PrimordixBig Bang Nucleosynthesis / Primordial HeliumSticker

Primordix is a baker with the worst kitchen in history and an ingredient that spoils.

In the first second after the Big Bang, the universe is too hot for nuclei. Protons and neutrons turn into each other freely through the weak force. As it cools, those reactions freeze out, and because a neutron outweighs a proton by 1.293 MeV, the last line on this page, neutrons end up outnumbered, roughly one for every five or six protons. Then the neutrons start to decay.

She can’t bake yet. Any deuterium she forms, the first step towards helium, is blasted apart by photons that outnumber baryons more than a billion to one. She has to wait until the oven cools, a few minutes in. By then decay has cut the ratio to about one neutron per seven protons. Then she bakes fast: almost every surviving neutron ends up in helium-4. The first two lines turn that into a helium share of about 0.25 by mass. Observations give 0.245 ± 0.003; standard theory with Planck’s baryon density predicts about 0.247. That is a quarter by mass; by number of nuclei, helium is only about 7.5%.

How fast her neutrons spoil is still argued. Experiments that trap ultracold neutrons in bottles give a mean life of about 878 s (Particle Data Group 2024: 878.4 ± 0.5 s). Experiments that count decays in a neutron beam give about 888 s. The gap is unresolved and remains an open question. Lithium-7 is another open problem: theory predicts about three times what old stars show.

Ralph Alpher, Hans Bethe and George Gamow proposed cooking the elements in a hot early universe in 1948; the modern calculation came later.

Three minutes. Neutrons won’t wait.