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(Dark) Matter! Luminous Matter Is Concentrated at the Center

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(Dark) Matter! Luminous Matter Is Concentrated at the Center Cosmology Two Mysteries and then How we got here Dark Matter Orbital velocity law Derivable from Kepler's 3rd law and Newton's Law of gravity r v2 M = r G M : mass lying within stellar orbit r r: radius from the Galactic center v: orbital velocity From Sun's r and v: there are about 100 billion solar masses inside the Sun's orbit! 4 Rotation curve of the Milky Way: Speed of stars and clouds of gas (from Doppler shift) vs distance from center Galaxy: rotation curve flattens out with distance Indicates much more mass in the Galaxy than observed as stars and gas! Mass not concentrated at center5 From the rotation curve, inferred distribution of dark matter: The Milky Way is surrounded by an enormous halo of non-luminous (dark) matter! Luminous matter is concentrated at the center 6 We can make measurements for other galaxies Weighing spiral galaxies C Compare shifts of spectral lines (in atomic H gas clouds) as a function of distance from the center 7 Rotation curves for various spiral galaxies First measured in 1960's by Vera Rubin They all flatten out with increasing radius, implying that all spiral galaxies have vast haloes of dark matter – luminous matter 1/6th of mass 8 This mass is the DARK MATTER: It's some substance that interacts gravitationally (equivalent to saying that it has mass)... It neither emits nor absorbs light in any form (equivalent to saying that it does not interact electromagnetically) Dark matter might conceivably have 'weak' (radioactive force) interactions 9 Gaggles of Galaxies • Galaxy groups > The Local group has 30-40 galaxies > 2 large ones (Milky Way & Andromeda) > Evidence that small ones being eaten by larger one all the time • Clusters of galaxies > Bigger than groups > The distinction between “Group” and “cluster” is kinda fuzzy • Super clusters of galaxies > Cluster of many clusters > We are in the Virgo super cluster • These are all gravitationally bound collections of galaxies 10 Evidence for dark matter in galaxy clusters Coma cluster L Fritz Zwicky, 1930's Looked at velocities of galaxies orbiting each other in clusters 11 Use the orbital velocity law once again, to determine the mass contained in the cluster visible Coma cluster r v2 M = r G Visible portion is only ~1/10 of cluster mass 12 Another way of weighing clusters: Look at x-rays radiated by the intra-cluster medium (hot gas) visible x-rays There is more matter in gas than stars - Tells us the temperature, and distribution of gas - Temperature tells us average speed of gas molecules - Average speed (in gravitational equilibrium) tells13 us how much mass there is Cluster masses obtained by x-ray measurements agree well with the galactic velocity method visible x-rays Even after accounting for the gas, we determine that clusters contain huge amounts of dark matter! 14 And yet more evidence of dark matter, using another tool GRAVITATIONAL LENSING Mass can bend light (prediction of general relativity) This results in multiple and/or distorted images of distant objects seen through a cluster of galaxies 15 Distorted images of galaxies behind a cluster When these images are analyzed quantitatively, we arrive at the same masses as derived from the other methods (orbital velocities, gas)... it all points to huge amounts of unseen mass! 16 Summary of evidence for Dark Matter Various techniques consistently indicate large amounts of dark matter Galactic rotation curves orbital velocities of galaxies within clusters x-ray emission from intra-cluster gas gravitational lensing 17 in 2006... The Bullet Cluster: two galaxy clusters that just collided and passed through each other X-ray emission from hot gas heated by collision Dark matter mapped by gravitational lensing 18 The Bullet Cluster gas dark matter Gas stayed in the region of the collision very clear (it interacted with the other gases) evidence Dark matter followed the galaxies } for DM (i.e. it didn’t interact even with itself) 19 Dark matter affects the structure of the entire universe, on very large scales, via its gravitational interaction It collapsed gravitationally and clumped up, just as the luminous matter did, but less effectively because it could not radiate away energy The universe ended up with tightly clumped luminous matter, more diffuse (but still clumped) dark matter 20 We have reason to believe that dark matter is largely something other than atoms The dark matter is not normal matter at all, but something entirely exotic and unknown. It must have mass and not interact much with normal stuff 21.
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