ASTR 20 Lecture Notes - Lecture 28: Gravitational Lens, Dark Matter, Cosmic Distance Ladder
Document Summary
Cepheid variable stars with longer periods have greater luminosities. Hubble measured the distance to the andromeda galaxy using cepheid variables as standard candles. Kepler"s laws tell us that objects orbiting a single massive object have a lower orbital velocity at larger distances. Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. We can measure the velocities of galaxies in a cluster from their doppler shifts. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars. Clusters contain large amounts of x-ray emitting hot gas. The temperature of hot gas tells us cluster mass: Gravitational lensing: the bending of light rays by gravity, can also tell us a cluster"s mass. All three methods of measuring cluster mass indicate similar amounts of dark matter.
