1912: Vesto Slipher discovered that the spectra of galaxies were
appeared to be moving away from us).
Distances to galaxies were unknown at the time.
Examples of "Spiral" Galaxies
Example of a "Redshifted" Spectrum
late 1920's: Edwin Hubble succeeded in measuring the distances to galaxies
using bright stars whose luminosity ("intrinsic brightness")
was thought to be well-known:
L (the luminosity) is known
b (the apparent brightness) is measured at the
d (the distance) is evaluated using the
Using this technique,
Hubble discovered that the more distant galaxies seemed to be
moving away from us at faster speeds than the more nearby galaxies.
Implication of Hubble's Law: The universe is
2-Dimensional Example of Expansion: Surface of
Air is blown into the
balloon at a constant rate such that 1 second after the balloon starts
expanding each of the dots is 1 cm away from its nearest neighbor. 3 seconds
after the balloon begins expanding, each dot is 3 cm away from its nearest
neighbor. If you lived on a dot, what would you say
is the distance to each of the other dots and what is the speed of each of
the other dots after 3 seconds have gone by?
speed = (distance traveled) / (time elapsed)
speed of any dot = (distance at 3 sec - distance at 1 sec) / (3 sec - 1 sec)
Hubble's Law DOES NOT imply that we are at the "center of
(See also the Cosmological Principle).
Hubble's Law DOES imply that the universe was denser in the
past! (i.e., the volume of the universe was smaller in the past)
On to Age of the Universe or back to