  Many galaxies contain supermassive black holes (SMBHs), whose formation and
history raise many puzzles. Pulsar timing arrays have recently discovered a
low-frequency cosmological "hum" of gravitational waves that may be emitted by
SMBH binary systems, and the JWST and other telescopes have discovered an
unexpectedly large population of high-redshift SMBHs. We argue that these two
discoveries may be linked, and that they may enhance the prospects for
measuring gravitational waves emitted during the mergers of massive black
holes, thereby opening the way towards resolving many puzzles about SMBHs as
well as providing new opportunities to probe general relativity.
