The imminent detection of gravitational waves from massive black-hole
  binaries with pulsar timing arrays
  Recent observations of massive galaxies indicate that they double in mass and
quintuple in size between redshift z = 1 and the present, despite undergoing
very little star formation, suggesting that galaxy mergers drive the evolution.
Since these galaxies will contain supermassive black holes, this suggests a
larger black hole merger rate, and therefore a larger gravitational-wave
signal, than previously expected. We calculate the merger-driven evolution of
the mass function, and find that merger rates are 10 to 30 times higher and
gravitational waves are 3 to 5 times stronger than previously estimated, so
that the gravitational-wave signal may already be detectable with existing data
from pulsar timing arrays. We also provide an explanation for the disagreement
with past estimates that were based on dark matter halo simulations.
