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Lecture

# Dark Matter, Dark Energy, and the Fate of the Universe

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Department
Astronomy
Course Code
ASTRON 100
Professor
Alexandra Pope

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Description
Dark Matter, Dark Energy, and the Fate of the Universe Names given to 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. Contents of Universe ● “Ordinary” Matter”: ~ 4.4% ○ Ordinary matter inside stars: ~ 0.6% ○ Ordinary matter outside stars: ~ 3.8% ● Dark Matter: ~ 23% ● Dark Energy: ~ 73% What is the evidence for dark matter in galaxies? ● We measure the mass of the solar system using the orbits of planets: ○ Orbital Period ○ Average Distance ● For circles: ○ Orbital velocity ○ Orbital radius Rotation Curve ● Aplot of orbital velocity versus orbital radius. ● The solar system’s rotation curve declines because the Sun has almost all the mass. ● The rotation curve orbital velocity rises with radius. ● The rotation curve of the Milky Way stays flat with distance. ● Mass must be more spread out than in the solar system. ● Mass in the Milky Way is spread out over a larger region than its stars. ● Most of the Milky Way’s mass seems to be dark matter. ● Mass within the Sun’s orbit: ○ 1.0 x 10^11 Msun ● Total mass of the Milky Way: ○ ~10^12 Msun ● The visible portion of the galaxy lies deep in the heart of a large halo of dark matter. How do we measure the rotation in other galaxies? ● We can measure the rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic hydrogen. ● Spiral galaxies all tend to have flat rotation curves, indicating large amounts of dark matter. ● Spiral galaxies: 10% visible matter + 90% dark matter ● Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. ● These galaxies also have dark matter. Evidence for dark matter in clusters of galaxies: galaxies velocities ● We can measure the velocities of galaxies in a cluster from their Doppler shifts. ● Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. ● These galaxies also have dark matter. ● The mass we find from galaxy motions in a cluster about 50 times larger than the mass in stars. Evidence for dark matter in clusters of galaxies: Hot Gas ● Clusters contain large amounts of X-ray emitting hot gas. ● Temperature of hot gas tells us the particle motions which tells us cluster mass: ○ 85% dark matter ○ 13% hot gas ○ 2% stars Evidence for dark matter in clusters of galaxies: Gravitational Lensing ● Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass. All three methods
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