Inflationary spectra and partially decohered distributions
Résumé
It is generally expected that decoherence processes will erase the quantum properties of the inflationary primordial spectra. However, given the weakness of gravitational interactions, one might end up with a distribution which is only partially decohered. Below a certain critical change, we show that the inflationary distribution retains quantum properties. We identify four of these: a squeezed spread in some direction of phase space, non-vanishing off-diagonal matrix elements, and two properties used in quantum optics called non-$P$-representability and non-separability. The last two are necessary conditions to violate Bell's inequalities. The critical value above which all these properties are lost is associated to the `grain' of coherent states. The corresponding value of the entropy is equal to half the maximal (thermal) value. Moreover it coincides with the entropy of the effective distribution obtained by neglecting the decaying modes. By considering backreaction effects, we also provide an upper bound for this entropy at the onset of the adiabatic era.