Two monster black holes roar, carve out enormous cavities around them

I recently wrote about the possibility that two huge black holes collided and merged in the center of the Andromeda Galaxy, sowing chaos. As you might expect, though, they need not collide to be trouble. They're supermassive black holes. Trouble is everywhere they are.

What might surprise you, though, is just how they can create discord. And this one I love: They're monsters. They roar.