Big Bang nucleosynthesis
/ NOO-klee-oh-SIN-thuh-sis /
Where did the lightest atoms in your body come from? The carbon and oxygen were forged inside stars, but the hydrogen — and most of the helium — is older than any star. It was made in the first few minutes after the Big Bang, when the entire universe was a furnace hot enough to weld protons and neutrons into the first atomic nuclei. This brief cooking session, lasting only about the first twenty minutes of cosmic history, is called Big Bang nucleosynthesis, and it is the recipe that set the universe's basic chemical ingredients.
Here is the gist. In the universe's first second, it was far too hot for any nucleus to survive — protons and neutrons existed as free particles in a roiling soup. As the cosmos expanded and cooled over the next few minutes, it passed through a brief window that was cool enough for protons and neutrons to stick together yet still hot enough to fuse them. In that window, protons and neutrons combined mainly into helium nuclei, with traces of deuterium (heavy hydrogen) and lithium. Then the universe cooled too far for fusion to continue, and the door slammed shut. The result, fixed within minutes, was a universe of about three-quarters hydrogen and one-quarter helium by mass, with only a whisper of anything heavier.
Big Bang nucleosynthesis is one of the great triumphs where particle physics, nuclear physics, and cosmology meet, because its predictions can be checked. Using the measured rates of nuclear reactions and the known physics of the early universe, the theory predicts the exact proportions of these light elements — and those predictions match what astronomers observe in the oldest, most pristine gas clouds with stunning accuracy. That agreement is powerful evidence that we genuinely understand the universe back to when it was seconds old, and it even helps pin down how much ordinary matter the universe contains, reinforcing the case that the rest of the matter must be dark.
The amount of deuterium — a rare heavy form of hydrogen — left over from the first few minutes is exquisitely sensitive to how much ordinary matter the universe holds. Measuring it in ancient gas clouds gives a count of ordinary matter that agrees beautifully with the entirely separate count from the cosmic microwave background.
The universe's first elements were cast in minutes.
Big Bang nucleosynthesis only made the lightest elements — hydrogen, helium, and a trace of lithium. Everything heavier, including the carbon, oxygen, and iron in your body, was forged much later inside stars and stellar explosions, not in the Big Bang.