Why is the universe still three-quarters hydrogen?

The universe is still about three-quarters hydrogen because of what happened in its first twenty minutes or so. Back then, the whole universe was hot and dense enough for nuclear fusion. Protons and neutrons fused into the very first atomic nuclei. This is called Big Bang nucleosynthesis.

The key was a shortage of neutrons. As the universe cooled, many neutrons decayed into protons, leaving roughly one neutron for every seven protons. Almost all the neutrons ended up locked inside helium-4, which is very hard to break apart. The protons left without a partner stayed single: those are hydrogen nuclei. Count it out and you get about 75 percent hydrogen and 25 percent helium by mass. Then expansion thinned everything out and fusion stopped. Everything heavier came later, from stars.

The idea goes back to George Gamow in the 1940s. His student Ralph Alpher did the detailed calculations, published in 1948. That paper is famous for a joke. Gamow added his friend Hans Bethe as an author, so the list would read Alpher, Bethe, Gamow, like alpha, beta, gamma. Bethe didn't object, and later did real work on the theory. Alpher, then a graduate student, felt the famous name overshadowed his own work, and still resented it in 1999.

The theory works well for hydrogen and helium, which make up over 99 percent of ordinary matter. But it can't build much beyond them, because no stable nucleus has a mass of 5 or 8. One loose thread remains: the theory predicts about three times more lithium-7 than astronomers find in the oldest stars. This lithium problem is still not solved.