Science
What is Redshift?
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.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains redshift.
Key things to understand
- 1Motion away stretches light waves toward the red end of the spectrum (a Doppler effect for light).
- 2The faster the source recedes, the greater the redshift.
- 3Distant galaxies show large redshifts, revealing the universe is expanding.
- 4It lets astronomers measure cosmic distances and look back in time.
- 5The opposite — an approaching source — causes a blueshift.
Frequently asked questions
- 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.