quark-hadron transition

When the energy density dropped below the level needed for the creation of quarks, the number of quarks in the universe became fixed. Before that time quarks had been independent particles, but afterwards they became subject to the strong nuclear force, which pulled them together in threes to form protons and neutrons. This is known as the quark-hadron phase transition.

Hadrons: heavy matter particles, in this case protons and neutrons.

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