Whispers from the dark side: Confronting light new physics with NANOGrav data

TL;DR

This paper investigates whether the recent NANOGrav gravitational wave background detection could be explained by new physics phenomena like phase transitions or axion-like particles, providing fits and constraints.

Contribution

It demonstrates that the observed GWB can be modeled by phase transitions or ALPs, offering new insights into early universe physics and PTA constraints.

Findings

Good fit with phase transition at 1-10 MeV

Best fit ALP parameters: F ≈ 5×10^{17} GeV, m ≈ 2×10^{-13} eV

PTAs can constrain early universe models effectively

Abstract

The NANOGrav collaboration has recently observed first evidence of a gravitational wave background (GWB) in pulsar timing data. Here we explore the possibility that this GWB is due to new physics, and show that the signal can be well fit also with peaked spectra like the ones expected from phase transitions (PTs) or from the dynamics of axion like particles (ALPs) in the early universe. We find that a good fit to the data is obtained for a very strong PT at temperatures around 1 MeV to 10 MeV. For the ALP explanation the best fit is obtained for a decay constant of $F \approx 5\times 10^{17}$ GeV and an axion mass of $2\times 10^{-13}$ eV. We also illustrate the ability of PTAs to constrain the parameter space of these models, and obtain limits which are already comparable to other cosmological bounds.