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What is redshift?

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Quick answer

Redshift is the stretching of light to longer, redder wavelengths when its source moves away from you. Because distant galaxies race away as the universe expands, their light is redshifted — which is how we know the cosmos is growing.

Key things to understand

  1. Motion away stretches light waves toward the red end of the spectrum (a Doppler effect for light).
  2. The faster the source recedes, the greater the redshift.
  3. Distant galaxies show large redshifts, revealing the universe is expanding.
  4. It lets astronomers measure cosmic distances and look back in time.
  5. The opposite — an approaching source — causes a blueshift.

People also ask

What does redshift tell us?
It reveals how fast and far away a galaxy is, and collectively showed that the universe is expanding from a Big Bang.
Is redshift the same as the Doppler effect?
It's the light version of it — like a siren dropping in pitch as it passes, light drops in frequency (reddens) as a source recedes.
What is cosmological redshift?
Redshift caused by the expansion of space itself stretching light as it travels, beyond the galaxy's own motion.

Part of the guide: Space & Astronomy →