The tally of merging black holes detected by the LIGO-Virgo gravitational-wave detectors continues to grow; the most recent data release brings the total to nearly 50 collisions! But how do these black-hole binaries form in the first place?
Two Formation Channels
Still from a simulation showing how black holes might dynamically form as they interact in the chaotic cores of globular clusters. [Carl Rodriguez/Northwestern Visualization]Before two black holes can collide in a burst of gravitational waves, they must first be bound together in an inspiraling binary pair.
There are two leading theories for how such pairs of black holes might arise in our universe. In isolated binary evolution, two massive stars of a stellar binary independently evolve into black holes. In dynamical encounters, single black holes pair up into binaries through gravitational interactions in the center of a dense, crowded star cluster.
Two Observational Clues
How can we determine which formation channel produced the black-hole binaries we’ve detected so far? Two observational signatures, in particular, could point to a dynamical merger:
Due to conservation of angular momentum, black holes in isolated binaries are expected to have aligned spins. Black holes that pair up via dynamical encounters, on the ...