Axion-Gauge Dynamics During Inflation as the Origin of Pulsar Timing Array Signals and Primordial Black Holes

TL;DR

This paper explores how axion-gauge dynamics during inflation could explain pulsar timing array signals and produce primordial black holes, highlighting potential signatures like chiral gravitational waves and constraints on different gauge models.

Contribution

It demonstrates that axion-$U(1)$ models can account for PTA signals and PBH production, and analyzes the limitations of axion-$SU(2)$ models in this context.

Findings

Axion-$U(1)$ models can explain PTA signals and produce PBHs.

Chiral gravitational wave signatures are a potential observational target.

Axion-$SU(2)$ models are less capable of explaining PTA signals and tend to overproduce scalar perturbations.

Abstract

We demonstrate that the recently announced signal for a stochastic gravitational wave background (SGWB) from pulsar timing array (PTA) observations, if attributed to new physics, is compatible with primordial GW production due to axion-gauge dynamics during inflation. More specifically we find that axion-$U(1)$ models may lead to sufficient particle production to explain the signal while simultaneously source some fraction of sub-solar mass primordial black holes (PBHs) as a signature. Moreover there is a parity violation in GW sector, hence the model suggests chiral GW search as a concrete target for future. We further analyze the axion-$SU(2)$ coupling signatures and find that in the low/mild backreaction regime, it is incapable of producing PTA evidence and the tensor-to-scalar ratio is low at the peak, hence it overproduces scalar perturbations and PBHs.