Multi-field inflation with large scalar fluctuations: non-Gaussianity and perturbativity

TL;DR

This paper investigates multi-field inflation models with hyperbolic field space that produce large scalar fluctuations, analyzing their non-Gaussianity, perturbativity, and implications for primordial black holes and gravitational waves.

Contribution

It provides new insights into scalar non-Gaussianity in hyperbolic multi-field inflation and validates results using two numerical methods, highlighting perturbativity constraints.

Findings

Large non-Gaussianity near the power spectrum peak, close to local form.

Validation of results through transport and $ abla$ formalism techniques.

Implications for perturbativity and phenomenology at interferometer scales.

Abstract

Recently multi-field inflation models that can produce large scalar fluctuations on small scales have drawn a lot of attention, primarily because they could lead to primordial black hole production and generation of large second-order gravitational waves. In this work, we focus on models where the scalar fields responsible for inflation live on a hyperbolic field space. In this case, geometrical destabilisation and non-geodesic motion are responsible for the peak in the scalar power spectrum. We present new results for scalar non-Gaussianity and discuss its dependence on the model's parameters. On scales around the peak, we typically find that the non-Gaussianity is large and close to local in form. We validate our results by employing two different numerical techniques, utilising the transport approach, based on full cosmological perturbation theory, and the $\delta N$ formalism, based on the separate universe approximation. We discuss implications of our results for the perturbativity of the underlying theory, focusing in particular on versions of these models with potentially relevant phenomenology at interferometer scales.