# The final fate of apparent horizons

How apparent horizons vanish in a binary black hole merger: by “weaving” back and forth in time

AEI researchers have resolved the long standing question of what happens to black hole apparent horizons in a binary black hole merger. Together with colleagues from the Memorial University of Newfoundland and Labrador, Canada, they showed that the two original black holes' horizons are annihilated by unstable horizon-like structures. Their results indicate that after the formation of a common (remnant) horizon, the world tubes of the initial apparent horizons can vanish by becoming unstable and then “weaving” back and forth in time.

## Paper abstract

We resolve the fate of the two original apparent horizons during the head-on merger of two non-spinning black holes. We show that following the appearance of the outer common horizon and subsequent interpenetration of the original horizons, they continue to exist for a finite period of time before they are individually annihilated by unstable MOTSs. The inner common horizon vanishes in a similar, though independent, way. This completes the understanding of the analogue of the event horizon's “pair of pants” diagram for the apparent horizon. Our result is facilitated by a new method for locating marginally outer trapped surfaces (MOTSs) based on a generalized shooting method. We also discuss the role played by the MOTS stability operator in discerning which among a multitude of MOTSs should be considered as black hole boundaries.