You'll recall from week 1 that the doppler effect occurred when sound changed pitch as a result of moving away from you. You also learned that light can have its own kind of doppler effect, which is called redshift or blueshift. Redshift occurs when an object emitting light is moving away from whatever is observing it. Blueshift occurs when an object emitting light is moving towards whatever is observing it.

As scientists observed light from many different galaxies, they found that many were redshifted, while a few were blueshifted. This means that most galaxies are moving away from our own. From their observations they concluded that the universe was expanding. The presence of redshift in light from other galaxies is part of the evidence for the Big Bang.