Once again, a gust of gravitational waves coming from the faraway collision of black holes has tickled the instruments of the Advanced Laser Interferometer Gravitational-Wave Observatory, bringing the count of definitive gravitational-wave detections up to three.
Before Advanced LIGO switched on in the fall of 2015 and almost immediately detected gravitational waves from a black-hole merger, no one knew whether it would see merging black holes, merging neutron stars, black holes merging with neutron stars or none of the above.
The three signals spotted by LIGO so far have all come from merging black holes, suggesting pairs of these ultradense, invisible objects abundantly populate the universe.
The LIGO team’s estimate for the abundance of black-hole mergers in the universe, based on its first two detections, favored one of two competing scenarios for how stars might end up colliding as black holes: The “Common-envelope” and “Chemically homogeneous” scenarios both involve pairs of massive stars that form near each other in the otherwise empty expanse of space, gravitationally collapse into black holes and collide.
The high rate of mergers disfavored a third scenario called “Dynamical formation,” which has the black holes forming far apart inside a dense stellar cluster.
Black holes that formed and evolved from a close-knit pair of stars – as in the common-envelope and chemically homogeneous scenarios – would spin in the same direction as their common axis, so misaligned spins would disfavor these scenarios and favor dynamical formation in a stellar cluster, which does not require any connection between the black holes’ spins.
“As LIGO’s sensitivity improves – we’re still a factor of three away from design sensitivity – and as we see more signals, we’ll get a much better understanding of the spins of the black hole population. contains hints of something interesting, but right now I’m looking forward to future detections before making a call.”