redshift
Redshift is what happens to light when its source is rushing away from you: the waves get stretched out, sliding toward the longer, redder end of the rainbow. You already know this trick with sound — an ambulance siren drops to a lower pitch the instant it speeds past you. Light does the same thing, only instead of a deeper note you get a redder color.
This turns out to be one of astronomy's most powerful tools. By splitting a star's or galaxy's light into its colors and measuring how far the pattern has shifted toward red, astronomers can gauge how fast that object is receding. For nearby objects this is a straightforward speed; for very distant galaxies the shift instead reflects the stretching of space itself, which we'll get to in a moment. Either way, the bigger the redshift, the faster the retreat.
Its most famous moment came in the 1920s, when Edwin Hubble noticed that almost every distant galaxy is redshifted, and the farther away a galaxy sits, the faster it flees. The only honest explanation was staggering: the whole universe is expanding, carrying the galaxies apart like raisins in a rising loaf of bread. A common mix-up: the galaxies aren't simply flying through space away from us — space itself is stretching, and the light waves stretch right along with it.
The light from a galaxy a billion light-years away arrives noticeably redder than it left — a fingerprint of the expanding universe.
The flip side is 'blueshift': when a source moves toward you, its light is squeezed to shorter, bluer waves. The Andromeda Galaxy is one of the rare neighbors heading our way, so its light is blueshifted — it's on a collision course with the Milky Way, a few billion years off.