Cosmic Perspective Chapter 23 Multiple Choice – Flashcards
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Why do we call dark matter "dark"?
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It emits no or very little radiation of any wavelength
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What is meant by "dark energy"?
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The agent causing the universal expansion to acelerate
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Why do we believe 90% of the mass of the Milky Way is in the form of dark matter?
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The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
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How do we know that there is much more mass in the halo of our galaxy than in the disk?
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Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
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What evidence suggests that the Milky Way contains dark matter?
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We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
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If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?
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Our understanding of gravity is not correct for galaxy-size scales.
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How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?
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Through observations of the 21 cm line of atomic hydrogen.
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The distribution of the dark matter in a spiral galaxy is
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approximately spherical and about ten times the size of the galaxy halo.
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How do we determine the amount of dark matter in elliptical galaxies?
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We measure the speeds of stars at different radii from the galactic center and determine how much mass is interior to the orbit.
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When we see that a spectral line of a galaxy is broadened, that is, spanning a range of wavelengths, we conclude that
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there are different Doppler shifts among the individual stars in the galaxy.
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A large mass-to-light ratio for a galaxy indicates that
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on average, each solar mass of matter in the galaxy emits less light than our Sun.
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What is the mass-to-light ratio for the inner region of the Milky Way Galaxy, in units of soar masses per solar luminosity?
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6
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Compared to the central regions of spiral galaxies, we expect elliptical galaxies to have
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higher mass-to-light ratios because stars in elliptical galaxies are dimmer than those in spirals.
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If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, and its stars account for only 5 solar masses per solar luminosity, how much of the galaxy's mass must be dark matter?
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95 percent
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Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?
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Measuring the amount of distortion caused by a gravitational lens.
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Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?
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The medium emits X rays, which are blocked by the Earth's atmosphere and require X-ray satellites in space in order to be observed.
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Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true?
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Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.
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Gravitational lensing occurs when
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massive objects bend light beams that are passing nearby.
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Which of the following is not evidence for dark matter?
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the expansion of the universe
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Which of the following particles are baryons?
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Protons
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Which of the following is an example of baryonic matter?
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You
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Measuring the amount of deuterium in the universe allows us to set a limit on
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the density of ordinary (baryonic) matter in the universe.
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the density of ordinary (baryonic) matter is between 1 and 10 percent of the critical density.
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Based on current evidence concerning the amount of deuterium in the universe, we can conclude that
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What do we mean when we say that a particle is a weakly interacting particle?
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It interacts only through the weak force and the force of gravity.
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Why can't the dark matter in galaxies be made of neutrinos?
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Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.
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Which of the following are candidates for dark matter?
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all of the above (brown dwarfs, Jupiter-size objects, WIMPs, faint red stars)
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Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into a disk?
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Since they do not interact with the electromagnetic force, they do not feel friction or drag and hence do no contract with the rest of the protogalactic cloud.
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Why isn't space expanding within systems such as our solar system or the Milky Way?
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Their gravity is strong enough to hold them together against the expansion of the universe.
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What are peculiar velocities?
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Velocities of distant objects that are not caused by the expansion of the universe.
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What do peculiar velocities reveal?
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The distribution of dark matter in large-scale structures.
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How do astronomers create three-dimensional maps of the universe?
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By using the position on the sky and the redshift to determine a distance along the line of sight.
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What does the universe look like on very large scales?
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Galaxies appear to be distributed in chains and sheets that surround great voids.
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What fraction of the mass needed in halt expansion is known to exist in the form of visible mass in the universe?
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1 percent
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Based on inventoried matter in the universe, including dark matter known to exist in galaxies and clusters, the actual density of the universe is what fraction of the critical density?
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26 percent
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If all the "dark matter" in the Universe were to be somehow instantaneously removed, which of the following would not happen?
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The Solar System would fly apart.
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Which model of the universe gives the youngest age for its present size?
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A recollapsing universe
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What is the ultimate fate of an open universe?
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All matter decays to a low-density sea of photons and subatomic particles.
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Recent measurements show that the expansion is accelerating, rather than slowing under the influence of gravity.
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The measurements show that the expansion is accelerating, rather than slowing under the influence of gravity.
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What is the evidence or an accelerating universe?
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White-dwarf supernovae are slightly dimmer than expected for a coasting universe.
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What might be causing the universe to accelerate?
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We don't know -- but we call it "dark energy."
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What is Einstein's cosmological constant?
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A repulsive force that counteracts gravity and was introduced to allow for a static universe.
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What is not a main source of evidence for the existence of dark matter?
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Massive blue stars
