Researchers from the Institute of Gravitational Physics of Max Planck in Germany and Memorial University of Newfoundland in Canada solved what is happening with visible horizons during the merger of two non-helping black holes. According to the article published in the magazine of Physical Review Letters, the initial horizons disappear, merging with other horizontal structures. Briefly about scientific work is described in a press release on PHYS.ORG.
Although the visible horizon can coincide with the horizon of the black hole event, these terms do not always describe the same thing, especially in the case of the fluctuating black hole. In this case, the visible horizon is necessarily located inside the horizon of events and is a border within which all light-like curves lead inside the black hole. Outside the visible horizon, the light can still move towards him, but it is not destined to leave the limits of a wider horizon of events, which in this case is called an absolute horizon. The absolute horizon may occur even in the absence of a black hole, for example, the cosmological horizon of events in the expanding universe.
When merging, the initial event horizons form a large horizon of events, which is described by the diagram, on the form of resembling pants. However, the picture of the fusion of visible horizons is more complicated. When black holes are approaching enough close to each other, a new visible horizon is instantly formed around them. However, the final fate of the initial horizons remained unknown.
The method to which scientists resorted for analysis consists of three key components. First, the researchers used high-precision techniques for modeling space-time, including space-time inside the black hole. Secondly, they applied a numerical method that allowed them to identify individual horizontal structures even in cases where they were very distorted. Thirdly, scientists simplified complex equations describing surfaces to equations describing curves. This allowed us to easily find new horizons that corresponded to the curves, and restore them to full surfaces. All this helped researchers to simulate the inner horizontal structure inside the black hole just formed.
The results of complex numerical simulation show that both initial horizons exist for some time inside the general visible horizon. However, they become unstable and ultimately gradually destroyed when interacting with numerous horizontal structures called MOTS (eng. Marginally Outer Trapad Surfaces). In general, scientists have learned to distinguish unstable MOTS from stable MOTS, which are the actual borders of the black hole.