The universe turns transparent
After 370,000 years the fog lifts, releasing the oldest light we can see.
- When
- Known to
- ± 20 million years
- Era
- Primordial
The universe cools to roughly 3,000 kelvin — cool enough for protons and electrons to combine into neutral hydrogen. With free electrons gone, photons stop scattering and travel freely for the first time. That light is still arriving: stretched by expansion into microwaves at 2.725 kelvin, it is the cosmic microwave background, and its faint temperature ripples are a photograph of the universe at this moment. Everything we know about the early universe's composition is read from it.