this era of the Universe, neutrons and protons were rapidly changing into each
other through the emission and absorption of neutrinos. Now the Universe has
expanded and cooled to the point where that process slows down, and at the end
of the slowing down, we are left with about seven protons for every neutron.
this happen? Particle physicists have known for a long time that a neutron just
sitting around will all by itself decay into a proton, an electron and an electron
antineutrino, but a proton won't decay into anything. (This process is illustrated
in the animation above.) If we hit a proton with a electron antineutrino at
high enough energy, we can make a neutron and a positron (an antielectron) come
out the other end. And if we hit a proton with an electron, we get a neutron
and an electron neutrino at the other end. So neutrons change into protons by
themelves, but the reverse process requires extra energy from some kind of collision.
Universe was sufficiently hot and dense, there were so many electrons and antineutrinos
hitting protons and changing them into neutrons that an equal numbers of protons
and neutrons are changing into each other at the same rate.
as the Universe kept expanding and cooling, the average energy level of the
particles dropped and so did the rate of neutrinos hitting protons and changing
them into neutrons. The neutrinos and antineutrinos decoupled
from the rest of the matter and radiation, and interactions between neutrinos
and other particles stopped being a very big factor in the dynamics of the Universe.
protons were no longer being changed to neutrons, but the neutrons were still
changing spontaneously all by themselves into protons. That eventually left
us with about seven times more protons than neutrons in the Universe.
a hydrogen nucleus, we only need one proton, no neutrons. To make a helium nucleus,
we need two protons and two neutrons. Therefore, a direct consequence of an
excess of protons over neutrons would be an excess of hydrogen over helium,
and that is what is observed today.This gives us a vital observational validation
for the Big Bang theoretical description of the early expanding Universe.