Cosmological Constraints on B-L Violation

This is a review of the cosmological bounds on $B-L$ violating interactions, and the loopholes in the argument that gives these constraints. If one assumes that the baryon asymmetry we observe today was present above the electroweak phase transition in equilibrium with the non-perturbative $B+L$ violating processes, then interactions that violate all three of ${B/3 - L_i}$ cannot simultaneously be in equilibrium. Otherwise the baryon asymmetry would be washed out. Therefore violation of at least one of the $B/3 - L_i$ must be small. This argument can be evaded by not having the observed baryon asymmetry present in the thermal bath (for instance, make it at the electroweak phase transition), by using a non-standard cosmological model, or possibly by some mass effects in models where the difference between lepton flavour asymmetries is conserved.