  We introduce a self-consistent stochastic coarse-graining method, which
includes both metric and scalar field fluctuations, to investigate the back
reaction of long wavelength perturbations in single-scalar driven inflation, up
to the second (one loop) order. We demonstrate that, although back reaction
cannot be significant during the last 70 e-foldings of inflation with a smooth
potential, there exist non-smooth inflaton potentials which allow significant
back reaction, and are also consistent with cosmological observations. Such
non-smooth potentials may lead to the generation of massive primordial black
holes, which could be further used to constrain/verify these models.
