On the Rarity of Double Black Hole Binaries: Consequences for Gravitational Wave Detection

Double black hole binaries are among the most important sources ofgravitational radiation for ground-based detectors such as LIGO or VIRGO. Evenif formed with lower efficiency than double neutron star binaries, they coulddominate the predicted detection rates, since black holes are more massive thanneutron stars and therefore could be detected at greater distances. Here wediscuss an evolutionary process that can very significantly limit the formationof close double black hole binaries: the vast majority of their potentialprogenitors undergo a common envelope (CE) phase while the donor, one of themassive binary components, is evolving through the Hertzsprung gap. Our latesttheoretical understanding of the CE process suggests that this will probablylead to a merger, inhibiting double black hole formation. Barring uncertaintiesin the physics of CE evolution, we use population synthesis calculations, andfind that the corresponding reduction in the merger rate of double black holesformed in galactic fields is so great (by ~500) that their contribution toinspiral detection rates for ground-based detectors could become relativelysmall (1 in 10) compared to double neutron star binaries. A similar processalso reduces the merger rates for double neutron stars, by factor of ~5,eliminating most of the previously predicted ultracompact NS-NS systems. Ourpredicted detection rates for Advanced LIGO are now much lower for double blackholes (~2/yr), but are still quite high for double neutron stars (~20/yr). Ifdouble black holes were found to be dominant in the detected inspiral signals,this could indicate that they mainly originate from dense star clusters (notincluded here) or that our theoretical understanding of the CE phase requiressignificant revision.Comment: 13 pages, significant additions/changes (ApJ