What Happens to Apparent Horizons in a Binary Black Hole Merger?

Area of Science:

• Black Hole Physics
• General Relativity
• Gravitational Waves

Background:

• Understanding black hole mergers is crucial for testing general relativity.
• The structure and evolution of apparent horizons during mergers remain complex.
• Marginally outer trapped surfaces (MOTSs) are key to defining black hole boundaries.

Purpose of the Study:

• To resolve the fate of the two original apparent horizons during a head-on merger of two nonspinning black holes.
• To elucidate the annihilation mechanism of these horizons.
• To complete the understanding of the apparent horizon analog of the event horizon's 'pair of pants' diagram.

Main Methods:

• Development of a new method for locating MOTSs using a generalized shooting method.
• Analysis of the stability operator of MOTSs to identify black hole boundaries.
• Numerical simulations of nonspinning black hole mergers.

Main Results:

• Original apparent horizons exist for a finite time after the formation of an outer common horizon.
• Individual annihilation of original apparent horizons occurs due to unstable MOTSs.
• The inner common horizon vanishes independently via a similar mechanism.
• The 'pair of pants' analogy for apparent horizons is now complete.

Conclusions:

• The study provides a complete picture of apparent horizon dynamics during black hole mergers.
• The new MOTS location method offers a powerful tool for future black hole research.
• The stability of MOTSs is critical for defining black hole boundaries in dynamic spacetimes.