Dark Matter

1. Dark Matter

2. Zwicky’s Coma • In 1933 Fred Zwicky measured the speed of Coma cluster galaxies. • Too fast for the visible stars • Cluster would fly apart • Either stars and physics are different, or there is dark matter. Image: D. Rowe

3. Vera’s Spin • In the 1970’s Vera Rubin studied the speed of stars in distant galaxies. • Outer stars too fast • Galaxies stayed together • Dark matter must be greater than visible stars. • Law of gravity same • Dark matter about 10 times larger

4. Orbital Check • The Sun orbits the center of the Milky Way. • 90 billion M inside orbit • Distant clusters should show lower speeds after the known mass ends. • Gas with high velocity outside Milky Way Sun’s orbit Mass within interstellar gas

5. Dark Halo • To account for the speed there must be mass outside the luminous part of the Milky Way. • Not visible in any wavelength dark matter hyperphysics.phy-astr.gsu.edu

6. Einstein Rings • Gravity bends light like a lens. • More mass = more bending • Dark matter is needed to account for the bending.

7. MACHOs • One candidate for dark matter sources were massive compact halo objects. • Dwarf stars • Brown dwarfs • Observable by microlensing • Observations rule out MACHOs.

8. WIMPs • The other possibility for dark matter are weakly interacting massive particles. • Heavier than atoms • Hard to detect • Dedicated experiments still haven’t seen WIMPs. 2004 experimental limits