4. What Redshift & Blueshift Tell Us

tells us
Picture of several galaxies. Box and circle show one of the farthest known galaxies. 
Photo provided by NASA. Public domain.

As astronomers observed the stars and made measurements they observed that most nearby galaxies were redshifted. This discovery puzzled scientists, because if the universe was always the same size, then there shouldn't be any shift.

Hubble's discovery that the spectral lines in the light coming from stars undergoes redshift is an important discovery because scientists can estimate distance from the apparent intensity of the stars' light. Hubble noticed that redshift increased with distance; in other words, galaxies farther away from us are moving away from us (and each other) at a higher speed. If galaxies are moving away from each other, then they must have been closer in the past. That means the universe would have been smaller long ago, and has been expanding. 

Redshifting in stars and galaxies is one of the key pieces of evidence that scientists use to support the the Big Bang Theory.