Chapter 16 Dark Matter Dark Energy and the Fate of the Universe Copyright 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 Unseen Influences 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 Copyright 2009 Pearson Education Inc Contents of Universe Normal matter 4 4 Normal matter inside stars 0 6 Normal matter outside stars 3 8 Dark matter Dark energy Copyright 2009 Pearson Education Inc 22 74 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 System Copyright 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 System Copyright 2009 Pearson Education Inc Who has the largest orbital velocity A B or C Motion on a Merry Go Round Copyright 2009 Pearson Education Inc Who has the largest orbital velocity A B or C Answer C Motion on a Merry Go Round Copyright 2009 Pearson Education Inc Rotation curve of merry goround rises with radius Rotation Curve for a Merry Go Round Copyright 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 Galaxy Copyright 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 Galaxy Copyright 2009 Pearson Education Inc Mass within Sun s orbit 1 0 1011MSun Total mass 1012MSun Copyright 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 B C D Copyright 2009 Pearson Education Inc Its mass is concentrated at the center It rotates like the solar system It s especially rich in dark matter It s just like the Milky Way Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk A B C D Copyright 2009 Pearson Education Inc Its mass is concentrated at the center It rotates like the solar system It s especially rich in dark matter It s just like the Milky Way 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 stars Copyright 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 Illustrated Copyright 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 B C D Measuring velocities of cluster galaxies Measuring total mass of the cluster s stars Measuring temperature of its hot gas Measuring distorted images of background galaxies Copyright 2009 Pearson Education Inc Thought Question What kind of measurement does not tell us the mass of a cluster of galaxies A B C D Measuring velocities of cluster galaxies Measuring total mass of the cluster s stars Measuring temperature of its hot gas Measuring distorted images of background galaxies Copyright 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 existence of dark matter Because gravity is so well tested most astronomers prefer option 1 Copyright 2009 Pearson Education Inc What might dark matter be made of Copyright 2009 Pearson Education Inc How dark is it Copyright 2009 Pearson Education Inc How dark is it not as bright as a star Copyright 2009 Pearson Education Inc Two Basic Options Ordinary Dark Matter MACHOS Massive Compact Halo Objects dead or failed stars in halos of galaxies Extraordinary Dark Matter WIMPS Weakly Interacting Massive Particles
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