Copyright © 2009 Pearson Education, Inc. Chapter 16 Dark Matter, Dark Energy, and the Fate of the UniverseCopyright © 2009 Pearson Education, Inc. 16.1 Unseen Influences in the Cosmos Our goals for learning: • What do we mean by dark matter and dark energy?Copyright © 2009 Pearson Education, Inc. What do we mean by dark matter and dark energy?Copyright © 2009 Pearson Education, Inc. Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate Unseen InfluencesCopyright © 2009 Pearson Education, Inc. • Normal matter: ~ 4.4% — Normal matter inside stars: ~ 0.6% — Normal matter outside stars: ~ 3.8% • Dark matter: ~ 22% • Dark energy: ~ 74% Contents of UniverseCopyright © 2009 Pearson Education, Inc. What have we learned? • What do we mean by dark matter and dark energy? — Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. — Dark energy is the name given to whatever might be causing the expansion of the universe to accelerate.Copyright © 2009 Pearson Education, Inc. 16.2 Evidence for Dark Matter Our goals for learning: • What is the evidence for dark matter in galaxies? • What is the evidence for dark matter in clusters of galaxies? • Does dark matter really exist? • What might dark matter be made of?Copyright © 2009 Pearson Education, Inc. What is the evidence for dark matter in galaxies?Copyright © 2009 Pearson Education, Inc. We measure the mass of the solar system using the orbits of planets. • Orbital period • Average distance Or for circles: • Orbital velocity • Orbital radius Encircled Mass as a Function of Distance for the Solar SystemCopyright © 2009 Pearson Education, Inc. Rotation curve A plot of orbital velocity versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass. Rotation Curve of the Solar SystemCopyright © 2009 Pearson Education, Inc. Who has the largest orbital velocity? A, B, or C? Motion on a Merry-Go-RoundCopyright © 2009 Pearson Education, Inc. Who has the largest orbital velocity? A, B, or C? Answer: C Motion on a Merry-Go-RoundCopyright © 2009 Pearson Education, Inc. Rotation curve of merry-go-round rises with radius. Rotation Curve for a Merry-Go-RoundCopyright © 2009 Pearson Education, Inc. The rotation curve of the Milky Way stays flat with distance. Mass must be more spread out than in the solar system. Rotation Curve of a Spiral GalaxyCopyright © 2009 Pearson Education, Inc. The mass in the Milky Way is spread out over a larger region than the stars. Most of the Milky Way’s mass seems to be dark matter! Encircled Mass as a Function of Distance for a Spiral GalaxyCopyright © 2009 Pearson Education, Inc. Mass within Sun’s orbit: 1.0 × 1011MSun Total mass: ~1012MSunCopyright © 2009 Pearson Education, Inc. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter.Copyright © 2009 Pearson Education, Inc. We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H.Copyright © 2009 Pearson Education, Inc. The broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. These galaxies also have dark matter.Copyright © 2009 Pearson Education, Inc. Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? A. Its mass is concentrated at the center. B. It rotates like the solar system. C. It’s especially rich in dark matter. D. It’s just like the Milky Way.Copyright © 2009 Pearson Education, Inc. Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? A. Its mass is concentrated at the center. B. It rotates like the solar system. C. It’s especially rich in dark matter. D. It’s just like the Milky Way.Copyright © 2009 Pearson Education, Inc. What is the evidence for dark matter in clusters of galaxies?Copyright © 2009 Pearson Education, Inc. We can measure the velocities of galaxies in a cluster from their Doppler shifts.Copyright © 2009 Pearson Education, Inc. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!Copyright © 2009 Pearson Education, Inc. Clusters contain large amounts of X ray–emitting hot gas. The temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% starsCopyright © 2009 Pearson Education, Inc. Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass.Copyright © 2009 Pearson Education, Inc. A gravitational lens distorts our view of things behind it. Gravitational Lensing IllustratedCopyright © 2009 Pearson Education, Inc. All three methods of measuring cluster mass indicate similar amounts of dark matter.Copyright © 2009 Pearson Education, Inc. Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies? A. Measuring velocities of cluster galaxies B. Measuring total mass of the cluster’s stars C. Measuring temperature of its hot gas D. Measuring distorted images of background galaxiesCopyright © 2009 Pearson Education, Inc. Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies? A. Measuring velocities of cluster galaxies B. Measuring total mass of the cluster’s stars C. Measuring temperature of its hot gas D. Measuring distorted images of background galaxiesCopyright © 2009 Pearson Education, Inc. Does dark matter really exist?Copyright © 2009 Pearson Education, Inc. Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.Copyright © 2009 Pearson Education, Inc. Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the
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