Ultra-low frequency gravitational waves from cosmological and astrophysical processes

Understand

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.