Resonant non-Gaussianity with equilateral properties

TL;DR

This paper shows how combining multiple resonant non-Gaussian signals can mimic equilateral shapes in the bispectrum, highlighting potential degeneracies in interpreting cosmological data.

Contribution

It demonstrates that superimposing resonant non-Gaussianities can produce equilateral shapes, revealing a degeneracy with models like DBI inflation.

Findings

Superposition of oscillatory contributions can synthesize equilateral shapes.

Detection of strong equilateral non-Gaussianity would rule out a resonant origin.

Resonant non-Gaussianities do not produce oscillations in the 2-point function.

Abstract

We discuss the effect of superimposing multiple sources of resonant non-Gaussianity, which arise for instance in models of axion inflation. The resulting sum of oscillating shape contributions can be used to "Fourier synthesize" different non-oscillating shapes in the bispectrum. As an example we reproduce an approximately equilateral shape from the superposition of ${\cal O}(10)$ oscillatory contributions with resonant shape. This implies a possible degeneracy between the equilateral-type non-Gaussianity typical of models with non-canonical kinetic terms, such as DBI inflation, and an equilateral-type shape arising from a superposition of resonant-type contributions in theories with canonical kinetic terms. The absence of oscillations in the 2-point function together with the structure of the resonant $N$-point functions, imply that detection of equilateral non-Gaussianity at a level greater than the PLANCK sensitivity of $f_{NL}\sim{\cal O}(5)$ will rule out a resonant origin. We comment on the questions arising from possible embeddings of this idea in a string theory setting.