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 orbitMass 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