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Chapter 22

Chapter 22

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 22: Dark Matter, Dark Energy and the Fate of the Universe 22.1 Unseen Influences in the Cosmos  astronomers don’t know what the universe is made of  have good reason to think the vast majority of the universe isn’t composed of atoms, but rather dark matter & dark energy  nature of dark matter & dark energy is unknown  dark matter: a name given to whatever unseen influence is causing observed gravitational effects  dark energy: a name given to whatever unseen influence is causing the universe expansion to accelerate 22.2 Evidence for Dark Matter  evidence for dark matter in galaxies? o distribution of mass in Milky Way  use orbital motion of a star to measure mass of Milky Way within star’s orbit  could do the same thing with orbits of stars at every distance from galactic centre  velocity hard to determine due to interstellar dust obscuring view  but can use Doppler shifts & radio lines with geometry  rotation curve: plots the rotational velocity of stars or gas clouds against distance from centre of galaxy & summarizes results of orbital velocity measurements  rotation curve of solar system drops off with distance from sun  inner planets orbit at faster speeds than outer planets  most mass in Milky Way not concentrated at centre  orbits of progressively more distant gas clouds encircle more & more mass  most of the mass lies within a spherical halo surrounding disk of galaxy  very little radiation detected from enormous mass, so qualifies as dark matter o dark matter in other spiral galaxies  determine amount of dark matter in galaxy by comparing its mass and luminosity  use luminosity to estimate amount of mass from stars  determine total mass of galaxy by applying law of gravity to observations of orbital velocities of stars & gas clouds  total mass larger than mass attributed to stars, excess mass must be dark matter  determine luminosity through telescope & inverse square law for light  determine total mass through orbital speeds of stars/gas clouds & Doppler shifts  combine observations to construct rotation curve o dark matter in elliptical galaxies  weigh inner parts by observing motions of stars which are disorganized, so no rotation curve  velocity of each star still depends on mass  compare spectral lines of different regions & find speeds of stars remain constant as we look farther form galaxy’s centre  most matter lies beyond distance where light trails off & so must be dark matter  what is the evidence for dark matter in clusters of galaxies? o orbits of galaxies in clusters PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013  calculate speeds of galaxies moving away from us through redshifts & determined the recession speed  subtract this speed from each galaxy’s speed to determine speeds of galaxies relative to cluster centre  only tells average radial component, speed toward/away from us  averaged them to get average orbital velocity to determine mass & and compared it to its luminosity  found clusters of galaxies have greater masses than luminosities suggest  most matter mustn’t be in form of stars but almost entirely dark o hot gas in clusters  observe xrays from hot gas that fills space between galaxies  tells about dark matter since temperature depends on total mass of cluster  temperature reflects average kinetic energies, so use xrays to determine particle speeds & the cluster’s total mass  gravity of dark matter seems to be binding galaxies of cluster together in same way gravity helps bind individual galaxies together o gravitational lensing  gravitational lensing: masses distort spacetime, so massive objects bend light beams passing nearby  light bending angle depends on mass of object doing the bending, so measure masses by observing how strongly they distort light paths  evidence of dark matter relies on understanding of gravity  seems following must be true: o dark matter really exists & we are observing effects of its gravitational attraction o there is something wrong with our understanding of gravity that is causing us to mistakenly infer the existence of dark matt
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