According to Wikipedia: Objects and radiation can also escape from the ergosphere. In fact the Penrose process predicts that objects will sometimes fly out of the ergosphere, obtaining the energy for this by "stealing" some of the black hole's rotational energy. If a large total mass of objects escapes in this way, the black hole will spin more slowly and may even stop spinning eventually. By this logic, won't all rotating black holes eventually stop spinning? Or are there processes by which a black hole can gain angular momentum? If so, what are these?