The final parsec problem
The final parsec problem is a fascinating concept that represents a lack in our understanding of black hole merger events. When two galaxies collide, it is exceedingly unlikely that the supermassive black holes at their centres themselves collide directly. In reality they likely slingshot past each other before being brought together by various braking mechanisms.
The most important braking mechanism is dynamical friction, in which the black hole transfers kinetic energy to nearby matter. This mechanism is sufficient to bring the two black holes into a stable binary orbit, the radius of which decreases over time, until there is so little material in their orbital paths that a merger would not occur in the current age of the universe.
Gravitational waves provide another mechanism for energy loss, but their effect does not become impactful until the two black holes are orbiting between 0.01 and 0.001 parsecs from each other. However, we know that black hole mergers have occurred. Therein lies the problem: by what mechanism do merging black holes close this final gap?.
proposed solutions
The only mechanism that we know to work is highly infrequent and involves the presence of a third black hole. In this case there are three more mechanisms which allow for energy dissipation:- The orbits may become highly eccentric, allowing two black holes to approach the distance at which dissipation of energy via gravitational waves becomes effective.
- Two black holes may pass their energy to the third and eject it from the system
- The three black holes will pass through a larger volume of space which will therefore contain more matter for dynamical friction.