Scientists have long known that when two black holes collide, they merge to create a more massive black hole. These events are so violent that they set the very fabric of space ringing with ripples called gravitational waves.
Now, new research suggests that the opposite may happen, too: single black holes may be able to split in two. But this can only happen if that progenitor black hole is spinning at maximum possible speed. That speed prevents the black holes from immediately falling back on each other.
There are physical laws in place that would usually prevent black holes from splitting in two; the team behind this research investigated whether there are conditions in which these laws can be temporarily bypassed.
One of the key aspects of physics that prevents black holes from bifurcating comes from thermodynamics. Black holes possess a characteristic called "entropy," which is essentially a measure of disorder within a system.
Imagine a toddler's crayon box. When entropy is low, the crayons are neatly arranged by color. As entropy is increased, the crayons are scattered and no longer in order. To get the crayons back in order, it requires a parent to put energy into the system and neatly reorder the crayons. In other words, the entropy of a system can't be reduced without some intervention and energy being added from outside that system. Parenting would be a little easier if it could.
A black hole splitting in two would represent a decrease in entropy, strongly disfavored by the laws of thermodynamics, but only if the black hole is not rotating. The situation could be different for a rapidly rotating black hole.
This team of scientists found that a loophole exists for such a black hole.
"The two resulting black holes must also be able to separate rather than immediately fall back together," team member Francesco Sannino of the University of Naples Federico II in Italy said in a statement. "When we include this requirement, the window largely closes again, except possibly for black holes rotating close to their maximum possible rate."
Black hole trousers?
The team calculated just how such a black hole split could occur, finding that the division of the black hole's outer boundary, or "event horizon," proceeds in an oddly trouser-like shape, with one resulting black hole slipping down each "leg."
They termed this "pair-of-pants topologies."
The researchers also determined that a split is more likely to take place if one of the resultant black holes is considerably smaller than its newly born twin.
"We cannot yet give a single probability for a black hole to split. It depends on how gravity behaves beyond Einstein's theory, and we are working to quantify this," Sannino said. "We expect small primordial black holes to be more prone to splitting than much larger astrophysical black holes."
Sannino added that such black hole splits may be detectable in high-energy light called gamma-rays.
"We may detect a split indirectly: gamma-ray telescopes such as Fermi-LAT can search for signals from tiny black-hole fragments that might form during mergers," the researcher concluded.
The team's research was published on Oct. 22 in the journal Physical Review D.
