  The fluctuations in the inflaton field at the end of inflation which seed the
density perturbations are prepared in a pure quantum state. It is generally
assumed that some physics causes this pure state to decohere so that it should
be treated probabilistically. We show that the entanglement entropy between the
universe inside our observable horizon and that outside our horizon is
sufficient to do this. For the modes which are super-Hubble at the end of
inflation, this entanglement entropy grows with volume inside the horizon,
rather than with the horizon's area, and is proportional to the number of
e-folds since Hubble crossing.
