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

TL;DR

This paper investigates the unique gravitational wave signatures produced by a brief turn in three-field inflation during the radiation era, providing analytic formulas and numerical validation, with implications for future detection.

Contribution

It introduces the first analysis of stochastic gravitational wave backgrounds from three-field inflation, deriving analytic expressions for power spectrum enhancements due to a sharp turn.

Findings

Analytic expression for power spectrum enhancement as a function of turn rate and torsion.

Identification of unique signatures in primordial spectra from three-field dynamics.

Numerical simulations confirm the analytic results with good agreement.

Abstract

The detection of a primordial stochastic gravitational wave background has the potential to reveal unprecedented insights into the early universe, and possibly into the dynamics of inflation. Generically, UV-complete inflationary models predict an abundance of light scalars, so any inflationary stochastic background may well be formed in a model with several interacting degrees of freedom. The stochastic backgrounds possible from two-field inflation have been well-studied in the literature, but it is unclear how similar they are to the possibilities from many-field inflation. In this work we study stochastic backgrounds from more-than-two field inflation for the first time, focusing on the scalar-induced background produced during the radiation era by a brief turn in three-field space. We find an analytic expression for the enhancement in the power spectrum as a function of the turn rate and the torsion, and show that unique signatures of three-field dynamics are possible in the primordial power spectrum and gravitational wave spectrum. We confirm our analytic results with a suite of numerical simulations and find good agreement in the shape and amplitude of the power spectra. We also comment on the detection prospects in LISA and other future detectors. We do not expect the moderately large growth of the inflationary perturbations necessary for detection to cause a breakdown of perturbation theory, but this must be verified on a case-by-case basis for specific microphysical models to make a definitive claim.