Abstract

Black-hole binaries are ideal targets to test strong-field gravitational interactions. Double black-hole systems are astrophysically relevant on both stellar-mass scale (as the final endpoint of massive binary-stars’ life) and galactic scale (where they form as a consequence of galaxy mergers). When the inspiral of spinning black-hole binaries is considered, Post-Newtonian theory predicts the existence of two one-parameter families of equilibrium solutions (“resonances”) in which the angular momentum and both spins share a common plane and precess at the same frequency. The two families are differentiated by either aligned or anti-aligned spin components in the orbital plane but both resonances have the capacity to attract generic non-resonant configurations. Spin-orbit resonances are relevant to shape astrophysical black-hole binary populations. Stellar-mass black-hole binaries are promising gravitational-wave sources for ground-based detectors: we show how the gravitational-wave signals emitted from the two resonant families consist in powerful, viable, probes of astrophysical processes in the binary formation and evolution. On the other hand, supermassive black-hole binaries may be ejected from their galaxies after merging events: the kick distribution (and therefore the black-hole occupation fraction) is deeply modified when spin-orbit resonances are properly considered.