C) matter consisting of black holes
D) matter that we have identified from its gravitational effects but that we cannot see in any
wavelength of light
2) Which of the following best summarizes what we mean by dark energy?
A) It is the energy contained in dark matter.
B) It is the energy of black holes.
C) It is a type of energy that is associated with the “dark side” of The Force that rules the
cosmos.
D) It is a name given to whatever is causing the expansion of the universe to accelerate with
time.
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) dark matter represents much more mass and extends much further from the galactic center
than the visible stars of the Milky Way
B) black holes are much more numerous than ordinary stars that give off light
C) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of
dark matter
D) luminous matter emits white light, much like the light reflected from icebergs
4) What is a rotation curve?
A) a precise description of the shape of a star‘s orbit around the center of the Milky Way Galaxy
B) a graph showing how orbital velocity depends on distance from the center of a spiral galaxy
C) a graph that shows a galaxy’s mass on the vertical axis and size on the horizontal axis
D) a curve used to decide whether a star’s orbit places it in the disk or the halo of a spiral galaxy
5) What is the primary way in which we determine the mass distribution of a spiral galaxy?
A) We apply Newton’s version of Kepler’s third law to the orbits of globular clusters in the
galaxy’s halo.
B) We calculate its mass-to-light ratio.
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.
6) What do we mean when we say that the rotation curve for a spiral galaxy is “flat”?
A) All the galaxy’s mass is concentrated in its flat, gaseous disk.
B) The disk of a spiral galaxy is quite flat rather than spherical like the halo.
C) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as
gas clouds located further inward.
D) The amount of light emitted by stars at different distances is about the same throughout the
galaxy.
7) Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what
does current evidence suggest?
A) Elliptical galaxies probably contain far more dark matter than spiral galaxies.
B) Elliptical galaxies probably contain about the same proportion of their mass in the form of
dark matter as do spiral galaxies.
C) Elliptical galaxies probably contain far less 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.
8) Which of the following is not one of the three main strategies used to measure the mass of a
galaxy cluster?
A) observing how the cluster bends light from galaxies located behind it
B) measuring the speeds of galaxies orbiting the cluster’s center
C) measuring the temperatures of stars in the halos of the galaxies
D) studying X-ray emission from hot gas inside the cluster
9) 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) The overall shape of the cluster is that of a lens.
D) It is an unusually large cluster that has a lot of gravity.
10) Which of the following statements best summarizes current evidence concerning dark matter
in individual galaxies and in clusters of galaxies?
A) Within individual galaxies, dark matter is always concentrated near the galactic center, and
within clusters it is always concentrated near the cluster center.
B) Dark matter is present in individual galaxies, but there is no evidence that it can exist between
the galaxies in a cluster.
C) Dark matter is the dominant form of mass in both clusters and in individual galaxies.
D) Dark matter is present between galaxies in clusters, but not within individual galaxies.
11) 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 has a negative charge.
C) It is made of subatomic particles that scientists call WIMPs.
D) It consists of atoms or ions with nuclei made from protons and neutrons.
12) 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) you, if you happened to be floating in the halo of the galaxy
B) brown dwarfs in the halo of the galaxy
C) planets in the halo of the galaxy
D) globular clusters in the halo of the galaxy
13) What do we mean when we say that particles such as neutrinos or WIMPs are weakly
interacting?
A) They are only weakly bound by gravity, which means they can fly off and escape from
galaxies quite easily.
B) They interact with other matter only through the weak force and not through gravity or any
other force.
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.
14) 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 current age of the universe
D) the total amount of mass in the universe
15) Which of the following best sums up current scientific thinking about the nature of dark
matter?
A) About 90% of dark matter consists of ordinary matter in the halo of the galaxy, and of the
other 10% of WIMPs.
B) Dark matter probably does not really exist, and rather indicates a fundamental problem in our
understanding of gravity.
C) There is no longer any doubt that dark matter is made mostly of WIMPs.
D) Most dark matter probably consists of weakly interacting particles of a type that we have not
yet identified.
16) 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 overall shape of the observable universe
C) the structure of any individual cluster of galaxies
D) the structure of any large galaxy
17) 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
18) 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) observations of the speeds of individual galaxies in clusters
C) observations of white dwarf supernovae
D) measurements of how galaxy speeds away from the Milky Way have increased during the
past century
19) 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 add further support to the idea that dark matter really exists and is made of non-ordinary
(nonbaryonic) matter, such as WIMPs.
C) 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.
D) They tell us that dark matter was produced during the era of nuclei.
20) Each of the four expansion models (recollapsing, critical, coasting, and accelerating) predict
different ages for the universe, given the current expansion rate. Why is this?
A) Each model predicts the universe is expanding slower now than in the past, thus
overestimating the age of the universe.
B) Each model predicts the universe is expanding faster now than in the past, thus
underestimating the age of the universe.
C) The age of the universe depends on the past expansion rate, and each model predicts a
different expansion rate in the past.
