Thermalization Process after Inflation and Effective Potential of Scalar
  Field
  We investigate the thermalization process of the Universe after inflation to
determine the evolution of the effective temperature. The time scale of
thermalization is found to be so long that it delays the evolution of the
effective temperature, and the resulting maximal temperature of the Universe
can be significantly lower than the one obtained in the literature. Our results
clarify the finite density corrections to the effective potential of a scalar
field and also processes of heavy particle production. In particular, we find
that the maximum temperature of the Universe may be at most electroweak scale
if the reheating temperature is as low as ${\cal O} (1)$ MeV, which implies
that the electroweak symmetry may be marginally restored. In addition, it is
noticeable that the dark matter may not be produced from thermal plasma in such
a low reheating scenario, since the maximum temperature can be smaller than the
conventional estimation by five orders of magnitude. We also give implications
to the Peccei-Quinn mechanism and the Affleck-Dine baryogenesis.
