Stochastic constant-roll inflation beyond the hilltop with the spectral method

Abstract

Stochastic inflation can be used to study large inflationary perturbations. This paper presents such a study for a quadratic hilltop potential, corresponding to constant-roll inflation. I solve the perturbation distribution using the spectral method, with detailed solutions of the eigenvalues and eigenfunctions of the Fokker-Planck operator. Contrary to previous studies of stochastic constant-roll inflation, the solution allows trajectories that cross the hilltop and get stuck near a reflecting boundary on the other side, tunneling out slowly in a way dictated by the lowest eigensolution. Despite their rarity, these trajectories turn out to dominate the mean first-passage time. For this reason, I argue the mean does not properly describe the inflationary background. Using the median instead, I compute the distribution of the coarse-grained $\Delta N$ distribution and show that its well-known exponential tail first flattens out and then forms a peak near a maximal $\Delta N$ value. I argue similar intricacies arise in primordial black hole models.