nanohertz gravitational waves

Nanohertz gravitational waves (nanohertz GWs) are gravitational waves with a frequency on the order of 10^-9 hertz, which corresponds to a wave period of about 30 years, so the term suggests waves with periods in the general range of 1 to 100 years. Speculated sources for this range include:

• quantum fluctuations in the early universe.
• binary SMBHs in galactic nuclei.
• compact objects captured by supermassive black holes.

Pulsar timing arrays (PTAs, e.g., NANOGrav) are designed to sense GWs in the range of 10^-5 Hz to 10^-10 Hz, i.e., periods from roughly a day to roughly 300 years. Likely a reason for this pursuit of the nanohertz range is due to the potential to detect them using basically existing equipment, though considerable resources have been devoted on the necessary observation and analysis, including development of specialized hardware for efficient extraction of the timing data.