At the center of the Milky Way Galaxy, there is a supermassive black hole. This is normal — at the center of almost every galaxy there is a black hole that is millions or billions times heavier than our sun. As galaxies move through the universe, they occasionally collide (the Milky Way is currently colliding with the Sagittarius Galaxy) and over a long time [1. But do not fear, the spaces between stars are so high that as galaxies collide, the inhabitants of a planet would never notice. When galaxies collide, their cores get closer and closer together, but the stars and planets on the outskirts of a galaxy, like our solar system, will never collide with anything. All that will happen is that our night sky will start to look a little different over time.], eventually the cores of these galaxies crash into each other, resulting in two supermassive black holes orbiting one another. The two black holes will get closer together, orbiting faster and faster as they fall into one another. This phenomenon should be common, based upon how frequently galaxies collide, however, we have never detected any close orbiting supermassive black holes. Due to this lack of evidence, there has been much debate in the scientific community regarding if it were even possible to for black hole orbits to be smaller than 1 parsec (about 3.25 light years) [2. The sun is about 8 light minutes from Earth, meaning it takes the light from the sun about 8 minutes to get to us. So if the sun disappeared right now, no one on Earth would notice for 8 minutes, therefore a light year is the distance that light would travel in a year. For perspective, Pluto is about 8 light hours away.] which is about the resolution of our modern telescopes.
That is, until now. The research done by scientists at Columbia University has confirmed the existence of a two supermassive black holes in orbit separated by 0.007 – 0.017 parsecs (8 – 20 light days)! It is difficult to detect this phenomenon by observation, because this occurs at the centers of colliding galaxies. The distances between galaxies is quite far, so our telescopes cannot see the separation between two supermassive black holes that are orbiting each other from one very massive supermassive black hole.
So how did they do it? Using quasars! Quasars are the dense region of hot gasses and materials surrounding a supermassive black hole. As the large amount of gravity surrounding a black hole attracts things to it, the material and gasses it attracts get shredded and fall into the black hole. These materials end up piling up around the black hole, and begin to form a hot, shredded disk, known as a quasar. These disks radiate enormous amounts of energy and light and rotate around the black hole, creating a rotating disk of brightly emitting material.
Now, if each of the supermassive black holes that are rotating one another have their own quasar, the quasars will be orbiting each other as well. The quasars are so bright that we can detect them with our telescopes. It is still difficult to tell the two quasars apart in the sky. However, what can be seen are the changes in light emitting from what appears to be a single quasar.
The scientists took advantage of the flickering light from the quasars to try and tell them apart. From their observations, they decided it was either one quasar rotating obscenely fast, or, as they showed, is in fact two supermassive black holes, that are rotating in close proximity!
This is the first discovery of supermassive black holes orbiting each other this fast. It’s so fast that Einstein’s theories of relativity apply! It is an important discovery because it settles a conversation about the formation and collision of galaxies. Furthermore, this discovery introduces some good methods for identifying other quasars fluctuations resulting from orbiting supermassive black holes. It’s also reassures our hypothesis of the frequency of galactic collisions with the discovery of these close orbiting supermassive black holes!