D) They predict different ages because we really have no way to accurately estimate the age of
the universe.
21) Which of the following best sums up current scientific thinking about the nature of dark
energy?
A) Dark energy most likely consists of a form of photons that we can’t see or detect.
B) Dark energy is most likely made up of weakly interacting particles that do not interact with
light.
C) Dark energy probably exists, but we have little (if any) idea what it is.
D) Dark energy is the source of the mind weapon used by Sith Lords in Star Wars.
18.6 Mastering Astronomy Concept Quiz
1) Why do we call dark matter “dark”?
A) It emits no radiation that we have been able to detect.
B) It absorbs all radiation, making it dark.
C) It contains large amounts of dark-colored dust.
D) It blocks out the light of stars in a galaxy.
2) Although most astronomers assume dark matter really exists, there is at least one other
possible explanation for the phenomena attributed to dark matter. What is it?
A) We could just be having a hard time understanding the observations because they involve
very distant galaxies.
B) The so-called dark matter is really just ordinary stars that are enshrouded in clouds of dust.
C) There could be something wrong or incomplete with our understanding of how gravity
operates on galaxy-size scales.
D) There could be something wrong with our understanding of how atoms produce light.
3) Based on observations of the cosmic microwave background, the overall composition of the
universe is approximately ________.
A) 5% ordinary (baryonic) matter, 27% non-ordinary (nonbaryonic) dark matter, 68% dark
energy
B) 100% ordinary (baryonic) matter
C) 1% ordinary (baryonic) matter, 99% non-ordinary (nonbaryonic) dark matter
D) 15% ordinary (baryonic) matter, 85% non-ordinary (nonbaryonic) dark matter
4) 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 orbital speeds would fall off sharply with increasing distance from the galactic center.
C) The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve
of a merry-go-round.
D) The rotation curve would look the same with or without the presence of dark matter.
5) 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 throughout the galaxy, with most located at large
distances from the galactic center.
B) Yes, they tell us that dark matter is concentrated near the center of the galaxy.
C) No, we cannot determine anything about the location of dark matter from the rotation curve.
D) Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.
6) It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a
spiral galaxy. Why?
A) We cannot observe spectral lines for elliptical galaxies.
B) Stars in elliptical galaxies are dimmer, making them harder to study.
C) Elliptical galaxies contain much less dark matter than spiral galaxies, so it’s much more
difficult to measure.
D) Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at
great distances from the centers of spiral galaxies.
7) 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 motions of galaxies.
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 from the X-rays it produces.
8) What does the temperature of the gas between galaxies in galaxy clusters tell us about the
mass of the cluster?
A) The temperature tells us the average speeds of the gas particles, which are influenced by
gravity, so we can use these speeds to determine the cluster mass.
B) Temperature is always directly related to mass, which is why massive objects are always
hotter than less massive objects.
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.
9) How does gravitational lensing tell us about the mass of a galaxy cluster?
A) The lensing broadens spectral lines, and we can use the broadening to “weigh” the cluster.
B) 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.
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) 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.
10) 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 are subatomic particles.
B) They travel at speeds close to the speed of light.
C) They tend to orbit at large distances from the galactic center.
D) They can neither emit nor absorb light.
11) On average, is the distance between galaxies increasing within clusters of galaxies?
A) Yes, and that is why clusters tend to grow in size with time.
B) No, because expansion of the universe affects only empty space, not space in which matter is
present.
C) No, because the universe is not old enough yet for these objects to have begun their
expansion.
D) No, because their gravity is strong enough to hold them together even while the universe as a
whole expands.
12) Which of the following statements about large-scale structure is probably not true?
A) Clusters and superclusters appear to be randomly scattered about the universe, like dots
sprinkled randomly on a wall.
B) Many cluster and superclusters are still in the process of formation as their gravity gradually
pulls in new members.
C) Galaxies and clusters have grown around tiny density enhancements that were present in the
early universe.
D) Voids between superclusters began their existence as regions in the universe with a slightly
lower density than the rest of the universe.
13) Based on current evidence, a supercluster is most likely to have formed in regions of space
where ________.
A) there was an excess concentration of hydrogen gas when the universe was very young
B) the density of matter was slightly higher than average 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
14) Based on current evidence, how does the actual average density of matter in the universe
compare to the critical density?
A) The actual density, even with dark matter included, is only about one third of the critical
density.
B) If we include dark matter, the actual density equals the critical density.
C) The actual density of matter is many times higher than the critical density.
D) The actual density of dark matter and luminous matter combined is no more than about 1% of
the critical density.
15) 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) We have moderately strong evidence that the acceleration is real, but essentially no idea what
is causing it.
C) The cause of the acceleration is well-understood, and attributed to the particles that make up
dark energy.
D) The acceleration probably is not real, and what we attribute to acceleration is probably just a
misinterpretation of the data.
16) 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 exists but dark matter does not.
C) Neither dark energy nor dark matter really exists.
D) Dark energy does not exist and there is much more dark matter than we are aware of to date.