Perturbative quantum damping of cosmological expansion

Perturbative quantum gravity in the framework of the Schwinger–Keldysh formalism is applied to compute lowest-order corrections to expansion of the Universe described in terms of the spatially flat Friedman–Lemaître–Robertson–Walker solution. The classical metric is approximated by a third degree polynomial perturbation around the Minkowski metric. It is shown that quantum contribution to the classical expansion, though extremely small, damps, i.e. slows down, the expansion (phenomenon of quantum friction).