How is the Matter Dominated Universe Created?



Motohiko Yoshimura


There are overwhelming evidences that the universe as a whole is made of matter and there is no substantial antimatter in a large vicinity of our galaxy. Since the law of microphysics is nearly symmetric under the combined symmetry, particle-antiparticle exchange plus parity, this poses a great mystery if one considers that antimatter can be equally created at very high temperatures in the early universe. Is the degree of matter-antimatter asymmetry quantified by the present baryon to entropy ratio of order 10-10 an initial condition at the beginning of the big bang, or how was it created?


Since late seventies the view on the baryon asymmetry has shifted, and now most physicists believe that this asymmetry is a reflection of a still unestablished, new law of microphysics. Namely, this small number of the asymmetry has to be explained by physics beyond the standard model of particle physics. There are three necessary ingredients to create the matter-antimatter asymmetry at an early universe; (1) baryon number non-conservation, (2) CP violation, (3) departure from equilibrium. I shall explain how these conditions are needed, and why the standard model fails to explain this asymmetry.


There are a few interesting theories beyond the standard model that have a potential to explain the asymmetry. One of them based on grand unified theory is the original suggestion and the simplest in physical interpretation. Recent measurements of precision electroweak parameters and a discovery of the neutrino oscillation hint towards such a unification scenario. On the other hand, the most important prediction of this type of theories, the proton decay, has yet to be experimentally discovered. The other options such as converted lepton asymmetry and supersymmetric scenario are also intriguing and may have experimental signatures in future.


Baryogenesis is thus one of the few keys that hint how to go beyond the standard model, along with a tiny neutrino mass.


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