  Since many or most galaxies have central massive black holes (BHs), mergers
of galaxies can form massive binary black holes (BBHs). In this paper, we study
the evolution of massive BBHs in realistic galaxy models, using a
generalization of techniques used to study tidal disruption rates around
massive BHs. The evolution of BBHs depends on BH mass ratio and host galaxy
type. BBHs with very low mass ratios (say, $\la$ 0.001) are hardly ever formed
by mergers of galaxies because the dynamical friction timescale is too long for
the smaller BH to sink into the galactic center within a Hubble time. BBHs with
moderate mass ratios are most likely to form and survive in spherical or nearly
spherical galaxies and in high-luminosity or high-dispersion galaxies; they are
most likely to have merged in low-dispersion galaxies (line-of-sight velocity
dispersion $\la$ 90 km/s) or in highly flattened or triaxial galaxies. The
semimajor axes and orbital periods of surviving BBHs are generally in the range
10^{-3}-10 pc and 10-10^5 yr; and they are larger in high-dispersion galaxies
than in low-dispersion galaxies, larger in nearly spherical galaxies than in
highly flattened or triaxial galaxies, and larger for BBHs with equal masses
than for BBHs with unequal masses. The orbital velocities of surviving BBHs are
generally in the range 10^2-10^4 km/s. The methods of detecting surviving BBHs
are also discussed. If no evidence of BBHs is found in AGNs, this may be either
because gas plays a major role in BBH orbital decay or because nuclear activity
switches on soon after a galaxy merger, and ends before the smaller BH has had
time to spiral to the center of the galaxy.
