  Supernova theory suggests that black holes of a stellar origin cannot attain
masses in the range of 50-135 solar masses in isolation. We argue here that
this mass gap is filled in by black holes that grow by gas accretion in dense
stellar clusters, such as protoglobular clusters. The accretion proceeds
rapidly, during the first 10 megayears of the cluster life, before the remnant
gas is depleted. We predict that binaries of black holes within the mass gap
can be observed by LIGO.
