At the center of almost every large galaxy lurks something supermassive and dark. Most often, that something is thought to be a black hole — a region where gravity is so strong that even light cannot escape. But what if it's not a black hole, but a dense ball of ultralight particles — a boson star? Such objects have not yet been discovered, but their properties can be calculated, and we can figure out how to tell them apart from black holes by the behavior of surrounding gas.

Gas falling onto a massive central object swirls into a disk that looks like a thick cosmic donut. The authors of the new study calculated the shape of such donuts for boson stars in different theories of gravity. It turns out that if a boson star transitions into a special "scalarized" state, when the field inside it intensifies, the space around it changes so dramatically that the disk can exist all the way down to the very center. In standard general relativity, a black hole has an innermost stable orbit: closer than that, gas inevitably falls inward. Here, no such boundary exists.

Such a split disk is not just a mathematical curiosity. If astronomers ever see a very compact, almost spherical gas cloud or a double ring structure at the center of a galaxy, it could be the first indirect evidence of a boson star instead of a black hole. For now, the calculations provide clear signposts for future observations.