Correlating features in the primordial spectra

TL;DR

This paper investigates how heavy fields during inflation influence primordial spectra, deriving a simple formula linking features in the power spectrum to the bispectrum, providing a potential signature of single-field inflation with heavy field effects.

Contribution

It introduces a formula connecting power spectrum features to bispectrum features caused by a varying sound speed from heavy fields during inflation.

Findings

Features in the bispectrum peak in the equilateral limit.

Enhanced signals are found in the squeezed limit consistent with single-field predictions.

Correlated features serve as a signature of heavy field effects in inflation.

Abstract

Heavy fields coupled to the inflaton reduce the speed of sound in the effective theory of the adiabatic mode each time the background inflationary trajectory deviates from a geodesic. This can result in features in the primordial spectra. We compute the corresponding bispectrum and show that if a varying speed of sound induces features in the power spectrum, the change in the bispectrum is given by a simple formula involving the change in the power spectrum and its derivatives. In this manner, we provide a uniquely discriminable signature of a varying sound speed for the adiabatic mode during inflation that indicates the influence of heavy fields. We find that features in the bispectrum peak in the equilateral limit and, in particular, in the squeezed limit we find considerable enhancement entirely consistent with the single field consistency relation. From the perspective of the underlying effective theory, our results generalize to a wide variety of inflationary models where features are sourced by the time variation of background quantities. A positive detection of such correlated features would be unambiguous proof of the inflaton's nature as a single light scalar degree of freedom embedded in a theory that is UV completable.