Black holes may gain extra charge when swallowing matter

New Scientist : General relativity predicts that black holes can be described completely by their mass, charge, and spin; other than by those three characteristics, it’s impossible to distinguish one black hole from another. But some alternative theories of gravity allow for other characteristics that would be unique to each black hole. Theorists call those new individualizing characteristics “hair.” Now Vitor Cardoso of the Technical University of Lisbon in Portugal and his colleagues have created a model of black holes that demonstrates the form one such hairy black hole might take. They used a variation of relativity that incorporates scalar-tensor theories . The model showed that as a black hole absorbed more matter, it would build up extra charge beyond what was predicted by general relativity. Cardoso says that there are two ways to test for that effect. The first would be to look for an imprint of the extra charge on gravitational waves , which can be created by the collision of two black holes. The other would be to look for alterations in the orbits of objects around the black hole, which could be caused by the extra charge.