An ancient glow, everywhere
The cosmic microwave background is relic radiation from when the young universe became transparent to light.
In 30 seconds
- The observed signal traces a time roughly 380,000 years after the Big Bang.
- Expansion has stretched that radiation to microwave wavelengths.
- Tiny variations help constrain models of cosmic history.
Make it concrete
Think of a message whose carrier wave stretches during a long journey. The message is ancient; the map colours are a way to display measured differences.
- 01Early light
- 02Stretched wavelengths
- 03Microwave map
Check your understanding
Is a CMB map a normal visible-light photograph of the Big Bang itself?
- The observed signal traces a time roughly 380,000 years after the Big Bang.
- Expansion has stretched that radiation to microwave wavelengths.
- Tiny variations help constrain models of cosmic history.
Go to the source
Sources reviewed: 13.09.2026