Can gravity be a thermal reservoir

It is shown, in an isotropic universe, that the quantum conformal fluctuations of the metric can play a role analogous to the fictitious field of Thermo Field Dynamics (TFD) of Takahashi and Umezawa, when a massless scalar field minimaly coupled to gravity is regarded as matter content. Moreover, by means of an extremum condition on the entropy and the energy, a planckian-like spectrum for the created particles is obtained