A) Dark energy most likely consists of a form of photons that we can't see or detect.
B) Dark energy is most likely made up of weakly interacting particles that do not interact with light.
C) Dark energy probably exists, but we have little (if any) idea what it is.
D) Dark energy is the source of the mind weapon used by Sith Lords in Star Wars.
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A) We are not measuring the orbital velocities of gas clouds and stars properly.
B) We are not measuring the distances to gas clouds and stars properly.
C) We are not attributing enough mass to the visible matter in the Milky Way.
D) We are not observing all the visible matter in the Milky Way.
E) There is something wrong with our understanding of how gravity works.
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A) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
B) The amount of light emitted by stars at different distances is about the same throughout the galaxy.
C) The disk of a spiral galaxy is quite flat rather than spherical like the halo.
D) All the galaxy's mass is concentrated in its flat, gaseous disk.
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A) The Solar System would fly apart.
B) The Milky Way would fly apart.
C) Clusters of galaxies would fly apart.
D) The Universe would expand forever.
E) all of the above
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A) We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
B) The cause of the acceleration is well-understood, and attributed to the particles that make up dark energy.
C) The acceleration is very important in the cosmos today, but the evidence indicates that it will eventually slow down, allowing the universe to recollapse.
D) The acceleration probably is not real, and what we attribute to acceleration is probably just a misinterpretation of the data.
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A) It interacts with other particles only through the weak force.
B) It interacts with other particles only through the weak force and the force of gravity
C) It interacts with other particles only through the weakest force, gravity.
D) It doesn't interact with any type of baryonic matter.
E) It interacts only with other weak particles, such as neutrinos.
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A) They are not as bright as a normal star and hence are not visible across great distances of space.
B) They do not emit light in the visible wavelength range.
C) They emit radiation only in the form of particles of dark matter.
D) They are made of particles of dark matter that release some radiation.
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A) Matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
B) Matter that may inhabit dark areas of the cosmos where we see nothing at all
C) Matter consisting of black holes
D) Matter for which we have theoretical reason to think it exists, but no observational evidence for its existence
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A) The one with more dark energy.
B) Dark energy doesn't affect the inferred age of the universe.
C) The one with less dark energy
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A) No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
B) No, because the universe is not old enough yet for these objects to have begun their expansion.
C) No, because expansion of the universe affects only empty space, not space in which matter is present.
D) Yes, and that is why clusters tend to grow in size with time.
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A) Through the comparison of computer models of galaxy formation with observations
B) By using a galaxy's position on the sky and its redshift to determine its distance along the line of sight
C) by using a galaxy's position on the sky and its brightness as a measure of distance along the line of sight
D) by interpreting the peculiar velocities of each galaxy
E) by carefully measuring the parallax of each galaxy
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A) 1 percent
B) 10 percent
C) 25 percent
D) 100 percent
E) 200 percent
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A) 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.
B) Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.
C) The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.
D) The question is nonsense-gas temperature cannot possibly tell us anything about mass.
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A) We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
B) We calculate its mass-to-light ratio.
C) We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
D) We count the number of stars we can see at different distances from the galaxy's center.
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A) Observations of white dwarf supernovae
B) Observations of the speeds of individual galaxies in clusters
C) Measurements of the rotation curve for the universe
D) Measurements of how galaxy speeds away from the Milky Way have increased during the past century
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A) The velocities and radii of galaxy orbits around the center of mass in the cluster.
B) The total amount of stars
C) The temperature of the intergalactic gas
D) The curvature of gravitationally-lensed galaxies
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