Primordial Stochastic Gravitational Wave Backgrounds from a Sharp Feature in Three-field Inflation II: The Inflationary Era

TL;DR

This paper investigates how sharp features in three-field inflation can generate a detectable stochastic gravitational wave background during inflation, with unique signatures largely independent of the number of fields, potentially observable by future GW detectors.

Contribution

It extends previous work to include the inflationary era in three-field inflation, analyzing isocurvature perturbations and their impact on the gravitational wave background.

Findings

Inflationary-era-sourced SGWB shape is largely independent of the number of fields.

Large isocurvature transients can significantly enhance the SGWB amplitude.

Some parameter space remains perturbatively controlled and detectable by GW experiments.

Abstract

We study the contribution of large scalar perturbations sourced by a sharp feature during cosmic inflation to the stochastic gravitational wave background (SGWB), extending our previous work to include the SGWB sourced during the inflationary era. We focus in particular on three-field inflation, since the third dynamical field is the first not privileged by the perturbations' equations of motion and allows a more direct generalization to $N$-field inflation. For the first time, we study the three-field isocurvature perturbations sourced during the feature and include the effects of isocurvature masses. In addition to a two-field limit, we find that the third field's dynamics during the feature can source large isocurvature transients which then later decay, leaving an inflationary-era-sourced SGWB as their only observable signature. We find that the inflationary-era signal shape near the peak is largely independent of the number of dynamical fields and has a greatly enhanced amplitude sourced by the large isocurvature transient, suppressing the radiation-era contribution and opening a new window of detectable parameter space with small adiabatic enhancement. The largest enhancements we study could easily violate backreaction constraints, but much of parameter space remains under perturbative control. These SGWBs could be visible in LISA and other gravitational wave experiments, leaving an almost universal signature of sharp features during multi-field inflation, even when the sourcing isocurvature decays to unobservability shortly afterwards.