宇宙暗物质dark_matter_intro.ppt

Dark Matter in the Universe ?A (mostly) Pictorial Introduction Edward Daw University of Sheffield A Spiral Galaxy-NGC3198 In Ursa Major (Great Bear / Plough / Big Dipper) constellation. Distance is 9.1Mpc, or 30 million light years. For comparison, distance from Earth to the galactic centre is a factor of ~1000 less, ~ 30 thousand light years 33 thousand light years Dust, Gas, and Debris Sombrero Galaxy - NGC 4594, in Virgo. Hubble Space Telescope Image /apod Horsehead Nebula /apod In Orion, 0.9m telescope, Kitt Peak National Observatory, U.S.A. Image is 2.7 by 1.8 light years, distance is 1.6 thousand light years Pink background is hydrogen emmission from IC434 emmission nebula. Question: Is the mass in the universe all observable through emmission or absorbsion of electromagnetic radiation ? Dark Matter ...is matter that does not shine or absorb light, and has therefore escaped direct detection by electromagnetic transducers like telescopes, radio antennas, x-ray satellites... It turns out that there is strong experimental evidence that there is more than 4 times as much dark matter as luminous matter in the observable universe Evidence for Dark Matter Use the fact that massive objects, even if they emit no light, exert gravitational forces on other massive objects. Study the motions (dynamics) of visible objects like stars in galaxies, and look for effects that are not explicable by the mass of the other light emmitting or absorbing objects around them. m1 m2 r12 Rotation of Stars around Galactic Centres We can measure how fast stars rotate around galactic centres by looking at the frequency shift of known spectral lines originating in the stars due to the Doppler effect. Star’s motion towards you, relative to the galactic centre alters wavelength of light Need World’s largest telescope... Some Results v r DISK RADIUS This is what we expect.... ...but here are some typical results Wavelength shifts are about a part in 106. These are hard measurements