Re: Anti-matter questions (fwd)

[Maurice Barnhill <mvb@UDel.Edu>]

Paul Camp wrote:
> > 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.

Let me add one thing that hasn't appeared in this thread yet.  CP is
known to be violated in the decays of the K0 meson.  K0 decay alone
certainly won't produce a baryon asymmetry, since it doesn't violate
baryon conservation, but the only unobserved *symmetry* breaking
required for a predominantly baryonic universe is baryon conservation. 
Whether the observed CP violation is produced by a mechanism that would
also allow an asymmetry between baryons and antibaryons is also not
known.

And of course you could always have the ugly solution: the universe
started out baryon dominated for no reason other than initial
conditions.
-- 
Maurice Barnhill, mvb@udel.edu                
http://www.physics.udel.edu/~barnhill/          
Physics Dept., University of Delaware, Newark, DE 19716