Gravitational radiation from supermassive black holes

If, as argued by some, the cores of most galaxies contain supermassive black holes, galaxy-galaxy collisions could lead to their coalescence and a consequent emission of gravitational radiation. For holes with masses $\ge {10}^6{M}_0$, the amplitude of this radiation should be sufficiently large (metric perturbation ∼${10}^{-18\mathrm{}}$-${10}^{-17\mathrm{}}$) as to be detectable at the present horizon distance ${\mathcal{R}}_H\sim {10}^{10}$ light years using current technology. It is shown here that there is solid evidence indicating that galaxy collisions could have been sufficiently frequent at early times (redshifts $z\sim 2-3$) to lead to a rate of potentially observable events as short as one every ∼1-100 yr.