Astronomy Chapter 18 Practice Questions – Flashcards
Unlock all answers in this set
Unlock answersquestion
Which of the following best summarizes what we mean by dark matter?
answer
matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
question
Which of the following best summarizes what we mean by dark energy?
answer
It is a name given to whatever is causing the expansion of the universe to accelerate with time.
question
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 _________.
answer
dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
question
What is a rotation curve?
answer
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
question
What is the primary way in which we determine the mass distribution of a spiral galaxy?
answer
We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
question
What do we mean when we say that the rotation curve for the Milky Way galaxy is "flat"?
answer
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
question
Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
answer
Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
question
Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
answer
measuring the temperatures of stars in the halos of the galaxies
question
When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
answer
It bends or distorts the light coming from galaxies located behind it.
question
Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
answer
Dark matter is the dominant form of mass in both clusters and in individual galaxies.
question
What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?
answer
It consists of atoms or ions with nuclei made from protons and neutrons.
question
Measuring the amount of deuterium in the universe allows us to set a limit on _________.
answer
the density of ordinary (baryonic) matter the universe
question
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
answer
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
question
Which of the following best sums up current scientific thinking about the nature of dark matter?
answer
Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
question
When we speak of the large-scale structure of the universe, we mean _________.
answer
the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
question
The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.
answer
observations of white dwarf supernovae
question
Which of the following best sums up current scientific thinking about the nature of dark energy?
answer
Dark energy probably exists, but we have little (if any) idea what it is.
question
Based on evidence both from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe's total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)?
answer
5%
question
According to current understanding, if the universe continues to expand forever, the last major source of light will come from _________.
answer
evaporation of black holes
question
Why do we call dark matter "dark"?
answer
It emits no radiation that we have been able to detect.
question
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?
answer
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
question
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?
answer
The orbital speeds would fall off sharply with increasing distance from the galactic center.
question
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?
answer
Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
question
It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
answer
Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
question
How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
answer
We detect this gas with X-ray telescopes.
question
Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
answer
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.
question
How does gravitational lensing tell us about the mass of a galaxy cluster?
answer
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.
question
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
answer
They travel at speeds close to the speed of light.
question
Is space expanding within clusters of galaxies?
answer
No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
question
Which of the following statements about large-scale structure is probably not true?
answer
Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
question
Based on current evidence, a supercluster is most likely to have formed in regions of space where _________.
answer
the density of dark matter was slightly higher than average when the universe was very young
question
Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur?
answer
The galaxies in clusters would begin to fly apart.
question
Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
answer
The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.
question
Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. Based on current understanding, which of the following would have to be true for this to be the case?
answer
Dark energy does not exist and there is much more matter than current evidence suggests.
question
Scientists do not know what dark energy is, yet they still claim that it is the dominant constituent of the mass-energy of the universe. What (if any) evidence supports this claim?
answer
There are two lines of evidence: measurements of the accelerating expansion, and the universal geometry inferred from the cosmic microwave background.
question
Which of the following statements correctly describes current understanding of the eventual fate of the universe?
answer
Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.
