# What happens “inside” a black-hole merger?

### AEI researchers find a new way to visualize a black-hole merger and shed new light on its interior structure

October 21, 2019

*pair-of-pants*picture for MOTSs from our numerical simulation. The tubes traced out by the individual MOTSs (colored red and purple) touch and penetrate each other. When the individual black holes get sufficiently close a common horizon is formed, which bifurcates into an inner branch (colored green) and an outer branch (colored blue). The outer branch settles down to the final equilibrium state, while the inner branch merges with the individual horizons precisely at the time when they touch.

## Paper abstract

We find strong numerical evidence for a new phenomenon in a binary black hole spacetime, namely the merger of marginally outer trapped surfaces (MOTSs). By simulating the head-on collision of two non-spinning unequal mass black holes, we observe that the MOTS associated with the final black hole merges with the two initially disjoint surfaces corresponding to the two initial black holes. This yields a connected sequence of MOTSs interpolating between the initial and final state all the way through the non-linear binary black hole merger process. In addition, we show the existence of a MOTS with self-intersections formed immediately after the merger. This scenario now allows us to track physical quantities (such as mass, angular momentum, higher multipoles, and fluxes) across the merger, which can be potentially compared with the gravitational wave signal in the wave-zone, and with observations by gravitational wave detectors. This also suggests a possibility of proving the Penrose inequality mathematically for generic astrophysical binary back hole configurations.