  A generalization of ``Termo Field Dynamics'' to a curved geometry is
proposed. In particular a neutral scalar field minimally coupled to gravity is
considered as matter content in a Robertson-Walker metric. A non linear
amplification in the particle creation is obtained, due to the altogether
action of thermal and geometric effects. As a consequence the frequencies in
the system look like red shifted with respect to the case where the thermal
creation is not taken into account.
