The final fate of apparent horizons

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

October 25, 2021

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.

A “snapshot” from the simulation showing a variety of MOTSs. The three dark lines correspond to three stable horizons of the remnant and the two individual black holes. Two of the shown surfaces exhibit self-intersections. Lighter colors indicate a larger number of negative eigenvalues of the stability operator (see paper for details).
 

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