Squeezed Thermal Vacuum and the Maximum Scale for Inflation

We consider the stimulated emission of gravitons from an initial state of thermal equilibrium, under the action of the cosmic gravitational background field. We find that the low-energy graviton spectrum is enhanced if compared with spontaneous creation from the vacuum; as a consequence, the scale of inflation must be lowered, in order not to exceed the observed CMB quadrupole anisotropy. This effect is particularly important for models based on a symmetry-breaking transition which require, as initial condition, a state of thermal equilibrium at temperatures of the order of the inflation scale.