Chapter 22
Conceptual Quiz
Part A
Why do we call dark matter "dark?"
Hint A.1
Study Section 22.1
ANSWER:

It contains large amounts of dark-colored dust. It blocks out the light of stars in a galaxy. It emits no visible light. It emits no radiation of any wavelength that we have been able to detect.
Part B
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?
Hint B.1
Study Section 22.1
ANSWER:

There could be something wrong with our understanding of how atoms produce light. We could just be having a hard time understanding the observations because they involve very distant galaxies. There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales. The so-called dark matter is really just ordinary stars that are enshrouded in clouds of dust.
Part C
If galaxy UGC 2885 has stars orbiting 40,000 light-years from the center at a velocity of 300 km/s, and its rotation curve is flat to large distances, about how fast will stars at 80,000 light-years from the center be orbiting?
Hint C.1
Study Section 22.2
ANSWER:

80 km/s 300 km/s 40 km/s 3,200 km/s
Part D
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?
Hint D.1
Study Section 22.2
ANSWER:

The orbital speeds would fall off sharply with increasing distance from the galactic center. The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve of a merry-go-round. The orbital speeds would rise upward with increasing distance from the galactic center, rather than remaining approximately constant. The rotation curve would look the same with or without the presence of dark matter.
Part E
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?
Hint E.1
Study Section 22.2
ANSWER:

Yes, they tell us that the mass is concentrated near the center of the galaxy. Yes, they tell us that the dark matter is spread throughout the galaxy, with most located at large distances from the galactic center; that is, out in the halo that surrounds the disk. Yes, they tell us that dark matter is spread uniformly throughout the galactic disk. No, we cannot determine anything about the location of dark matter from the rotation curve.
Part F
It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
Hint F.1
Study Section 22.2
ANSWER:

Stars in elliptical galaxies are dimmer, making them harder to study. We cannot observe spectral lines for elliptical galaxies. Elliptical galaxies contain much less dark matter than spiral galaxies, so it's much more difficult to measure. Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
Part G
We define the Sun to have a mass-to-light ratio of 1 (in solar units). Therefore, a red giant star that has the same mass as the Sun but is 1,000 times as luminous as the Sun has a mass-to-light ratio of:
Hint G.1
Study Section 22.2
ANSWER:

1,000 0.1 0.001 10
Part H
Why did the intracluster medium go undetected until the 1960s?
Hint H.1
Study Section 22.2
ANSWER:

It makes up such a tiny fraction of the total mass in clusters of galaxies that our technology was not up to the task of detecting it. It is so rarely found in clusters of galaxies that we had not yet been lucky enough to point our telescopes at a cluster that has it. It consists entirely of dark matter, so we couldn't see it. It is most prominent at X-ray wavelengths and we did not have X-ray telescopes before the 1960s.
Part I
Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
Hint I.1
Study Section 22.2
ANSWER:

The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass. Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects. The question is nonsense --- gas temperature cannot possibly tell us anything about mass. 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.
Part J
How does gravitational lensing tell us about the mass of a galaxy cluster?
Hint J.1
Study Section 22.2
ANSWER:

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. 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. 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. The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
Part K
We have evidence that at least some dark matter consists of objects that astronomers call MACHOs (massive compact halo objects). What's the evidence?
Hint K.1
Study Section 22.2
ANSWER:

Recent telescopic observations with visible light have succeeded in identifying many of the objects known as MACHOs. The rotation curve of the Milky Way Galaxy shows that some dark matter must be made of MACHOs. We have detected gravitational lensing of distant objects that appears to be caused by compact but unseen objects in the halo of our galaxy. Radio observations have detected numerous MACHOs in the galactic halo.
Part L
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is NOT one of their characteristics?
Hint L.1
Study Section 22.2
ANSWER:

They can neither emit nor absorb light. They travel at speeds close to the speed of light. They are subatomic particles. They tend to orbit at large distances from the galactic center.
Part M
Why isn't space expanding within clusters of galaxies?
Hint M.1
Study Section 22.3
ANSWER:

Expansion of the universe affects only empty space, not space in which matter is present. The universe is not old enough yet for these objects to have begun their expansion. Space IS expanding within clusters of galaxies, which is why clusters tend to grow in size with time. Their gravity is strong enough to hold them together even while the universe as a whole expands.
Part N
Which of the following statements about galaxies and large-scale structures such as voids, clusters, superclusters, sheets and filaments is probably NOT true?
Hint N.1
Study Section 22.3
ANSWER:

Galaxies and clusters and the structures in which they are embedded form from tiny density enhancements that were present in the early universe. Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall. Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members. Voids began their existence as regions in the universe with a slightly lower density than the rest of the universe.
Part O
Based on current evidence, a supercluster is most likely to have formed in regions of space where _______.
Hint O.1
Study Section 22.3
ANSWER:

the acceleration of the expansion is proceeding faster than elsewhere. the density of dark matter was slightly higher than average when the universe was very young. a place where supermassive black holes were present in the very early universe. there was an excess concentration of hydrogen gas when the universe was very young.
Part P
According to current evidence, how does the actual average density of matter in the universe compare to the critical density?
Hint P.1
Study Section 22.4
ANSWER:

The total actual density of both dark matter and luminous matter together is no more than about 1% of the critical density. The density of luminous matter is much less than the critical density, but when we include dark matter the actual density probably equals the critical density. The actual density of matter is many times higher than the critical density. The actual density, even with dark matter included, is less than about a third of the critical density.
Part Q
Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant today (such as 24 km/s/Mly), the age of the universe is OLDEST if what is true?
Hint Q.1
Study Section 22.4
ANSWER:

The expansion rate is slowing dramatically with time (a recollapsing universe). The expansion rate has remained nearly constant with time (a coasting universe). The expansion rate has slowed by the amount expected for a universe with the critical density (a critical universe). The expansion rate has been increasing with time (an accelerating universe).
Part R
Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
Hint R.1
Study Section 22.4
ANSWER:

We have strong observational evidence that the acceleration is real, but essentially only a name (dark energy) for what is causing it. The acceleration is very important in the cosmos today, but we think the acceleration will eventually slow down. The acceleration was something a few astronomers thought they saw, but we no longer think it is real and assume that the original claims were based on errors in interpreting the data. We are certain we understand the cause of the acceleration: the cause is dark energy.
Part S
Suppose it turns out that dark matter is made up of an unstable form of matter that has a very long half-life of say, 50 billion years. What is a possible consequence of dark matter decaying to energy?
Hint S.1
Study Section 22.4
ANSWER:

Stars would cease to exist. The universe would cease its expansion. Clusters of galaxies would not be able to confine their galaxies and the stars and gas in the outer parts of galaxies would fly off. Solar systems would expand and disperse.
Part T
Some people think it would be better if we lived in a recollapsing universe that would eventually stop expanding and start contracting, rather than in an accelerating universe. For this to be the case, which of the following would have to be true (based on current understanding)?
Hint T.1
Study Section 20.4.
ANSWER:

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