Chapter 22
Basic Skills/Definitions Quiz
Part A
Which of the following best summarizes what we mean by dark matter?
Hint A.1
Study Section 22.1.
ANSWER:

Matter that may inhabit dark areas of the cosmos where we see nothing at all Matter for which we have theoretical reason to think it exists, but no observational evidence for its existence Matter consisting of black holes Matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
Part B
Which of the following best summarizes what we mean by dark energy?
Hint B.1
Study Section 22.1.
ANSWER:

It is the energy contained in dark matter. It is a name given to whatever might be responsible for causing the expansion of the universe to accelerate with time. It is a type of energy that is associated with the "dark side" of the force that rules the cosmos. It is the energy of black holes.
Part C
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 ______.
Hint C.1
Study Section 22.2
ANSWER:

Black holes are much more interesting than ordinary stars that give off light. The unseen dark matter represents much more mass and extends much further from the galactic center than the luminous matter in the Milky Way. The luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter. Luminous matter emits white light, much like the light reflected from icebergs.
Part D
What is a rotation curve?
Hint D.1
Study Section 22.2.
ANSWER:

A graph showing how orbital velocity depends on distance from the center for a spiral galaxy A graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis A curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy A precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
Part E
What is the main way that we determine the mass distribution of a spiral galaxy?
Hint E.1
Study Section 22.2
ANSWER:

Constructing its rotation curve, which we make by looking for Doppler shifts in the 21-cm emission from atomic hydrogen in its disk. Counting the number of stars we can see at different distances from the galaxy's center. Measuring its mass-to-light ratio. Applying Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
Part F
What do we mean when we say that the rotation curve for a spiral galaxy is "flat?"
Hint F.1
Study Section 22.2
ANSWER:

All the galaxy's mass is concentrated in its flat, gaseous disk. Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. The disk of a spiral galaxy is quite flat, rather than being spherical like the halo. The amount of light emitted by stars at different distances is about the same throughout the galaxy.
Part G
Although we know less about dark matter in elliptical galaxies than we do in spiral galaxies, what does current evidence suggest?
Hint G.1
Study Section 22.2
ANSWER:

Elliptical galaxies probably contain far more dark matter than spiral galaxies. Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers. Elliptical galaxies probably contain a similar proportion of their mass in the form of dark matter as do spiral galaxies. Elliptical galaxies probably contain far less dark matter than spiral galaxies.
Part H
In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that ______.
Hint H.1
Study Section 22.2
ANSWER:

the higher the amount of mass relative to light (higher mass-to-light ratio), the older the galaxy or cluster. the higher amount of mass relative to light (higher mass-to-light ratio), the lower the proportion of dark matter. the amount of light should be at least one solar luminosity for each solar mass of matter (mass-to-light ratio less than or equal to 1). the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter.
Part I
Which of the following is NOT one of the three main strategies used to measure the masses of galaxy clusters?
Hint I.1
Study Section 22.2
ANSWER:

Observing how the clusters bend light as gravitational lenses. Measuring the temperatures of stars in the halos of the galaxies. Studying the X-ray emission from hot gas inside the clusters. Measuring the speeds of galaxies orbiting the cluster's center.
Part J
What do we mean by the intracluster medium in galaxy clusters?
Hint J.1
Study Section 22.2
ANSWER:

The famous oracle who is able to commune with the galaxies by lighting candles and speaking to the dead. It is the gas and dust found within the cluster's individual galaxies. The hot, X-ray emitting gas that fills the space between galaxies in the cluster. It is material found between the clusters of galaxies that make up a supercluster.
Part K
When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
Hint K.1
Study Section 22.2
ANSWER:

It magnifies the effects of gravity that we see in the cluster. The overall shape of the cluster is that of a lens. It is an unusually large cluster that has a lot of gravity. It bends or distorts the light coming from galaxies located behind it.
Part L
Which of the following statements best summarizes what studies of the masses of individual galaxies and of clusters of galaxies have told us about dark matter?
Hint L.1
Study Section 22.2.
ANSWER:

Dark matter is present between galaxies in clusters, but not within individual galaxies. Dark matter is the dominant form of mass in both clusters and in individual galaxies. Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center. Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
Part M
What is the distinguishing characteristic of what we call ordinary or baryonic matter?
Hint M.1
Study Section 22.2
ANSWER:

It is made of subatomic particles that scientists call WIMPs. It emits a great deal of light. It consists of atoms or ions with nuclei made from protons and neutrons. It can attract other matter through the force of gravity.
Part N
Some dark matter may consist of what astronomers call MACHOs (massive compact halo objects). Which of the following would NOT be considered a type of MACHO if it were found in the halo of the galaxy?
Hint N.1
Study Section 22.2
ANSWER:

Dim, low-mass stars. Neutrinos. Planet-sized objects that do not orbit a star. Brown dwarfs.
Part O
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
Hint O.1
Study Section 22.2
ANSWER:

They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily. They interact with other matter ONLY via the weak force and not via gravity or any other force. The light that they emit is so weak that it is undetectable to our telescopes. They interact with other matter via the weak force but do not interact with the electromagnetic force and hence cannot emit light.
Part P
Which of the following best sums up current scientific thinking about the nature of dark matter?
Hint P.1
Study Section 22.2
ANSWER:

Aside from some suspicions that it dark matter is made largely of undiscovered subatomic particles, we really we don't have a very good idea of what it is. Dark matter probably does not really exist. We are certain that dark matter is made mostly of WIMPs. Dark matter consists 90% of MACHOs and 10% of WIMPs.
Part Q
What do astronomers mean by the large-scale structure of the universe?
Hint Q.1
Study Section 22.3
ANSWER:

The overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe. Any individual cluster of galaxies is considered a large-scale structure. The overall shape of the observable universe. Any individual galaxy is considered a large-scale structure.
Part R
What do we mean by the critical density of the universe?
Hint R.1
Study Section 22.4
ANSWER:

It is the density of dark matter in the universe. It is the density necessary in a particular region of space in order for a supercluster to form in that region. The average density the universe would need in order for its gravity to ultimately win out over the current rate of expansion (if it weren't accelerating). It is the actual average density of the universe.
Part S
Evidence from distant supernovae (as well as from studies of the cosmic microwave background) seems to suggest that the universe is doing something that goes against what nearly all astronomers expected just a decade ago. What is this phenomenon, which is variously attributed to things called dark energy, the cosmological constant, or quintessence?
Hint S.1
Study Section 22.4
ANSWER:

The expansion of the universe seems to be slowing down. The expansion of the universe seems to be constant. The expansion of the universe seems to be getting even faster. The expansion of the universe is only true for us, as observers in the Milky Way.
Part T
What exactly is dark energy ?
Hint T.1
Study Section 22.1
ANSWER:

A mysterious form of matter which does not interact with light. A mind weapon used by Sith Lords in Star Wars. Photons, or radiation, that we can't see or detect. We really don't know, but since it's not a form of matter, we use "dark energy" as the all-purpose explanation for the observed acceleration of the expansion of the universe.