Horizons of the Kastor-Traschen multi-black-hole cosmos

Recently a cosmological multi-black-hole solution has been found which in some cases describes coalescing black holes. We analyze the global structure of these solutions. For the case of two black holes of equal mass, we investigate numerically the location of the marginal surfaces, which generate trapping horizons as they evolve. In the contracting chart, we show that for black holes with a subcritical total mass a new black-hole trapping horizon appears which encloses the original black-hole trapping horizons. For a supercritical total mass, there is an eternal naked singularity, and two subcases: one in which a new black-hole trapping horizon appears but later vanishes, and the other where such a horizon does not appear. The first subcase suggests a relationship between the dynamics of the trapping horizon and the existence of the naked singularity.