  We explore chemically homogeneous evolution (CHE) as a formation channel for
massive merging binary black holes (BBHs). We develop methods to include CHE in
a rapid binary population synthesis code, Compact Object Mergers: Population
Astrophysics and Statistics (COMPAS), which combines realistic models of binary
evolution with cosmological models of the star-formation history of the
Universe. For the first time, we simultaneously explore conventional isolated
binary star evolution under the same set of assumptions. This approach allows
us to constrain population properties and make simultaneous predictions about
the gravitational-wave detection rates of BBH mergers for the CHE and
conventional formation channels. The overall mass distribution of detectable
BBHs is consistent with existing gravitational-wave observations. We find that
the CHE channel may yield up to ~70% of all gravitational-wave detections of
BBH mergers coming from isolated binary evolution.
