Primordial gravitational waves in the nano-Hertz regime and PTA data -- towards solving the GW inverse problem

TL;DR

This paper explores the possibility that the nano-Hertz gravitational wave background detected by pulsar timing arrays originates from new physics phenomena like cosmic strings or phase transitions, proposing models and analyzing their compatibility with data.

Contribution

It introduces simple, concrete models for primordial gravitational waves from various cosmological sources and confronts them with pulsar timing and cosmological data to identify viable parameter spaces.

Findings

Certain models are compatible with current PTA data

Predictions made for future astrophysical tests

Constraints on model parameters established

Abstract

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.