# The interior of a binary black hole merger

## TL;DR

This paper provides numerical evidence of a new phenomenon in binary black hole mergers involving the merging of MOTSs, enabling tracking of physical quantities through the merger and potential implications for gravitational wave observations.

## Contribution

It demonstrates the merger of MOTSs during black hole collisions and introduces a method to track physical quantities across the merger process.

## Key findings

- MOTSs merge during binary black hole collisions.
- A MOTS with self-intersections forms immediately after the merger.
- Physical quantities can be tracked through the merger process.

## 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.