Why are there so many black hole collisions? — A surprising solution to the mystery.
Every time astronomers discover a collision between two black holes, the data shows that they collide while orbiting each other in unusual orbits. Now, a team from Copenhagen has decided to investigate why this happens.
Scientists recording the waves emitted in such collisions since 2015 have noticed that in the case of the most massive pairs of black holes, both components of the system usually move in orbits that do not resemble circles. The question then arose as to why this happens.
The first such case was a collision of black holes that emitted gravitational waves cataloged as GW 190521. In this particular collision, scientists noticed that the black holes collided with each other were larger than expected, the collision also emitted visible light, and in addition, the black holes were not orbiting each other in circles. Even for scientists working in a brand new field of science — gravitational wave astronomy — these were three surprises in one.
A surprising solution to the mystery of black hole collisions
Scientists from Copenhagen have developed a model that shows that collisions of this type occur at the centers of galaxies. Why there? In every large galaxy, there is a supermassive black hole at the center with a mass of millions or even billions of solar masses. Around such a black hole, a very dense, flat disk of gas always forms. And it is in this environment that the density and velocities of the black holes are so high that instead of politely circling around their common centers of mass, the black holes smash through their surroundings occasionally colliding with each other like balls on a pool table. In such an environment, orderly systems of black holes orbiting in circles cannot exist.
What’s more, the dense gas that fills the disk also contributes to the formation of chaotic black hole systems. It intercepts black holes from their surroundings, so to speak, and over time gradually brings them closer to the central supermassive black hole itself. As a result, there are suddenly quite a few black holes at a sufficiently small distance from it. At such a location, there is a high probability that a system of two black holes orbiting the center of mass will encounter a third black hole, which will begin to gravitationally interact with it and introduce perturbations into the flight trajectory of both black holes.
In a normal, quieter environment, all three of these components could form a three-dimensional spatial system. However, the researchers note that in the thin disk around a supermassive black hole, everything tends to happen in two-dimensional space. Data analysis shows that the chance of a system of two black holes moving in elliptical orbits in such a situation increases a hundredfold. This in turn makes it likely that almost half of the black hole collisions in such an environment will occur on elliptical orbits. Contrary to appearances, therefore, these elliptical orbits are not as unique as they might seem.
What’s more, the scientists note that there are plenty of black holes in the gaseous disk surrounding the supermassive black hole. This, in turn, may explain the surprisingly large masses of the two black holes whose collision was recorded in 2019. After all, the two black holes could already be the effects of merging smaller black holes.
Scientists are not sure what is actually a more interesting object to study — colliding black holes or the structure of thin gas disks around supermassive black holes. Now it has become clear that we are dealing with collisions of multiple black holes in gas disks. This in turn means that in the coming years, scientists will keep a close eye on such objects and will certainly discover more collisions of black holes, especially when gravitational wave detectors come into operation to record the collisions of smaller objects. The universe is constantly rippling, you just have to listen to it properly.