The first three minutes
Almost everything interesting about the early universe happened before the toast in your kitchen would have finished. The first three minutes set the hydrogen-to-helium ratio that still holds today.
A rough timetable
| Time | Temperature | What happens |
|---|---|---|
| 10⁻³² s | ~10²⁸ K | inflation ends, expansion continues |
| 10⁻⁶ s | ~10¹³ K | quarks bind into protons and neutrons |
| 1 s | ~10¹⁰ K | neutrinos stop interacting and go free |
| 3 min | ~10⁹ K | nuclei of helium and lithium form |
| 380,000 yr | ~3,000 K | atoms form, light escapes |
Why the universe is a quarter helium
At high temperature, protons and neutrons convert into one another freely. As things cool, that traffic stops and the ratio freezes at roughly seven protons per neutron. Pair off every available neutron into helium and you get helium making up about 25 per cent of ordinary matter by mass.
That number is a genuine prediction, calculated from nuclear physics alone — and it matches what we measure in the oldest stars.
Nucleosynthesis is the strongest quantitative evidence we have that the early universe was hot and dense. Change the physics and the ratio breaks.
Where it stops
Nothing heavier than lithium forms, because there is no stable nucleus with mass 5 or 8 to build through, and by then the universe is too cool and too thin. Carbon, oxygen and iron had to wait for stars — which is why the atoms in your body are second-hand.
Cover image: GSFC — Public domain (NASA) (source).