astron lec23 dark matter

3 ways in which inflation solves mysteries:

1. opposite sides of universe were in thermal equilibrium in smaller version before inflation
2. quantum mechanics sets initial fluctuations which get stretched out in inflation
3. rapid expansion flattens universe at early times, imagine a balloon expanding, the surface gets flatter

dark matter

undetected form of mass that emits little or no light, but whose existence we infer from its gravitational influence, attractive force

dark energy

unknown form of energy that seems to be causing the expansion of universe to accelerate, repulsive force

orbital velocity law

mass enclosed = r x v^2 / G

rotation curve/orbital speed flat curve meaning

orbital speed is big even at large distances

why is Milky Way rotation curve flat

as radius increases, mass and orbital speed increase due to dark matter

fraction of Milky Way’s mass that is dark matter

90%

how many stars in our galaxy

100 billion stars

evidence for dark matter in clusters of galaxies

mass from galaxy motions in a cluster is 50x more than mass in stars

gravitational lensing

light from galaxy is bent by massive thing’s gravity, can tell us about its mass

3 methods of measuring cluster mass

galaxy velocities, X-ray gas temperatures, and gravitational lensing

X-ray gas temperatures

clusters have lots of hot gas emitting X-rays, temperature of hot gas tells us about mass

the Bullet Cluster

gravitational lensing of blue on either side is not lined up with majority of normal matter, pink in middle, which is hard to have happen without dark matter

possibilities for dark matter

weakly interacting massive particles (WIMPs), they don’t interact with anything

effect of dark matter overtime

gravity of dark matter pulls mass into denser regions, universe gets lumpier overtime