Dark Matter Zwickys Coma In 1933 Fred Zwicky

Dark Matter

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

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

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

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

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

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

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