Black hole horizons in a binary black hole merger

Apparent horizons are routinely used in numerical relativity to describe black holes in simulations of dynamical systems. Advances in numerical methods allowed us to follow these objects into the interior of merging black holes, revealing how the two original horizons connect with the remnant horizon.
In this talk, I will present results on head-on mergers, showing that the evolution of apparent horizons is much more intricate than previously thought: In the interior of the newly formed common horizon, the original horizons are individually annihilated by unstable horizon-like structures. This completes our picture of how two black holes become one and provides the analog of the famous pair-of-pants diagram of the event horizon now for the apparent horizon.