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:
- 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.
- 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.
- 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.