Electrons and positrons annihilate
At six seconds the last great annihilation floods the universe with light.
- When
- Known to
- ± 20 million years
- Era
- Primordial
Once the universe cools below the threshold for producing electron–positron pairs, the existing ones annihilate rather than being replaced. The energy released reheats the photons — which is why the cosmic microwave background today is warmer than the neutrino background that had already decoupled. The small surplus of electrons that survives exactly balances the protons, leaving the universe electrically neutral overall.