A) We detect this gas with X- ray telescopes.
B) We infer its existence by observing its gravitational effects on the galaxy motions.
C) The hot gas shows up as bright pink in visible- light photos of galaxy clusters.
D) We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.
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A) Dark energy is the source of the mind weapon used by Sith Lords in Star Wars.
B) Dark energy is most likely made up of weakly interacting particles that do not interact with light.
C) Dark energy most likely consists of a form of photons that we can't see or detect.
D) Dark energy probably exists, but we have little (if any) idea what it is.
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A) people
B) basketballs
C) black holes
D) red giants
E) weakly interacting massive particles
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A) density of dark matter in the universe
B) density of water
C) average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist
D) actual average density of the universe
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A) They interact with other matter only through the weak force and not through gravity or any other force.
B) They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily.
C) The light that they emit is so weak that it is undetectable to our telescopes.
D) They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
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A) the one with less dark matter
B) the one with more dark matter
C) Dark matter doesn't affect the inferred age of the universe.
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A) We're pretty sure that whatever is causing the acceleration is pure evil.
B) Dark energy and dark matter are just two forms of the same thing.
C) The term dark energy was the one that caught on with other astronomers.
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A) We are not observing all the visible matter in the Milky Way.
B) We are not measuring the distances to gas clouds and stars properly.
C) We are not measuring the orbital velocities of gas clouds and stars properly.
D) We are not attributing enough mass to the visible matter in the Milky Way.
E) There is something wrong with our understanding of how gravity works.
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A) It blocks out the light of stars in a galaxy.
B) It contains large amounts of dark- colored dust.
C) It emits no radiation that we have been able to detect.
D) It is dark brown or dark red in color.
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A) a massive object bends light beams that are passing nearby.
B) a telescope lens is distorted by gravity.
C) dark matter builds up in a particular region of space, leading to a very dense region and an extremely high mass- to- light ratio.
D) a massive object causes more distant objects to appear much larger than they should, and we can observe the distant objects with better resolution.
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A) It interacts only with other weak particles, such as neutrinos.
B) It interacts with other particles only through the weakest force, gravity.
C) It doesn't interact with any type of baryonic matter.
D) It interacts with other particles only through the weak force and the force of gravity.
E) It interacts with other particles only through the weak force.
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A) the density of ordinary (baryonic) matter is about 5 percent of the critical density.
B) neutrons outnumber protons 7 to 1 in the universe.
C) we live in a critical- density universe.
D) ordinary (baryonic) matter makes 75 percent of the mass of the universe.
E) most of the deuterium that was created during the era of nucleosynthesis has since been destroyed.
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A) Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
B) Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.
C) Dark matter is present between galaxies in clusters, but not within individual galaxies.
D) Dark matter is the dominant form of mass in both clusters and in individual galaxies.
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A) luminous matter emits white light, much like the light reflected from icebergs
B) dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way
C) black holes are much more interesting than ordinary stars that give off light
D) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter
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A) Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.
B) Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
C) Yes, they tell us that dark matter is concentrated near the center of the galaxy.
D) No, we cannot determine anything about the location of dark matter from the rotation curve.
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A) When we observe in different wavelengths, such as infrared or radio, we see objects that don't appear in visible- light observations.
B) We see many dark voids between the stars in the halo of the Milky Way.
C) When we look at the galactic center, we are able to observe a large black hole that is composed of dark matter.
D) 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.
E) We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way.
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A) We measure how fast the galaxy rotates as a whole.
B) We measure the broadening of the galaxy's absorption lines to infer the speeds of stars at different distances from the galactic center.
C) We count the number of stars in the galaxy. Combining this with the galaxy's volume, we can calculate the galaxy's density.
D) We measure the orbital velocities of star- forming gas clouds in the outer portion of the galaxy.
E) We search for dark lanes of dust and black holes within the galaxy.
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A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
B) There is no longer any doubt that dark matter is made mostly of WIMPs.
C) Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.
D) About 90% of dark matter consists of ordinary matter in the halo of the galaxy, and of the other 10% of WIMPs.
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A) a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis
B) a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
C) a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
D) a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy
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