  We analyze the temperature dependence of CP violation effects in the Standard
Model by determining the effective action of its bosonic fields, obtained after
integrating out the fermions from the theory and performing a covariant
gradient expansion. We find non-vanishing CP violating terms starting at the
sixth order of the expansion, albeit only in the C odd/P even sector, with
coefficients that depend on quark masses, CKM matrix elements, temperature and
the magnitude of the Higgs field. The CP violating effects are observed to
decrease rapidly with temperature, which has important implications for the
generation of a matter-antimatter asymmetry in the early Universe. Our results
suggest that the cold electroweak baryogenesis scenario may be viable within
the Standard Model, provided the electroweak transition temperature is at most
of order 1 GeV.
