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Baryogenesis is the name given to the still-unexplained origin of the total asymmetry of matter vs. antimatter in the present universe. All the things we see in the universe, stars, galaxies, etc., appear to be made of matter, not antimatter. The particles which make up this matter are called baryons, i.e. protons, neutrons and their heavier, exotic cousins.

On the other hand, the standard model of particle physics doesn't obviously contain a mechanism which would create the asymmetry we see today. In 1967 A.D. Sakharov showed that three conditions must have been met in order for baryogenesis to occur:

  1. There must be some baryon-number violating process at the elementary particle level. The standard model doesn't contain such a process; however, most Grand Unified Theories do.
  2. This process must violate charge (C) and charge-times-parity (CP) symmetry. While the electroweak theory violates charge symmetry (and parity) maximally, the amount of CP violation (it was recently proven there really is direct CP violation in kaon decays) is too small to generate the observed matter-antimater assymetry even if the previous and next conditions held.
  3. This asymmetry must have been generated during a time of disequilibrium in the universe (otherwise the process would work both forwards and backwards resulting in no net change).
Sakharov's article, originally published in Russian I believe, is reprinted in Kolb and Turner's book, The Early Universe.

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