How does nucleosynthesis provide evidence of the big bang?

In a comparatively cool universe, matter production would primarily consist of hydrogen. In fact, a large portion of the universe's material is composed of 74% hydrogen and 26% helium, raising the question, "Where did all the helium come from?"

The early universe had a very high temperature, no atoms, and was made up of photons, electrons, protons, and neutrons. The big bang can provide an answer to this question.

With a half-life of roughly ten minutes, protons decay into neutrons, while neutrons are unstable and can transform back into protons when given sufficient energy. Protons are effectively stable.

Protons and neutrons were roughly equal in number in the early universe. As the universe cooled, neutrons began to decay more quickly than they were being created. Another effect of cooling was the ability of protons and neutrons to fuse to form hydrogen and helium isotopes.

This process of fusion, known as "big bang nucleosynthesis," is thought to provide evidence for the big bang by explaining the universe's relative helium abundance.

The Big Bang theory predicts that light elements like hydrogen and helium were produced during the early universe through nucleosynthesis. The observed abundance of these elements is consistent with the hypothesis that the universe went through a hot and dense phase, which is in line with the Big Bang model's initial conditions.