Copyright © 2009 Pearson Education, Inc. Chapter 16 Dark Matter, Dark Energy, and the Fate of the UniverseCopyright © 2009 Pearson Education, Inc. Havasupai IndiansCopyright © 2009 Pearson Education, Inc. Do you believe the blood should be returned A. Yes B. NoCopyright © 2009 Pearson Education, Inc. 16.3 Structure Formation Our goals for learning: • What is the role of dark matter in galaxy formation? • What are the largest structures in the universe?Copyright © 2009 Pearson Education, Inc. What is the role of dark matter in galaxy formation?Copyright © 2009 Pearson Education, Inc. Gravity of dark matter is what caused protogalactic clouds to contract early in time. ⇒Copyright © 2009 Pearson Education, Inc. WIMPs can’t contract to the center because they don’t radiate away their orbital energy.Copyright © 2009 Pearson Education, Inc. Dark matter is still pulling things together. After correcting for Hubble’s law, we can see that galaxies are flowing toward the densest regions of space.Copyright © 2009 Pearson Education, Inc. What are the largest structures in the universe?Copyright © 2009 Pearson Education, Inc. Maps of galaxy positions reveal extremely large structures: superclusters and voids.Copyright © 2009 Pearson Education, Inc. Models show that the gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time. Time in billions of years 0.5 2.2 5.9 8.6 13.7 Size of expanding box in millions of light-years 13 35 70 93 140Copyright © 2009 Pearson Education, Inc. Models show that gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time. Large-Scale Structure of the UniverseCopyright © 2009 Pearson Education, Inc. Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs.Copyright © 2009 Pearson Education, Inc. What have we learned? • What is the role of dark matter in galaxy formation? — The gravity of dark matter seems to be what draws gas together into protogalactic clouds, initiating the process of galaxy formation. • What are the largest structures in the universe? — Galaxies appear to be distributed in gigantic chains and sheets that surround great voids.Copyright © 2009 Pearson Education, Inc. 16.4 The Fate of the Universe Our goals for learning: • Will the universe continue expanding forever? • Is the expansion of the universe accelerating?Copyright © 2009 Pearson Education, Inc. Will the universe continue expanding forever?Copyright © 2009 Pearson Education, Inc. Does the universe have enough kinetic energy to escape its own gravitational pull? Fate of a Launched CannonballCopyright © 2009 Pearson Education, Inc. Fate of universe depends on the amount of dark matter. Critical density of matter Not enough dark matter Lots of dark matterCopyright © 2009 Pearson Education, Inc. Amount of matter is ~25% of the critical density, suggesting fate is eternal expansion. Not enough dark matterCopyright © 2009 Pearson Education, Inc. But expansion appears to be speeding up! Dark energy? Not enough dark matterCopyright © 2009 Pearson Education, Inc. Estimated age depends on both dark matter and dark energy. old older oldestCopyright © 2009 Pearson Education, Inc. Thought Question Suppose that the universe has more dark matter than we think there is today. How would that change the age we estimate from the expansion rate? A. Estimated age would be older B. Estimated age would be the same C. Estimated age would be youngerCopyright © 2009 Pearson Education, Inc. Thought Question Suppose that the universe has more dark matter than we think there is today. How would that change the age we estimate from the expansion rate? A. Estimated age would be older B. Estimated age would be the same C. Estimated age would be youngerCopyright © 2009 Pearson Education, Inc. Is the expansion of the universe accelerating?Copyright © 2009 Pearson Education, Inc. The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded.Copyright © 2009 Pearson Education, Inc. An accelerating universe is the best fit to supernova data.Copyright © 2009 Pearson Education, Inc. What have we learned? • Will the universe continue expanding forever? — Current measurements indicate that there is not enough dark matter to prevent the universe from expanding forever. • Is the expansion of the universe accelerating? — An accelerating universe is the best explanation for the distances we measure when using white dwarf supernovae as standard
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