Swarms of Black Holes

Black holes are almost the worst thing to try to observe, as they intrinsically give off no radiation at all. No particles and no photons. Nothing to see. The only way they are noticed is when they disturb something else, like a planet, a cloud, a star, another black hole, and either disrupt it, merge with it, or fling it somewhere where it can be noticed.

This being the case, exactly how would someone distinguish between a large black hole, if such things exist, and a cluster or swarm of smaller ones? One justification for this being a reasonable question, physics-wise, is that black holes are really, really small. They are the size of neutron stars, or small planets. If you have a bunch of them, how do they ever find each other in order to merge with one another to make a giant black hole? The collision cross-section is so small they would just fly around for longer than the age of the universe and the age of the next universe and lots longer than that, provided they had some kinetic energy of the order of the stars that formed them.

Maybe you might imagine that they grow by collecting gas. What happens to the gas nearby when a bit of it falls into the black hole? Out of the envelope of the black hole comes radiation, particulate and photonic, connected to the absorption process, just like a solar wind. So, the gas blows away, and what is left to fall into the black hole? There is also something called angular momentum that keeps the dust from falling inward, just as with a planetary disk and the galactic disk. It makes the gas spin around the black hole. So, building up a massive black hole with millions of solar masses is a difficult process.

How about swarms of black holes? Small black holes get made in the supernova process all the time. A star with a mass greater than about ten solar masses is going to collapse inwards after it uses up its hydrogen and then helium and then other elements in the fusion process, and suddenly there is no heat being generated inside to maintain the internal pressure. Collapse, and you have a neutron star, which finally gets big enough to generate a space-time envelope capturing all photons and matter inside it. A black hole. There should be lots and lots of these. Large stars, maybe about .05% of the total population, can undergo this transformation, and they only live twenty million years or so. If they are .05% now, and they live 1/500th of the age of the galaxy, there should have been 25% of the current stellar population passing through the black hole transition over the life of the galaxy. That is one lot of black holes.

Even if this number is crazy large, and there is only 10 billion black holes around, that is still a lot. The number of stars in the galaxy is 200 billion or so, and it is hard to figure out how instead of 25% of this there is only 5%, but even if the number of black hole survivors is 10 billion, and they are all around us, it is not hard to see why these heavier objects would not preferentially drift into the center of the bulge and make a swarm. They are so small that a swarm of a million would not produce any visible dual black hole collisions in the history of mankind. The swarm might be concentrated into a tenth of a light year, and perhaps could have excluded other objects, such as heavy stars created nearby and which drifted into the center, by the radiation pressure caused by various absorption events. A heavy star has a huge cross-section, compared to a black hole, and would have received much more momentum after an event than another black hole. Compare the size of Rigel with that of Earth, if you want a picture of this.

Why wouldn’t such a swarm of black holes be detectable by gravitational wave detectors? Huge masses flying around in the gravitational field of the swarm should make some disturbances. After all, the curvature of space-time is at its extreme around a black hole, meaning that there is a spreading disturbance outward from it. The difficulty with this is the wavelength. A black hole traveling across the swarm, a distance of a couple of tenths of a light year, for example, might take a hundred years, meaning the signal is of the order of a billionth of a hertz. Perhaps it interacts with other black holes on the way across and generates signals only a thousandth of a hertz. This is simply not within our detection capability unless we wait centuries. Gravitational wave detectors are good for quick events, like black hole collisions or mergers with stars, or something else fast. What makes things even worse is that the signals from the motion of individual black holes in a swarm are counteracted by the signals from motion of others, so the amplitude of the signal is very low as well. So, detecting a swarm of black holes in the center of the galaxy as opposed to a single massive black hole is probably not going to happen.

Possibly it will be within equipment capabilities to determine the difference between these two scenarios by some other means. It isn’t clear how gas being absorbed by a ten solar mass black hole would give off different radiation in detectable bands that gas being absorbed by a million solar mass black hole, but perhaps there would be some differences. It also isn’t clear how the residual gas surrounding the black hole would be different if it was being cleared by a million solar mass black hole as opposed to being cleared by a hundred thousand individual black holes of ten solar masses each. Perhaps there would be more turbulence if the clearing was being done by a swarm of rapidly moving black holes as opposed to one stationary one in the center. Perhaps the total distance cleared would be more, being the total of the smaller clearing distance plus the mean dispersion radius of the black hole swarm as opposed to the larger clearing distance of the supermassive black hole. Some more complicated computations are necessary.

The implications of this for alien civilization are not clear at all. Would an alien civilization arise in the bulge somewhat near the swarm, or would an alien civilization ever conceive of a reason to travel near to the swarm? Is there any mechanism by which they could extract energy from such a maelstrom? Not obvious at all.