Matter-Antimatter Asymmetry in the Universe and an Arrow for Time

In the Big Bang model, the Universe starts out as very hot with all particlespecies in equilibrium. In these circumstances, creation and annihilation ofmatter and antimatter is equally probable. As the Universe expands it coolsdown and when the temperature of the Universe goes below a temperatureof O(T ∼ 1GeV) the inverse annihilation process 2γ → p+ ¯p is energeticallyblocked. Hence, around this temperature, the direct process p + ¯p → 2γbegins to turn all protons and antiprotons into photons. This process isvery efficient and, if one starts in a matter - antimatter symmetric Universe,one expects the Universe now to have very little matter (or antimatter) left.In detail, [1] one predicts a ratio of the baryon to photon densities now oforderη