The baryon to photon ratio is determined during baryogenesis , , . It is not known when baryogenesis occurred. Sakharov  noted that three conditions are required: different interactions for matter and anti-matter (CP violation), interactions which change the baryon number, and departure from thermodynamic equilibrium. This last condition may be satisfied in a first order phase transition, the GUT transition at 10-35 s, or perhaps the electroweak transition at 10-11 s. If baryogenesis occurred at the electroweak scale, then future measurements may lead to predictions for , but if, alternatively, baryogenesis is at the GUT or inflation scale, it will be very hard to predict (J. Ellis personal communication).
The matter/anti-matter asymmetry of the universe (the value) is attracting discussion in the popular science press because of the inauguration of major experiments to study CP violation in B mesons (, ; Economist, May 8 1999, 85-87).