Can two black holes merge? But could the reverse be possible: could a single black hole split in two?
Astronomers have already detected collisions between black holes thanks to the gravitational waves they produce. After their encounter, the two objects become one. A team of physicists wondered whether the laws of physics allowed the opposite scenario.
Numerical simulation of two black holes in the process of merging. The red sheets represent gravitational waves propagating after the black holes approach each other.
Credit: NASA/Ames Research Center/Christopher E. Henze, public domain.
Wikimedia image
Before answering, it is important to remember that a black hole is not a solid ball that could be cut with a tool. It is a region of space where gravity is so strong that even light can no longer escape once it has crossed its boundary. This boundary is called the event horizon.
In Einstein's usual description of gravity, a non-rotating black hole cannot split. Such a separation would decrease a physical quantity related to the total area of its horizons. Yet the laws used to describe these black holes prohibit this decrease.
The researchers then examined rotating black holes. This time, some calculations seem to make separation less impossible. But one obstacle remains: the two pieces would have to be able to move away from each other instead of immediately being attracted back together.
When this condition is taken into account, almost all possibilities disappear. Only a small possibility remains for black holes rotating extremely fast, almost at the limit allowed by the equations. Even in this case, the researchers did not demonstrate that a separation could actually occur.
The team also studied more unusual hypotheses, in which space would have additional dimensions or gravity would follow rules different from Einstein's. Some of these models allow fragmentation scenarios. But they rely on many assumptions.
No astronomer has ever seen a black hole split, and no mechanism capable of causing the event has currently been established in our Universe. This study is instead intended to help understand where known laws prohibit separation and where they might allow an exception. The researchers are particularly interested in what this would imply for very small black holes formed in the early Universe.
Fun fact: the title of the study refers to a pair of trousers. If one drew the evolution of a black hole splitting over time, its boundary would form a kind of tube dividing into two, like the legs of a pair of trousers.