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Whenever a physical reaction occurs it
must do so in a manner that conserves baryons and leptons.
That could not always have been the case else the universe would not
have a baryon-antibaryon asymmetry. Of course, as far as known,
experimentally confirmed physical law goes, this is correct. However,
Sakharov pointed out (in 1967, a decade *before* grand unification
attempts) that in order for the universe to have evolved a baryon
asymmetry, several conditions would have to be satisfied:
1. Baryon number violation. If all interactions involve baryon
conservation then the present observed asymmetry can only reflect
asymmetric initial conditions. Note that most GUTS predict this
effect.
2. C and CP violation. In the absence of this condition, Baryon
nonconservation reactions will produce baryon and antibaryon excesses
at the same rate. This condition is necessary to give the reactions
an "arrow" in one direction.
3. Nonequilibrium conditions. In equilibrium, all the chemical
potentials associated with nonconserved quantum numbers vanish.
Masses of particles and antiparticles are guaranteed to be equal due
to CPT invariance. This implies that the number of baryons equals the
number of antibaryons. (Kolb and Turner's The Early Universe has the
details of all this.)
Having pointed out that *something* must be asymmetric to give us the
universe we see, one must also hasten to add that nobody really knows
what it was. As far as known physical law is concerned, Leigh is
right on target. Everything else is speculation. However, I can't
miss a chance to refer to Sakharov's paper since I think it is one of
the most remarkable examples of prescience I've ever seen.