Solution of the Dyson-Schwinger equation on a de Sitter background in the infrared limit

We propose an ansatz which solves the Dyson-Schwinger equation for the real scalar fields in a Poincaré patch of de Sitter space in the infrared limit. The Dyson-Schwinger equation for this ansatz reduces to the kinetic equation, if one considers scalar fields from the principal series. Solving the latter equation we show that under the adiabatic switching on and then off of the coupling constant, the Bunch-Davies vacuum relaxes in future infinity to the state with the flat Gibbons-Hawking density of out-Jost harmonics on top of the corresponding de Sitter invariant out vacuum.