The Final Parsec Problem (or Last Parsec Problem)
is an astrophysical problem regarding the merger
of two Supermassive Black Holes, e.g., after a Galaxy Merger.
When the Black Holes are extremely close,
they merge because Gravitational Waves significantly bleed their orbital energy.
Beyond such extreme closeness, close encounters with stars can remove energy,
but the odds of encountering a star
decreases as the orbit decreases, to a point
where the black holes would remain in virtually the same orbit,
i.e., the Timescale for further decrease is far too long.
Assuming they sometimes do merge, it is not
clear how their orbits continue to become smaller
through the final Parsec.