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Linné on line arrow Physics and the Cosmos arrow Macrocosmos arrow Big Bang arrow Hadron formation

Hadron formation

The interaction that mainly governs the behaviour of quarks is the strong force. The theory for the strong force is called quantum chromodynamics (QCD). It predicts that quarks cannot exist as free particles but only as bound states consisting of several quarks in a combination that is neutral for the strong force.

However, quarks can exist as quasi free particles if there is enough high energy radiation that can break up the bound states by interacting with them. This was the case in the early stages of the evolution of the Universe, but at this stage the energy of the radiation is so low that it cannot break up the bound states as before. That is why the quarks that used to be "free" will be bound in hadrons.

On the other hand, all hadrons are not stable and some of them will decay. In the end of this era there will only be stable hadrons left (protons and neutrons). The neutron is actually unstable, but its lifetime is much longer than the age of the universe at this time and therefore it can be considered as stable.

Apart from the evolution that has been described so far in this phase, there is also another change that is of principle importance but rather weak. Earlier the energy in the Universe was dominated by radiation but at the end of this epoch it is mainly in the form of matter.

Compared to earlier epochs, the latest phase has taken a very long time. It lasted from 10-34 s until approximately 100 s. This evolution is typical since from now on larger and larger objects are going to be formed and naturally this takes longer time.

In the next phase the nucleons (neutron and protons) will be combined into atomic nuclei.