The challenge of massive black hole binary coalescence; from galaxy formation to gravitational waves

I will review recent progress in understanding how a pair of massive black holes can evolve from galactic scale separations, kpc and above, to milliparsec separations, where gravitational wave emission takes over
and leads to signals that should be detected by the Laser Interferometer
Space Antenna (LISA). I will show how, contrary to common belief, the
orbital decay of massive black holes in gaseous environments can be
less efficient and more stochastic than in stellar backgrounds, where
the only two processes at play are dynamical friction and 3-body
encounters with passings stars. Novel supercomputer simulations that attempt to study orbital decay in realistic galactic hosts show that coalescence timescales
can become very short, about 10 Myr, in high redshit massive galaxies that are on their way to become quenched spheroids, but can be longer than a
Gyr in massive star forming gas-rich disks as those observed at z > 1.
Hosts of LISA black holes are expected to have a different nature. The complex interplay between
the physical properties of galactic nuclei and the orbital decay process
poses a huge challenge to computational model, but at the same time
holds the promise for LISA and other gravitational wave detection
experiments to become a powerful probe of galaxy formation and evolution.