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.
Redshift has a mirror image, blueshift, which happens when a light source moves toward an observer instead of away, and both fall under the same Doppler principle. There is also a distinct effect called gravitational redshift, unrelated to motion at all, in which light climbing out of a strong gravitational field loses energy and shifts toward the red end of the spectrum simply because of how gravity affects the passage of time nearby. This gravitational version was one of general relativity’s early testable predictions, and it has since been confirmed both in laboratory experiments on Earth and in light escaping from extremely dense objects like white dwarfs. Cosmological redshift, the kind Hubble observed in distant galaxies, is technically a third, related but distinct phenomenon, caused by the stretching of space itself rather than ordinary motion through it.
Astronomers express how strongly a galaxy’s light has been redshifted using a single number called z, and the highest confirmed values now reach beyond z equals 13, corresponding to galaxies whose light has been travelling toward us for over 13.4 billion years, seen as they existed when the universe was only a few hundred million years old. Finding and confirming galaxies at such extreme redshift has become one of the signature achievements of the James Webb Space Telescope.