Application of Gradient Expansion to Inflationary Universe
  Using the long wave perturbation scheme(gradient expansion), the effect of inhomogeneity on the inflationary phase is investigated. We solved the perturbation equation of which source term comes from inhomogeneity of a scalar field and a seed metric. The result indicates that sub-horizon scale inhomogeneity strongly affects the onset of inflation. 
Yasusada Nambu and Atsushi Taruya