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 $\sim 10^{-2}/c_s$ of the Hubble time during inflation, where $c_s$ 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.

• Received 24 February 2014

DOI:https://doi.org/10.1103/PhysRevD.89.083531