Infrared behavior of quasilocal systems at finite temperature

We adopt the method developed recently for treating the infrared behavior of finite-size and quasilocal systems to study the symmetry behavior of dynamical quantum fields at finite temperature in cosmological spacetimes. We find that for slowly varying fields at high temperature the critical temperature is increased by an amount proportional to the second-order time derivative (''deceleration'') of the background field. This work can be viewed as a preliminary step towards a fuller investigation of critical dynamics in the early Universe involving nonequilibrium quantum fields