  We investigate the dynamics of stellar-mass black holes (BH) in star clusters
focusing on the dynamical formation of BH-BH binaries, which are very important
sources of gravitational waves (GW). We examine the properties of these BH-BH
binaries through direct N-body computations of Plummer clusters, having
initially N(0) <= 10^5 low mass stars and a population of stellar mass BHs,
using the state-of-the-art N-body integrator "NBODY6". We find that the stellar
mass BHs segregate rapidly into the cluster core to form a central dense
sub-cluster of BHs in which BH-BH binaries form via 3-body encounters. While
most of the BH binaries finally escape from the cluster by recoils due to
super-elastic encounters with the single BHs, we find that for clusters with
N(0) >= 5 X 10^4, typically a few of them dynamically harden to the extent that
they can merge via GW emission within the cluster. Also, for each of such
clusters, there are a few escaped BH binaries that merge within a Hubble time,
most of the mergers happening within a few Gyr of cluster evolution. These
results imply that the intermediate-aged massive clusters constitute the most
important class of star clusters that can produce dynamical BH-BH mergers at
the present epoch. The BH-BH merger rates obtained from our computations imply
a significant detection rate (~ 30/yr) for the "Advanced LIGO" GW detector that
will become operative in the near future. Finally, we briefly discuss our
ongoing development on this work incorporating the formation of BHs in star
clusters from stellar evolution. In particular, we highlight the effect of
stellar metallicity on the BH sub-cluster driven expansion of a star cluster's
core.
