PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013
Lecture 31: April 5 , 2013
Hubble’s Law is empirical, an observed relationship between velocity and distance
galaxies are moving away from us, and their recessional velocity is proportionate to how far away
the farther away they are, the faster their velocity
the expansion rate of the universe appears to be the same everywhere
in a cosmological redshift, the light/photons of redshifts are being stretched as the universe
o the size of the universe changes between the time a photon was emitted by a galaxy & the
time it is observed by us
o this changes our understanding of what distance is
o what we are really saying is, how old is this light that we are seeing?
o look back time is a measure of how far the light has travelled
o when the light leaves its objects, the universe was smaller then when it gets to us
can galaxies recede faster than light?
o galaxies aren’t moving faster than the speed of