The first elements are forged
Between ten seconds and twenty minutes, the universe makes all the helium it will ever need.
- When
- Known to
- ± 20 million years
- Era
- Primordial
Protons and neutrons fuse into the first atomic nuclei: about 75% hydrogen and 25% helium-4 by mass, with traces of deuterium, helium-3 and lithium-7. The window is brief — expansion cools the universe below fusion temperatures within minutes — and nothing heavier than lithium is made, because no stable nucleus has mass number 5 or 8. Everything heavier waits for stars. The measured abundances match the prediction closely, and are among the strongest evidence for the Big Bang model.