Hubble's Law

1912: Vesto Slipher discovered that the spectra of galaxies were redshifted (galaxies 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 telescope

d (the distance) is evaluated using the equation above

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 expanding!

2-Dimensional Example of Expansion: Surface of a Balloon

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 the universe"!
(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 start.