Astro2020 science white paper: The gravitational wave view of massive
  black holes
  Coalescing, massive black-hole (MBH) binaries are the most powerful sources
of gravitational waves (GWs) in the Universe, which makes MBH science a prime
focus for ongoing and upcoming GW observatories. The Laser Interferometer Space
Antenna (LISA) -- a gigameter scale space-based GW observatory -- will grant us
access to an immense cosmological volume, revealing MBHs merging when the first
cosmic structures assembled in the Dark Ages. LISA will unveil the yet unknown
origin of the first quasars, and detect the teeming population of MBHs of $10^4
- 10^7$ solar masses. forming within protogalactic halos. The Pulsar Timing
Array, a galactic-scale GW survey, can access the largest MBHs the Universe,
detecting the cosmic GW foreground from inspiraling MBH binaries of about 10^9
solar masses. LISA can measure MBH spins and masses with precision far
exceeding that from electromagnetic (EM) probes, and together, both GW
observatories will provide the first full census of binary MBHs, and their
orbital dynamics, across cosmic time. Detecting the loud gravitational signal
of these MBH binaries will also trigger alerts for EM counterpart searches,
from decades (PTAs) to hours (LISA) prior to the final merger. By witnessing
both the GW and EM signals of MBH mergers, precious information will be
gathered about the rich and complex environment in the aftermath of a galaxy
collision. The unique GW characterization of MBHs will shed light on the deep
link between MBHs of $10^4-10^{10}$ solar masses and the grand design of galaxy
assembly, as well as on the complex dynamics that drive MBHs to coalescence.
