Scientists in Germany and Canada have uncovered the internal structure of merging black holes unraveled what happens to the visible horizons during the merger of two non-rotating black holes. According to an article published in the journal Physical Review Letters, the original horizons disappear, merging with other horizon-like structures. A summary of the scientific work is described in a press release on Phys.org.
While the visible horizon may coincide with the event horizon of a black hole, these terms do not always describe the same thing, especially in the case of an oscillating black hole. In this case, the visible horizon is necessarily located within the event horizon and is a boundary, within which all light-like curves lead into the black hole. Outside the visible horizon, light can still move away from it, but it is not destined to leave the wider event horizon, which in this case is called the absolute horizon. An absolute horizon can arise even in the absence of a black hole, for example, the cosmological event horizon in an expanding Universe.
When merged, the original event horizons form a large event horizon, which is described by a diagram that resembles a pants. However, the picture of the merging of the visible horizons is more complex. When black holes get close enough to each other, a new visible horizon instantly forms around them. However, the ultimate fate of the original horizons remained unknown.
The method that scientists have used to conduct the analysis has three key components. First, the researchers used high-precision techniques to model spacetime, including the spacetime inside a black hole. Second, they applied a numerical method that allowed them to identify individual horizon-like structures even in cases where they were highly distorted. Third, scientists have simplified complex equations describing surfaces to equations describing curves. This made it easy to find new horizons that fit the curves and restore them to full surfaces. All this helped the researchers to model the internal horizon-like structure inside the newly formed black hole.
The results of complex numerical simulations show that both original horizons exist for some time within the common visible horizon. However, they become unstable and eventually degrade gradually when interacting with numerous horizon-like structures called MOTS (marginally outer trapped surfaces). Overall, scientists have learned to distinguish unstable MOTS from stable MOTS, which are the actual boundaries of a black hole.