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Proton to neutron ratio is fixed

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. Prior to 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.
. How does 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.
. When the 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.
. However, 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.
. So the 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.
. To make 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.


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