01:750:109 Chapter Notes - Chapter 12.2: Before Present, Cosmological Principle, Photon
The Implications of Hubble's Law
●Hubble's Law: tells us that the universe is expanding and also gives us a way to measure galactic
distances
○Also other implications: tracing the motions of galaxies back through time indicates that
all matter in the observable universe started very close together and that the entire
universe we observe today came into being at a single moment, apx 14 billion yrs ago
In what sense is the universe expanding?
●Galaxies moving away from us and more distant ones moving away faster
●Cosmological Principle:
idea that the matter in the universe is evenly distributed, without a
center or an edge
●Universe expanding, but galaxies not (held together by gravity)
How do distance measurements tell us the age of the universe?
Lookback Time and Cosmological Redshift
●Galaxy's lookback time
is the time we are seeing the galaxy as it looked x yrs ago
○"400 million ly away" means the galaxy has a look back time of 400 million ly. We are
seeing the galaxy as it looked 400 million yrs ago
●One way to interpret a galaxy's redshift is in terms of how fast the galaxy is moving away from
us (the v
in Hubble's law)
●Another way to interpret it is thinking of galaxies as remaining stationary relative to the space in
which they sit, while the space btwn them grows
○From this perspective, the expansion of the universe causes photon wavelengths to
become longer (redder) with time
●Therefore, we have a choice when we interpret the redshift of a distant galaxy:
○Redshift caused by the Doppler effect as the galaxy moves away from us
○Redshift as being a cosmological redshift that arises from photon stretching in an
expanding universe
■For more distant galaxies, this one more useful
●A galaxy's redshift tells us how much space has expanded during the time that the galaxy's light
reached to us
●A galaxy's lookback time tells us the distance its light has traveled
The Horizon of the Universe
●The cosmological horizon is a boundary in time, not in space
●Observable universe ends at the cosmological horizon
