Chapter 22

Which of the following best summarizes what we mean by dark matter?
matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
Which of the following best summarizes what we mean by dark energy?
It is a name given to whatever is causing the expansion of the universe to accelerate with time.
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 _________.
dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
What is a rotation curve?
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
What is the primary way in which we determine the mass distribution of a spiral galaxy?
We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
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 ______.
the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter
Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
measuring the temperatures of stars in the halos of the galaxies
When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
It bends or distorts the light coming from galaxies located behind it.
Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
What is the distinguishing characteristic of what we call ordinary or baryonic matter?
It consists of atoms or ions with nuclei made from protons and neutrons.
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?
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
Which of the following best sums up current scientific thinking about the nature of dark matter?
Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
When we speak of the large-scale structure of the universe, we mean _________.
the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
The critical density of the universe is the _______.
average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
Observations of white dwarf supernovae.
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
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?
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
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?
The orbital speeds would fall off sharply with increasing distance from the galactic center.
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?
Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
We detect this gas with X-ray telescopes.
Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
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.
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
They travel at speeds close to the speed of light.
Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.
the density of dark matter was slightly higher than average when the universe was very young
Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
The actual density, even with dark matter included, is less than about a third of the critical density.
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)?
ark energy does not exist and there is much more dark matter than we are aware of to date.