First atomic nuclei form

In the first minutes after the Big Bang, about 13.8 billion years ago, the whole universe acted like a nuclear furnace. Between roughly ten seconds and twenty minutes after the start, it was hot and dense enough for protons and neutrons to fuse into the first atomic nuclei, but cooling fast enough that the process soon stopped. The result was a cosmos made of about 75 percent hydrogen and 25 percent helium by mass, with tiny amounts of deuterium (heavy hydrogen), helium-3 and lithium. Almost nothing heavier formed.

The timing was set by temperature. In the first second, the universe was so hot that any nucleus that formed was instantly smashed apart, and protons and neutrons kept turning into each other. As it cooled, neutrons became rarer, ending up about one for every seven protons, because free neutrons slowly decay. Fusion first had to build deuterium, a fragile pairing of one proton and one neutron, which survived only once the temperature fell below about a billion degrees. After that, almost every remaining neutron was quickly locked into helium-4, which explains why helium ended up at about a quarter of the mass. By then the universe was too thin and cool to fuse heavier elements.

In 1948 Ralph Alpher and George Gamow worked out that a hot early universe should have produced light elements this way, and later calculations refined the predictions. Measurements of the oldest stars and gas clouds match them closely, making this one of the strongest pieces of evidence for the Big Bang. The amount of deuterium also reveals how much ordinary matter exists, about 5 percent of the universe's contents. One puzzle remains: old stars contain about three times less lithium than predicted.