Redshift is the stretching of a light wave’s wavelength toward the longer, redder end of the spectrum. It happens whenever the source of light is moving away from an observer, similar to how a car’s engine sound drops in pitch as it drives away, an effect known as the Doppler shift.
On a cosmic scale, nearly every distant galaxy shows redshift, and the further away a galaxy is, the more its light is redshifted. This was the key evidence Edwin Hubble used in 1929 to show that the universe is expanding, since it means galaxies are not just moving away from us individually, the space between everything is stretching.
Redshift is one of astronomy’s most useful measuring tools. By measuring how redshifted a distant galaxy’s light is, astronomers can estimate both its distance and how long ago that light was actually emitted, letting them look billions of years into the universe’s past.