Challenges in Interpreting the NANOGrav 15-Year Data Set as Early Universe Gravitational Waves Produced by ALP Induced Instability

TL;DR

This paper investigates whether a tachyonic instability in a dark gauge field, induced by an axion-like particle, can explain the NANOGrav 15-year gravitational wave observations, highlighting significant cosmological constraints and suppression issues.

Contribution

It analyzes the viability of ALP-induced gauge field instabilities as a gravitational wave source within current observational constraints, extending previous work with newer data.

Findings

Larger axion mass and decay constant favored with 15-year data

Model faces strong constraints from $ abla N_{ m eff}$ and dark matter overproduction

Gravitational wave signal is significantly suppressed, requiring excessive dark radiation

Abstract

In this paper, we study a possible early universe source for the recent observation of a stochastic gravitational wave background at the NANOGrav pulsar timing array. The source is a tachyonic instability in a dark gauge field induced by an axion-like particle (ALP), a known source for gravitational waves. We find that relative to the previous analysis with the NANOGrav 12.5-year data set, the current 15-year data set favors parameter space with a relatively larger axion mass and decay constant. This favored parameter space is heavily constrained by $\Delta N_{\rm eff}$ and overproduction of ALP dark matter. While there are potential mechanisms for avoiding the second problem, evading the $\Delta N_{\rm eff}$ constraint remains highly challenging. In particular, we find that the gravitational wave magnitude is significantly suppressed with respect to the gauge boson dark radiation, which implies that successfully explaining the NANOGrav observation requires a large additional dark radiation, violating the cosmological constraints. Satisfying the $\Delta N_{\rm eff}$ constraint will limit the potential contribution from this mechanism to the observed signal to at most a percent level.