Rick's Cosmology Tutorial: Chapter 6 Abstract

The Shifting Neutron-Proton Ratio in the Early Universe: Neutrino Decoupling

Knowing the temperature of the universe at a given time, we can find the ratio of the neutron and proton abundances in terms of their mass difference by assuming thermal equilibrium. This thermal equilibrium depends upon their being an active mechanism which can interconvert neutrons and protons. It is shown that such a mechanism exists before about 1 second. It consists of the nucleon-lepton reactions mediated by the weak nuclear force.

It is shown that the expansion of the universe causes these weak force mediated reactions to become 'frozen out' at around 1 second. This 'freeze out' is primarily responsible for the relative numbers of neutrons and protons in the universe today. Moreover, after a few minutes, the neutrons find sanctuary in helium nuclei, which prevents them from disappearing due to beta decay.

Hence we derive the primordial composition of the universe to be about 75% hydrogen and 25% helium-4. The agreement of this 'prediction' with the observed abundances provides impressive evidence in favour of a hot dense origin for the universe.

Read Chapter 6 (pdf): The Neutron-Proton Ratio: Neutrino Decoupling

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