Failure of the stochastic approach to inflation beyond slow-roll

Abstract

After giving a pedagogical review we clarify that the stochastic approach to inflation is generically reliable only at zeroth order in the (geometrical) slow-roll parameter ${\epsilon }_1$ if and only if ${\epsilon }_2^2\ll 6/{\epsilon }_1$, with the notable exception of slow-roll. This is due to the failure of the stochastic $\Delta N$ formalism in its standard formulation. However, by keeping the formalism in its regime of validity, we showed that, in ultra-slow-roll, the stochastic approach to inflation reproduces the power spectrum calculated from the linear theory approach.

v3: typo in the appendix corrected. Results unchanged. Published version in JCAP