- Neutral hydrogen strongly absorbs and scatters ultraviolet light and many other photon energies.
- This trapped or absorbed photons emitted by the first stars and galaxies.
- Reionization begins (~400 million years onward):
- UV radiation from the first stars and quasars ionized the neutral hydrogen.
- Once ionized, the gas no longer absorbed light so strongly.
- The universe became transparent to starlight, allowing photons to travel freely across cosmic distances.
