Self-interacting dark matter solves the final parsec problem of supermassive black hole mergers

Understand

Evidence for a stochastic gravitational wave (GW) background, plausibly originating from the merger of supermassive black holes (SMBHs), is accumulating with observations from pulsar timing arrays.

• An outstanding question is how inspiraling SMBHs get past the "final parsec" of separation, where they have a tendency to stall before GW emission alone can make the binary coalesce.
• We argue that dynamical friction from the dark matter (DM) spike surrounding the black holes is sufficient to resolve this puzzle, if the DM has a self-interaction cross section of order $\,$cm$^2$/g.
• The same effect leads to a softening of the GW spectrum at low frequencies as suggested by the current data.