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We study the Brownian motion of a field where there are boundaries in the inflationary field space. Both the field and the boundary undergo Brownian motions with the amplitudes of the noises determined by the Hubble expansion rate of the corresponding de Sitter spacetime. This setup mimics models of inflation in which curvature perturbation is induced from inhomogeneities generated at the surface of the end of inflation. The cases of the drift-dominated regime as well as the diffusion-dominated regime are studied in detail. We calculate the first hitting probabilities as well as the mean number of e-folds for the field to hit either of the boundaries in the field space. The implications for models of inflation are reviewed.
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