Decoherence of inflationary primordial fluctuations

We study the process whereby quantum cosmological perturbations become classical within inflationary cosmology. By setting up a master- equation formulation we show how quantum coherence for super- Hubble modes can be destroyed by its coupling to the environment provided by sub- Hubble modes. We identify what features the sub- Hubble environment must have in order to decohere the longer wavelengths, and identify how the onset of decoherence ( and how long it takes) depends on the properties of the sub- Hubble physics which forms the environment. Our results show that the decoherence process is largely insensitive to the details of the coupling between the sub- and super- Hubble scales. They also show how locality implies, quite generally, that the decohered density matrix at late times is diagonal in the field representation ( as is implicitly assumed by extant calculations of inflationary density perturbations). Our calculations also imply that decoherence can arise even for couplings which are as weak as gravitational in strength.