Primordial Gravitational Waves from Scalar Backreaction in Axion-SU(2) Inflation

TL;DR

This paper numerically investigates how strong scalar backreaction in axion-SU(2) inflation affects gravitational wave production, revealing potential signals detectable by LISA.

Contribution

It provides the first numerical analysis of scalar backreaction effects during spectator chromo-natural inflation with decaying spectator sector.

Findings

Scalar backreaction amplifies perturbations and alters background dynamics.

The gauge field decays rapidly due to an effective quartic potential.

Predicted gravitational wave signals may peak at interferometer scales and be detectable by LISA.

Abstract

In this work, we perform the first numerical study of strong scalar backreaction in spectator chromo-natural inflation (SCNI) in the case where the spectator sector decays during inflation. The tachyonic instability in scalar fluctuations, activated as the system crosses the $m_Q = \sqrt{2}$ threshold, amplifies perturbations and may significantly alter the background dynamics. The strong scalar backreaction regime introduces an effective quartic term in the potential for the gauge field background that rapidly drives it to zero, accelerating the axion-gauge system decay. We describe the dynamics of such decay and derive the gravitational wave spectrum for a set of benchmark parameters. Interestingly, the signal may peak at interferometer scales and lie within LISA's projected sensitivity.