It sounds like the stuff of nightmares: feed a black hole, and it might just sprout hair. That’s the bizarre-sounding conclusion of calculations for the way black holes behave when they consume matter. Though the hair is metaphorical, the results could open a window into physics beyond the traditional picture of the universe.

With an immense pull and the ability to gobble up stuff, no one can deny that black holes are captivating. They are also hugely important as places where new physics, such as an extension to Einstein’s theory of general relativity, might be on display. Most recently, this potential was explored via a very strange question: do black holes have hair?

The hair in question isn’t physical, but a metaphor for individuality. To understand where it comes from, first think about the generic black holes predicted by Einstein’s relativity. These can be described completely by their mass, charge and spin. Defined by only those three values, black holes all look basically the same from the outside, even if different objects formed them, or they swallowed different stuff.

This lack of individuality has led them to be called “bald”. But many physicists hope that black holes are not bald, and instead have other properties that arise from parameters other than those three. These would be known as hair – and would offer clues as to why Einstein’s equations aren’t the whole story, something that needs to be solved more generally if relativity is ever to be reconciled with quantum mechanics.


Getting wiggy with it

Now Vitor Cardoso of the Technical University of Lisbon in Portugal and his colleagues have a new way to give black holes hair. They added some better-known but still unproven extensions to general relativity – called scalar-tensor theories – to black-hole models and found that in-falling matter can induce extra charge on a black hole.

As the charge does not arise from any predictions of general relativity, it counts as hair. “The black hole grows a wig,” says Cardoso.

“It is an interesting result,” says Avi Loeb of Harvard University, who was not involved in the new work. “I am most curious if we can apply it to real black holes.”

Cardoso says that an imprint of the extra charge should show up in gravitational waves, ripples in space-time that can be created when black holes collide and merge.

Black hole silhouette

“It would be one of the clearest smoking guns of deviation from general relativity,” says Cardoso. Although gravitational waves have yet to be directly detected, most physicists are confident they exist and should be found soon. For his part, Cardoso predicts upgrades to current detectors will reveal the waves – and perhaps black hole hair – within the next five years.

Another way to test the theory would be to look closely at the silhouettes of black holes as they feed using direct imaging techniques, says Loeb. The extra charge from a hairy black hole would affect the orbits of objects nearby, like the very massive stars that orbit the supermassive black hole in the centre of our galaxy. “A hairy black hole would have a different silhouette from the one predicted by general relativity,” says Loeb.

It is still a theoretical scenario, but if the extra charge can be detected, then an already weird object will be well on the way to solving one of the biggest problems in modern physics – thanks to its hair.

Journal reference: Physical Review Letters, doi.org/n3c