Squeezed thermal vacuum and the maximum scale for inflation

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Abstract
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 an initial condition, a state of thermal equilibrium at temperatures of the order of the inflation scale.
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