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Lecture 31

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McGill University
PHYS 183
Tracy Webb

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 they are  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 expands  look-back time 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
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