Unravelling Cosmological Perturbations
  We explain in detail the quantum-to-classical transition for the cosmological
perturbations using only the standard rules of quantum mechanics: the
Schrodinger equation and Born's rule applied to a subsystem. We show that the
conditioned, i.e. intrinsic, pure state of the perturbations, is driven by the
interactions with a generic environment, to become increasingly localized in
field space as a mode exists the horizon during inflation. With a favourable
coupling to the environment, the conditioned state of the perturbations becomes
highly localized in field space due to the expansion of spacetime by a factor
of roughly exp(-c N), where N~50 and c is a model dependent number of order 1.
Effectively the state rapidly becomes specified completely by a point in phase
space and an effective, classical, stochastic process emerges described by a
classical Langevin equation. The statistics of the stochastic process is
described by the solution of the master equation that describes the
perturbations coupled to the environment.
