Supermassive Black-hole Demographics & Environments With Pulsar Timing
  Arrays
  Precision timing of large arrays (>50) of millisecond pulsars will detect the
nanohertz gravitational-wave emission from supermassive binary black holes
within the next ~3-7 years. We review the scientific opportunities of these
detections, the requirements for success, and the synergies with
electromagnetic instruments operating in the 2020s.
