While the era of gravitational wave (GW) astronomy is now well underway,we have only just begun to explore the full GW spectrum. In contrast to the ~100-Hz GWs emitted by stellar mass binary systems and detected by LIGO, more massive sources such supermassive black hole binary (SMBHB) systems throughout the Universe are expected to produce GWs at 10 orders of magnitude lower frequency. Detecting these very low-frequency GWs requires different observational techniques. In particular, high-precision timing of a set of millisecond pulsars is sensitive to GW with period comparable to the experiment duration — for a decade-long data set, GW frequencies in the few-nHz range are measured. GWs induce timing fluctuations that are correlated between pulsars with a unique angular signature, allowing them to be distinguished from other astrophysical or instrumental effects, a concept known as a pulsar timing array (PTA).
Our group, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), has been running such a PTA experiment for the past 15 years, using the Arecibo and Green Bank radio telescopes to monitor a set of pulsars and analyze their timing data for signs of GWs. Over this time there has been dramatic growth in the number of pulsars observed, a crucial factor for improved GW sensitivity. Additionally there have been significant advances in the capabilities of the instrumentation, data analysis methods, and the size and organization of the collaboration. All these factors have contributed to dramatically improved results that are now informing the SMBHB population, and exhibiting unexplained noise that may be the initial sign of a GW signal. In this talk I will present an overview of the project and the recent NANOGrav 12.5-Year Data Set results, and discuss prospects for GW detection in both the near future and longer term.