chapter22 - Chapter 22 Dark Matter Dark Energy and the Fate...

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Chapter 22 Dark Matter, Dark Energy, and the Fate of the Universe
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22.1 Unseen Influences in the Cosmos Our goals for learning What do we mean by dark matter and dark energy?
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What do we mean by dark matter and dark energy?
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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 Influences
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“Normal” Matter: ~ 4.4% Normal Matter inside stars: ~ 0.6% Normal Matter outside stars: ~ 3.8% Dark Matter: ~ 25% Dark Energy ~ 71% Contents of Universe
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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
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22.2 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?
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What is the evidence for dark matter in galaxies?
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We measure the mass of the solar system using the orbits of planets Orb. Period Avg. Distance Or for circles: Orb. Velocity Orbital Radius
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Rotation curve A plot of orbital velocity versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass
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Who has the largest orbital velocity? A, B, or C?
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Who has the largest orbital velocity? A, B, or C? Answer: C
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Rotation curve of merry-go- round rises with radius
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Rotation curve of Milky Way stays flat with distance Mass must be more spread out than in solar system
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Mass in Milky Way is spread out over a larger region than the stars Most of the Milky Way’s mass seems to be dark matter!
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Mass within Sun’s orbit: 1.0 x 10 11 M Sun Total mass: ~10 12 M Sun
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The visible portion of a galaxy lies deep in the heart of a large halo of dark matter
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We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H
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Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter
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Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting These galaxies also have dark matter
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