Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz to microhertz regime. An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with recent NANOGrav limits which may hint that a gravitational-wave background detection is imminent.
Nicola Bartolo, Daniele Bertacca, Michele Liguori, Sabino Matarrese, Alvise Raccanelli, Angelo Ricciardone