Most galaxies are believed to harbour a massive black hole at their center, and most galaxies are believed to merge with others during their life-time, perhaps many times. Such a merger will usually lead to a starburst at the common center of the merged galaxy. Also, each time this happens, two black holes come close to each other, and may merge.

The torque on the stars leads to angular momentum loss to let the two black holes merge. The black hole merger then leads to ubiquitous emission of gravitational waves, which transport angular momentum. The dependence of this angular momentum loss on the spin parameters of the two black holes and their orbit is strong, since the angular momentum transport by gravitational waves ist minimized as soon as the second black hole crosses the innermost stable orbit surface. It ensues that the orbital angular momentum of the second black hole can dominate over the instrinsic angular momentum of the primary black hole.

It follows that black hole mergers can increase the spin of a black hole, and also lead to a spin flip, as implied by observations of radio galaxies. So, the spin evolution ob black holes may be punctuated by mergers, that increase the spin, and activity phases, that decrease the spin.This has important consequences for the gravitational wave emission during the very last stage of a black hole merger.

This work was done with Mihaela Chirvasa (now MPI Grav. Physics), Gopal Krishna (Tata Inst., Pune), Paul Wiita (Univ. Atlanta), and Christian Zier (now RRI, Bangalore).