  Pulsar timing arrays (PTAs) can be used to detect and study gravitational
waves in the nanohertz band (i.e., wavelengths of order light-years). This
requires high-precision, decades-long data sets from sensitive, instrumentally
stable telescopes. NANOGrav and its collaborators in the International Pulsar
Timing Array consortium are on the verge of the first detection of the
stochastic background produced by supermassive binary black holes, which form
via the mergers of massive galaxies. By providing Northern hemisphere sky
coverage with exquisite sensitivity and higher frequency coverage compared to
the SKA, a Next-Generation Very Large Array (ngVLA) will be a fundamental
component in the next phase of nanohertz GW astrophysics, enabling detailed
characterization of the stochastic background and the detection of individual
sources contributing to the background, as well as detections of (or stringent
constraints on) cosmic strings and other exotica. Here we summarize the
scientific goals of PTAs and the technical requirements for the ngVLA to play a
significant role in the characterization of the nanohertz gravitational wave
universe.
