  Double black hole binaries are among the most important sources of
gravitational radiation for ground-based detectors such as LIGO or VIRGO. Even
if formed with lower efficiency than double neutron star binaries, they could
dominate the predicted detection rates, since black holes are more massive than
neutron stars and therefore could be detected at greater distances. Here we
discuss an evolutionary process that can very significantly limit the formation
of close double black hole binaries: the vast majority of their potential
progenitors undergo a common envelope (CE) phase while the donor, one of the
massive binary components, is evolving through the Hertzsprung gap. Our latest
theoretical understanding of the CE process suggests that this will probably
lead to a merger, inhibiting double black hole formation. Barring uncertainties
in the physics of CE evolution, we use population synthesis calculations, and
find that the corresponding reduction in the merger rate of double black holes
formed in galactic fields is so great (by ~500) that their contribution to
inspiral detection rates for ground-based detectors could become relatively
small (1 in 10) compared to double neutron star binaries. A similar process
also reduces the merger rates for double neutron stars, by factor of ~5,
eliminating most of the previously predicted ultracompact NS-NS systems. Our
predicted detection rates for Advanced LIGO are now much lower for double black
holes (~2/yr), but are still quite high for double neutron stars (~20/yr). If
double black holes were found to be dominant in the detected inspiral signals,
this could indicate that they mainly originate from dense star clusters (not
included here) or that our theoretical understanding of the CE phase requires
significant revision.
