The mathematics allows us to work out their relative abundances: about 75 percent hydrogen (one proton), 25 percent helium (two protons, two neutrons), and trace amounts of deuterium (a heavy form of hydrogen, with one proton and one neutron), helium-3 (a light form of helium with two protons and one neutron), and lithium (three protons, four neutrons).11 Detailed astronomical observations of atomic abundances have confirmed that these ratios are spot-on, a triumph of mathematics and physics in illuminating the detailed processes that happened within minutes of the big bang.
