[ywofedam] Supermassive black holes shouldn't rotate

Supermassive black holes grow by swallowing other matter.  However, due to frame-dragging, a rotating black hole will preferentially swallow matter approaching it at the wrong angle, against its rotation, so with the "wrong" angular momentum, so swallowing it would cause it to rotate slower.  Matter approaching it with the "right" angular momentum will be more likely to receive a kick in velocity due to frame dragging, and that kick may allow it to escape being swallowed.  Also, that kick will consume some of the black hole's rotational momentum also slowing the black hole's rotation. (Similar to the idea of dynamical relaxation causing evaporation.)

This ought to result in supermassive black holes with almost no rotation.  But quasars are thought to be powered by a supermassive black hole's rotational energy (Blandford-Znajek process).