Bounds on nonadiabatic evolution in single-field inflation

We examine the regime of validity of N-point spectra predictions of single-field inflation models that invoke transient periods of nonadiabatic evolution. Such models generate oscillatory features in these spectra spanning frequencies up to the inverse time scale of the transient feature. To avoid strong coupling of fluctuations in these theories this scale must be at least ∼10-2/cs of the Hubble time during inflation, where cs is the inflaton sound speed. We show that, in such models, the signal-to-noise ratio of the bispectrum is bounded from above by that of the power spectrum, implying that searches for features due to nonadiabatic evolution are best focused first on the latter.