At the heart of many galaxies, including our own, lies a supermassive black hole millions of times the mass of our sun. Scientists have yet to observe the merger of two such black holes, but using simulations, they are trying to learn what such collisions might look like. Simulations like the one shown here require combining relativity, electromagnetism, and, yes, fluid dynamics to capture what happens during the in-spiral.
Supermassive black holes like these are surrounded by gas disks that flow around them. Magnetic and gravitational forces heat the gas, causing it to emit UV light and, at times, high energy X-rays, both of which may be observable.
Gravitational wave detectors, similar to LIGO, may also measure evidence of supermassive black hole mergers, but physicists expect that will require a next-generation observatory, like the space-based LISA to be launched in the 2030s. (Image and video credit: NASA Goddard; research credit: S. d’Ascoli et al.; submitted by @lh7)