Big Bang nucleosynthesis
/ BBN /
Where did the universe's hydrogen and helium come from? Stars forge heavy elements like carbon and iron, but stars cannot explain why the cosmos is overwhelmingly the two lightest elements in a very specific ratio — and why that ratio is the same everywhere we look. Big Bang nucleosynthesis is the answer: in the first few minutes after the Big Bang, the entire universe acted as a nuclear furnace, cooking the lightest atomic nuclei out of raw protons and neutrons.
The timing is sharp. In the first second the universe was too hot for any nucleus to survive — it was a soup of free protons, neutrons, and other particles. As it expanded and cooled, between about one and twenty minutes after the start, the temperature passed through a narrow window where protons and neutrons could fuse and stick. In that brief window, most neutrons ended up locked into helium, producing a universe that was about 75 percent hydrogen and 25 percent helium by mass, with trace amounts of deuterium (heavy hydrogen) and a little lithium. After roughly twenty minutes the expansion had cooled and thinned things too far for fusion to continue, freezing in these primordial abundances. Heavier elements could not form then; they had to wait for stars.
Big Bang nucleosynthesis is one of the three great pillars of evidence for the Big Bang, and a stunning success of the theory. The predicted abundances — 25 percent helium, a particular trace of deuterium — match what we measure in the most pristine, least star-processed gas in the universe, across a precision of several decimal places. This agreement also weighs the ordinary matter of the cosmos: the measured deuterium pins down how much normal (baryonic) matter exists, and it comes out far too little to be the universe's dark matter, an early and independent hint that dark matter is something exotic. The one nagging puzzle is lithium, where prediction and observation still disagree — an honest open problem.
The theory predicts that out of every 12 hydrogen nuclei, about 1 helium nucleus should form, giving roughly 25 percent helium by mass. Measurements of the oldest, most metal-poor galaxies and gas clouds find almost exactly this — a fingerprint of the universe's first three minutes.
The universe's hydrogen, helium, and a little lithium were cooked in its first minutes.
Nucleosynthesis made only the lightest elements — hydrogen, helium, traces of deuterium and lithium. Everything heavier, including the carbon and oxygen in you, was forged later inside stars. The lithium prediction still mismatches observation, an open puzzle.