  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.

General Relativity and Quantum Cosmology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Theory;Quantum Physics