The International Pulsar Timing Array: A Galactic-Scale Gravitational Wave Observatory*

The International Pulsar Timing Array (IPTA) is a network of researchers and students composed of the NANOGrav Physics Frontiers Center (PFC) and colleagues from Australia, Europe, and India working to directly detect gravitational waves at frequencies 11 orders of magnitude smaller than LIGO through high-precision timing of radio pulsars using the world's largest radio telescopes. This experiment requires observations of pulsars distributed across the sky, necessitating observations with radio telescopes across the globe and an efficient framework for international collaboration. The most promising GW sources in the low-frequency band are supermassive binary black holes at the cores of merging galaxies. Hence, a detection of GWs will provide new insights into galaxy growth and evolution through cosmic time, transforming our understanding of the Universe. Recent IPTA analyses indicate the presence of a common spectral signature in IPTA data consistent with a stochastic background of gravitational waves. However, it does not yet show the quadrupolar spatial correlations necessary to claim a detection. In this talk, I will provide an overview of our experiment, describe the scientific benefits and challenges of international collaboration, highlight the most recent gravitational-wave analysis results, and discuss the very promising future of the IPTA experiment.