Astronomy Ch. 18 Final – Flashcards

Which of the following best summarizes what we mean by dark matter? matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light matter consisting of black holes matter that may inhabit dark areas of the cosmos where we see nothing at all matter for which we have theoretical reason to think it exists, but no observational evidence for its existence

Which of the following best summarizes what we mean by dark energy? It is a type of energy that is associated with the "dark side" of the force that rules the cosmos. It is the energy contained in dark matter. It is the energy of black holes. It is a name given to whatever is causing the expansion of the universe to accelerate with time.

What is a rotation curve? a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy a graph showing how orbital velocity depends on distance from the center for a spiral galaxy a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"? The disk of a spiral galaxy is quite flat rather than spherical like the halo. All the galaxy's mass is concentrated in its flat, gaseous disk. Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. The amount of light emitted by stars at different distances is about the same throughout the galaxy.

Which of the following is NOT one of the three main strategies used to measure the mass of galaxy clusters? studying X-ray emission from hot gas inside the cluster measuring the speeds of galaxies orbiting the cluster's center observing how the cluster bends light from galaxies located behind it measuring the temperatures of stars in the halos of the galaxies

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies? Dark matter is present between galaxies in clusters, but not within individual galaxies. Dark matter is the dominant form of mass in both clusters and in individual galaxies. Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster. Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting? They respond to the weak force but not to the electromagnetic force, which means they cannot emit light. They interact with other matter only through the weak force and not through gravity or any other force. They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily. The light that they emit is so weak that it is undetectable to our telescopes.

Measuring the amount of deuterium in the universe allows us to set a limit on _________. the acceleration of the universe the density of ordinary (baryonic) matter in the universe the current age of the universe the total amount of mass in the universe

Which of the following best sums up current scientific thinking about the nature of dark matter? There is no longer any doubt that dark matter is made mostly of WIMPs. Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified. About 90 percent of dark matter consists of ordinary matter in the halo of the galaxy, and the other 10 percent of WIMPs. Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.

The critical density of the universe is the _______. density of water density of dark matter in the universe actual average density of the universe average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist

What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating? measurements of how galaxy speeds away from the Milky Way have increased during the past century observations of the speeds of individual galaxies in clusters observations of white dwarf supernovae measurements of the rotation curve for the universe

Which of the following best sums up current scientific thinking about the nature of dark energy? Dark energy is most likely made up of weakly interacting particles that do not interact with light. Dark energy probably exists, but we have little (if any) idea what it is. Dark energy is the source of the mind weapon used by Sith Lords in Star Wars. Dark energy most likely consists of a form of photons that we can't see or detect.