Page "Big Bang" Paragraph 8

The Universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing.

Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form.

After about 10 < sup >− 11 </ sup > seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments.

At about 10 < sup >− 6 </ sup > seconds, quarks and gluons combined to form baryons such as protons and neutrons.

The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons.

The temperature was now no longer high enough to create new proton – antiproton pairs ( similarly for neutrons – antineutrons ), so a mass annihilation immediately followed, leaving just one in 10 < sup > 10 </ sup > of the original protons and neutrons, and none of their antiparticles.

A similar process happened at about 1 second for electrons and positrons.

After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the Universe was dominated by photons ( with a minor contribution from neutrinos ).