Generically large nongaussianity in small multifield inflation

TL;DR

This paper shows that small multifield inflation models without hybrid mechanisms generally produce large squeezed nongaussianity unless the field directions have equal slopes, especially relevant if the tensor-to-scalar ratio is very low.

Contribution

It identifies the conditions under which small multifield inflation naturally leads to large nongaussianity, highlighting the importance of equal slopes in field directions.

Findings

Large nongaussianity arises unless field slopes are equal.

Small two-field models can match observed perturbations but produce large nongaussianity.

Unequal slopes in field directions lead to excludably large nongaussianity.

Abstract

If forthcoming measurements of cosmic photon polarization restrict the primordial tensor-to-scalar ratio to $r < 0.01$, small field inflation will be a principal candidate for the origin of the universe. Here we show that small multifield inflation, without the hybrid mechanism, typically results in large squeezed nongaussianity. Small multifield potentials contain multiple flat field directions, often identified with the gauge invariant field directions in supersymmetric potentials. We find that unless these field directions have equal slopes, large nongaussianity arises. After identifying relevant differences between large and small two-field potentials, we demonstrate that the latter naturally fulfill the Byrnes-Choi-Hall large nongaussianity conditions. Computations of the primordial power spectrum, spectral index, and squeezed bispectrum, reveal that small two-field models which otherwise match observed primordial perturbations, produce excludably large nongaussianity if the inflatons' field directions have unequal slopes.