Astronomy Colloquium

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) aims to detect and characterize low-frequency gravitational waves by timing an array of millisecond pulsars. In 2023, together with international colleagues in the International Pulsar Timing Array (IPTA), we reported the first evidence for a stochastic gravitational-wave background. This signal is consistent with arising from a population of supermassive black hole binaries, but longer, more sensitive datasets are needed to determine its origin and use it to constrain the astrophysics of galaxy mergers and evolution. I will describe our next "20-yr" dataset, which contains multi-frequency observations of 80 millisecond pulsars with four radio telescopes. I will describe the properties of the dataset, the expected constraints it will provide on the stochastic gravitational-wave background, and the prospects for detecting an individual supermassive black-hole binary source. Finally, I will outline our strategy for future NANOGrav and IPTA data analyses, with particular attention to the transformational impact of the upcoming DSA radio telescope. By increasing both array sensitivity and the number of precisely timed pulsars, the DSA will accelerate progress toward robust detections of individual nanohertz gravitational-wave sources and enable detailed characterization of the gravitational-wave universe.