For a phenomenon so widely discussed in research and pop culture, scientists still know very little about black holes. While astronomers are practically certain that black holes exist, it's not easy to spot something that emits no light. A great new episode of PBS Space Time breaks down the clever ways that astrophysicists can locate and "see" black holes.
Even though black holes don't emit any light, they do have material surrounding them that does. Like everything else in the universe, black holes exist with neighbors—albeit neighbors that they are consuming. When nearby gas gets sucked into a black hole, it still has to travel from its point of origin to its final resting place. It does so quickly and without delay, and if there is a lot of matter then it starts to build up like a traffic jam.
Like any traffic jam, even one on the road to annihilation, things start to heat up. With the kinetic energy of the matter smashing into each other, we get something visible, what we call an accretion disk. If enough material builds up, and enough energy and light is released from the volatile accretion disk, then it is known as a .
Accretion disks are the oldest known things to exists in the universe, give or take a random gamma-ray burst. The Hubble Telescope has spotted some that scientists believe to be at least 13 billion years old, almost as old as the universe itself. But as the matter surrounding a black hole gets devoured, the pressure weakens, and the light gradually fades. Again, like any traffic jam, things eventually clear up.