Ultra-low frequency gravitational waves from cosmological and astrophysical processes

TL;DR

This paper discusses how ultra-low frequency gravitational waves, detectable by pulsar timing arrays, can reveal insights into black hole evolution and cosmological events, and explores combining multiple observational bounds to distinguish their origins.

Contribution

It introduces a method to combine various observational bounds to differentiate between astrophysical and cosmological sources of ultra-low frequency gravitational waves.

Findings

PTA observations are nearing detection of the stochastic GW background.

Combining bounds from CMB, BBN, and astrometry can help identify GW origins.

Current PTA data cannot yet distinguish between astrophysical and cosmological GW sources.

Abstract

Gravitational waves (GWs) at ultra-low frequencies (${\lesssim 100\,\mathrm{nHz}}$) are key to understanding the assembly and evolution of astrophysical black hole (BH) binaries with masses $\sim 10^{6}-10^{9}\,M_\odot$ at low redshifts. These GWs also offer a unique window into a wide variety of cosmological processes. Pulsar timing arrays (PTAs) are beginning to measure this stochastic signal at $\sim 1-100\,\mathrm{nHz}$ and the combination of data from several arrays is expected to confirm a detection in the next few years. The dominant physical processes generating gravitational radiation at $\mathrm{nHz}$ frequencies are still uncertain. PTA observations alone are currently unable to distinguish a binary BH astrophysical foreground from a cosmological background due to, say, a first order phase transition at a temperature $\sim 1-100\,\mathrm{MeV}$ in a weakly-interacting dark sector. This letter explores the extent to which incorporating integrated bounds on the ultra-low frequency GW spectrum from any combination of cosmic microwave background, big bang nucleosynethesis or astrometric observations can help to break this degeneracy.