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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow Big Bang arrow Formation of atomic nuclei in the early Universe

Formation of atomic nuclei in the early Universe

In this epoch atomic nuclei are formed out of the protons and neutrons which, together with the electrons, are the foundation of all known matter.

However, it is only the lightest elements (He and Li) that are formed in this epoch. There is also some production of heavier elements but it is not important. The reason is that the amount of matter is rather small and the heavier elements that are created are short lived and will therefore decay before they can form even heavier, stable nuclei (compare with the production of heavier elements in the solar cycle).

Since this epoch is very long (about 300 000 years) compared to the life time of the neutrons (about 15 minutes) all free neutrons will have decayed at the end of this epoch. In the decay of a neutron a proton, an electron, and an antielectron-neutrino are formed. This reaction can also go in the opposite direction as long as the energy of the particles is high enough. When this is no longer true, the neutrinos cannot interact with other particles, and one says that they are decoupled. After this stage the neutrinos are only affected by the expansion of the universe (compare with the cosmic microwave background radiation). This means that today there is a remnant of neutrinos from the early Universe. However, due to their very low energy and weak interaction it will be extremely difficult to prove their existence.

When the energy of the particles has decreased even more, the electrons will be bound to the atomic nuclei and electrically neutral atoms will be formed.