The merger of a black hole with a cosmological horizon

Abstract In recent years there have been many studies on exactly solvable black hole mergers, based on
a model by Emparan and Martínez where the mass of one black hole is blown up to infinity.
Here we replace the large black hole by a cosmological horizon, and study how it merges with
a black hole in the Schwarzschild-de Sitter spacetime by considering an observer positioned at
future null infinity. We describe the geometry of the horizon over time, including the role that
caustics play in the merger process, and also examine the growth of the horizon area. We argue
that in the limit of zero cosmological constant, the system reduces to the Emparan-Martínez
Schwarzschild merger. This allows us to regularise the increase in the area during the merger,
which otherwise diverges.