Astro2020 Project White Paper The NANOGrav Program for Gravitational Waves and Fundamental Physics The North American Nanohertz Observatory for Gravitational Waves July 10, 2019 Thematic areas: Multi-messenger astronomy and astrophysics; Cosmology and fundamental physics; Formation and evolution of compact objects. Contact author: Scott Ransom (NANOGrav Chair), NRAO, scott.ransom@nanograv.org Authors: A. Brazier (Cornell), S. Chatterjee (Cornell), T. Cohen (NMT), J. M. Cordes (Cornell), M. E. DeCesar (Lafayette), P. B. Demorest (NRAO), J. S. Hazboun (UW Bothell), M. T. Lam (WVU, RIT), R. S. Lynch (GBO), M. A. McLaughlin (WVU), S. M. Ransom (NRAO), X. Siemens (OSU, UWM), S. R. Taylor (Caltech/JPL, Vanderbilt), and S. J. Vigeland (UWM) for the NANOGrav Collaboration (∼ 50 institutions, 100+ individuals) Related NANOGrav Science White Papers · Gravitational Waves, Extreme Astrophysics, and Fundamental Physics with Precision Pulsar Timing, J. Cordes et al. · Supermassive Black-hole Demographics & Environments with Pulsar Timing Arrays, S. Taylor et al. · Fundamental Physics with Radio Millisecond Pulsars, E. Fonseca et al. · Physics Beyond the Standard Model with Pulsar Timing Arrays, X. Siemens et al. · Multi-messenger Astrophysics with Pulsar Timing Arrays, L. Kelley et al. · The Virtues of Time and Cadence for Pulsarsand Fast Transients, R. Lynch et al. · Twelve Decades: Probing the ISM from kiloparsec to sub-AU scales, D. Stinebring et al. About the cover image: Characteristic GW strain across the full spectrum. Binary black hole sources are shown in each frequency band, probed by current (PTA and ground-based) and planned (LISA) detectors. S. Taylor, C. Mingarelli, adapted from gwplotter.org (Moore, Cole, Berry 2014); the allowed range of the stochastic background is scaled to be compatible with the NANOGrav 2018 upper limit.