Astronomy Chapter 16 Study Quizzes – Flashcards
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Which of the following best summarizes what we mean by dark matter?
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matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
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Which of the following best summarizes what we mean by dark energy?
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It is a name given to whatever is causing the expansion of the universe to accelerate with time.
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The text states that luminous matter in the Milky Way seems to be much like the tip of the iceberg. This refers to the idea that ___________.
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dark matter represents much more mass and extends much farther from the galactic center for a spiral galaxy
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What is a rotation curve?
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a graph showing how orbital velocity depends on distance from the center for a spiral galaxy.
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What is the primary way in which we determine the mass distribution of a spiral galaxy?
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We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center
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What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
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Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward
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Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
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Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies
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Which of the following is NOT one of the three main strategies used to measure the mass of galaxy clusters?
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measuring the temperatures of stars in the halos of the galaxies
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When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
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It bends or distorts the light coming from galaxies located behind it
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Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
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Dark matter is the dominant form of mass in both clusters and in individual galaxies
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What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)?
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It consists of atoms or ions with nuclei made from protons and neutrons
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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?
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globular clusters in the halo of the galaxy
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What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
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They respond to the weak force but not to the electromagnetic force, which means they cannot emit weak light
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Which of the following best sums up current scientific thinking about the nature of dark matter?
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Most dark matter probably consists of weakly interacting particles of a type that have not yet identified
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When we speak of the large scale structure of the universe, we mean _________.
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the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe
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The critical density of the universe is the _________.
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average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist.
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What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
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Observations of white dwarf supernovae
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Which of the following best sums up current scientific thinking about the nature of dark energy?
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Dark energy probably exists, but we have little (if any) idea what it is.
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Why do we call dark matter "dark"?
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It emits no radiation that we have been able to detect
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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?
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There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales
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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?
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The orbital speeds would fall off sharply with increasing distance from the galactic center.
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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?
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Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
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It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
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Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
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How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
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We detect this gas with X-ray telescopes
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Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
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The temperature tells us the average speed of the gas particles, which are held in the cluster by gravity, so we can use those speeds to determine the cluster mass
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How does gravitational lensing tell us about the mass of a galaxy cluster?
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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
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What evidence tells us that at least some dark matter consists of objects made of ordinary matter that orbit in the halo of the galaxy?
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We have detected gravitational lensing of distant objects that appears to be caused by unseen objects in the halo of our galaxy
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If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
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They travel at speeds close to the speed of light
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Is space expanding within clusters of galaxies?
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No, because their gravity is strong enough to hold them together even while the universe as a whole expands
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Which of the following statements about large-scale structure is probably NOT true?
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Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall
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Based on current evidence, a supercluster is most likely to have formed in regions of space where ______________.
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the density of dark matter was slightly higher than average when the universe was very young
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Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
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The actual density, even with dark matter included, is less than a third of the critical density
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Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
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We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
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Some people wished we live 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)?
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Dark energy does not exist and there is much more dark matter than we are aware to date.
