ASTR 100 Lecture Notes - Lecture 17: Orbital Speed, Radiography, Future Of An Expanding Universe
Document Summary
Galaxy rotation curves: orbital speed of stars vs speed from the center. Do not decline as they would if total mass was only in the stars/observable: mathematic calculations are incorrect. Galaxy clusters: estimated speed of galaxies in clusters from doppler effect. Mass found from galaxy motions is 50x larger than the mass in stars. Temperature of this mass/gas tells us: 85% dark matter, 13% hot gas, 2% stars. Gravitational lensing: mass in galaxy clusters bends light via gravity; can tell us a cluster"s mass. Mass curves the fabric of spacetime: gr: newton"s gravitational formula only predicts some bending, not as much as gr. 1919: gravity bends starlight at a total eclipse of the sun. Einstein improved newton"s theory and predicted twice the bend: confirmed during eclipse by arthur eddington. All methods of measuring cluster mass find similar amounts of dark matter. What is dark matter: machos: ordinary matter that we can"t see.
