Big Bang nucleosynthesis

Where did the raw material of the cosmos come from? The stars forged carbon, oxygen, and iron, but they inherited a universe already stocked with hydrogen and helium. That head start was made in the first few minutes after the Big Bang, when the whole universe was a nuclear furnace. Big Bang nucleosynthesis is the theory of how the lightest elements were cooked in that brief window, and its predictions match what we see to remarkable precision.

Between roughly one second and about twenty minutes after the beginning, the universe cooled through the temperature range, about a billion kelvin, where protons and neutrons can bind into nuclei but are not immediately blasted apart again. Neutrons and protons fused into deuterium, and deuterium into helium-4, with tiny traces of helium-3 and lithium-7. Because the neutron-to-proton ratio had frozen at about 1 to 7, nearly all the neutrons ended up locked in helium-4, giving a universe about 75 percent hydrogen and 25 percent helium by mass, plus trace deuterium at the level of a few parts in 100,000. The yields depend sharply on the density of ordinary matter, so measuring the abundances weighs the baryons.

BBN is one of the three great pillars of the Big Bang, alongside the expansion and the CMB, and it is a genuine early-universe test: it probes the cosmos at one second old, long before any galaxy or star existed. The measured abundances of deuterium and helium agree with theory and pin the baryon density to the same value the CMB independently gives, a striking consistency. A lingering wrinkle is the lithium problem: the predicted amount of lithium-7 is about three times what is observed, an unresolved tension.