With respect to accretion by large BH: the bigger the hole, the better the vacuum cleaner effect. Specifically, the really big BH’s don’t have a lot of matter in their neighborhood for verry long.
Since we’ve only been looking for a hundred years or so it’s not surprising that we don’t see radiation from the big giants. Of course galactic centers do tend to get busy.
I first remember the Universe = Black Hole from David Goodstein’s Mechanical Universe series on PBS. I loved it then. It’s still pretty tasty…
Except that there isn’t really a vacuum cleaner effect, at least not the way most people think of it. Orbitting a black hole at any distance is exactly like orbitting anything else with the same mass, at that distance. A stellar-mass black hole doesn’t suck any more than a star does. The only difference is that you can get closer to a black hole than you can to anything else with the same mass.
Once you get to within a few times the radius of the event horizon, you start to see significant gravitational radiation, which will steal energy from your orbit, and cause it to eventually decay into the hole. And since larger holes have larger event horizons, they’ll have a longer range for this. But on the other hand, it’ll also take longer for an orbit to decay into a larger hole.
You’re telling me that even though I said it incorrectly I’m right? The event horizon radius at the center of galaxies has to be substantial and your reply gives substance to the vacuum cleaner-effect. I originally posted that term with tongue firmly in cheek. It’s my own guess as to why quasars are bright and Sagitarrius doesn’t cast shadows.
(Now the good cop) I love your posts. I welcome anything you have to add to make a smarter man of me…
Not really. How long it takes a black hole to eat up all of its available fuel depends more on the matter around the hole than on the size of the hole itself. And if you put matter in an orbit around a black hole at a distance proportional to the hole’s size, the big hole will eat it slower, not quicker. And the one at the core of our Galaxy is still eating, as evidenced by the fact that it’s still the brightest thing in the sky in radio waves. The primary reason that it doesn’t cast shadows is that there’s a lot of gunk between here and the center of the Galaxy. Radio waves can cut through the gunk easily enough, but visible light not so much. So I suppose you could say that it is casting a shadow, but that the entire vicinity of the Solar System is in such a shadow.