Lec30 - 22 Dark Matter and the Fate of the Universe It is...

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22. Dark Matter and the Fate of the Universe Thomas Paine (1737 – 1809) American patriot, from The Age of Reason It is difficult beyond description to conceive that space can have no end; but it is more difficult to conceive an end. It is difficult beyond the power of man to conceive an eternal duration of what we call time; but it is more impossible to conceive a time when there shall be no time.
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Determining Mass Distribution • In Spiral Galaxies • measure the Doppler shift of the 21-cm radio line at various radial distances • construct a rotation curve of the atomic Hydrogen gas (beyond visible disk) • calculate the enclosed mass using Kepler’s Law
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Galactic rotation
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Summary: Rotation speed vs. mass V 2 ~ M/r V ~ Sqrt[M/r] M(r) ~ V 2 r V ~ Δλ
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Dark Matter in Spiral Galaxies Vera Rubin showed that the rotation curves of spirals are flat at large distances from their centers • indicates that (dark) matter is distributed far beyond disk Vera Rubin
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Dark Matter in Elliptical Galaxies Elliptical Galaxies No gas measure average orbital speeds of stars at various distances use broadened absorption lines • Results again indicate dark matter lies beyond the visible galaxy Can't measure total amount of dark matter, since we can see only motions of stars.
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Mass-to-Light Ratio… • Measure both mass [M ] and luminosity [L ] in Solar units • Within orbit of Sun, M/L = 6 M / L for Milky Way • typical for inner regions of spiral galaxies • for inner regions of elliptical galaxies, M/L = 10 M / L • not surprising since ellipticals contain dimmer stars • For outer regions of galaxies… • M/L ~= 50 M / L , even higher in dwarf galaxies • Thus most matter in galaxies is not in stars !
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Measuring Mass of Cluster of Galaxis 3 independent ways to measure galaxy cluster mass: 1. measure the speeds and positions of the galaxies orbiting within the cluster 2. measure the temperature and distribution of the hot gas between galaxies 3. observe how clusters bend light as gravitational lenses
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Mass from Galaxy Orbits Pioneered by Fritz Zwicky. assume the galaxies orbit about the cluster center measure the orbital velocities of the galaxies measure each galaxy’s distance from the center apply Kepler’s Law to calculate mass of cluster Zwicky found M/L>100 for clusters. his proposals of dark matter were met with skepticism
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Lec30 - 22 Dark Matter and the Fate of the Universe It is...

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