Cosmology for the Curious by Delia Perlov & Alex Vilenkin

Author:Delia Perlov & Alex Vilenkin
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

(13.1)

A neutron is more massive than a proton and electron combined, so it costs extra energy to convert a proton into a neutron. As the universe expands and cools, energetic electrons needed to make the conversion become more and more rare, and neutrons become less and less numerous relative to the protons. At about one second ABB, the rates of conversion reactions become too slow to keep up with the expansion of the universe, so proton to neutron conversions effectively cease. At that time, there are approximately 6 protons for each neutron.

Isolated neutrons are unstable1 and decay into protons and lighter particles with an average lifetime of 15 min:

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