Nucleosynthesis began around the time 10^{-3} s, when protons and neutrons could finally remain together in the deuteron without flying apart due to the interaction of radiation. Calculate the ratio of protons to neutrons when the temperature of the universe was about 10^{10} K.

Strategy The ratio of protons to neutrons is purely a statistical distribution based on the available energy and the masses of the proton and neutron. The ratio is determined by the Maxwell-Boltzmann distribution from thermodynamics and the difference in masses. Because the proton has lower mass, we expect more protons to exist.