Black hole swarms are collections of black holes, as many as you want up to millions, occupying a small galactic space, like a light year or so. Because black holes are only the size of a planet like Earth, there is virtually no chance two of them will collide. So a swarm, if formed, will simply go on buzzing around for the rest of the age of the universe.
The evidence for massive black holes, thought to occupy the centers of many galaxies, is indirect and matches the evidence that a swarm of black holes would display. So there is no simple way to tell which it is that occupies the dead center of galaxies.
Here’s a little astronomical background. Globular clusters are collections of stars, held together by mutual gravitational attraction. They look like spherical balls of stars, and if you could watch one closely for millennia, you would see the individual stars moving like Brownian motion, going every which way, and having their straight-line orbits disturbed by near stars. The central area of the globular cluster is denser, as most stars are not simply orbiting it a circle around the center, but dive into it, coming out on the other side. The denseness of the center is caused by the relative numbers of stars that happen to be passing through it at any given time, as compared to the number per cubic lightyear which are in the further out regions.
The less kinetic energy a particular star has, the longer it will linger in the center. If there were a lot of stars with not much kinetic energy, the center of the globular cluster would be even denser, as those low KE stars would be spending a lot of time there and increasing the mass density near the center. Then the higher mass in the center would pull in even more stars, again adding to the local density.
Globular clusters exist in which many stars have somehow lost much of their KE and spend their time in the central region of the cluster. The astronomical name for this phenomenon is core collapse. Essentially the core of the cluster has collapsed in upon itself and has grown denser, because somehow kinetic energy was transferred from one subset of stars, the central ones, to the rest, which still go flying out to the edges of the cluster before turning around and coming back in. The process for this KE transfer is gradual and statistical. An ordinary star transfers kinetic energy continually by its gravitational interactions with other stars, and if one gets lucky, it can dump most of its KE on the way in, and then stay in the center. When the center becomes more dense, these interactions become more frequent. Can core collapse happen by this process alone? Once it happens, and say 20% of the stars are restricted to the center, it might stay that way, but the difficulty is in getting it to happen in the first place.
Since the 80’s, astrophysicists have been estimating how long this takes by looking at the number of close interactions a sample star might have, and how likely it was that this could result in a significant reduction in KE, thereby providing another candidate for the central stars in a core-collapse globular cluster. This approach in interesting, but it ignores the fact that stars interact with many other stars at the same time. If there was a clot of a thousand stars, the gravitational force on a sample star could be much greater, and this relaxation time would be shorter. Stars don’t clot like that, but they do have density fluctuations and perhaps even waves of density. Density waves have not been studied much, but they are the likely culprit for the beautiful spirals on galaxies we see. Thus relaxation times might be much shorter than the one-on-one calculation indicates, if there were turbulent agglomerations of stars, density fluctuations and density waves in a globular cluster.
A second feature is the segregation of stars by mass. In a potential field, heavier stars with the same average energy don’t move as far out of the field, simply because they have less velocity for the same energy. On the average, there should be more heavier ones in the center than at the fringes, in a large globular cluster. This means they might be more subject to becoming core-collapse participants than lighter stars. This does not mean that all O stars are found in the center of globular clusters, and M stars are found at the edge, but it does mean that there is a tendency for this to happen, and the relative ratio of O’s to M’s would be different as one goes further out from the center of the cluster.
If an M star interacts with several O stars during its passage through the core of the cluster, it may pick up even more speed than another O might, and then spend even more time out of the core region. And recall that O stars become black holes when they age, meaning that there would be black holes preferentially in the core of a core-collapsed globular cluster. They would not be visible, but would contribute to the severity of the core collapse. Since the lifetime of O and other stars which produce black holes are quite short from a universe time viewpoint, there could be quite a lot of them there.
What works for a globular cluster works even more for an elliptical galaxy or the bulge or bar in a spiral galaxy. There is a mechanism for large stars, destined to become black holes, or more likely, already made black holes, with masses ten to a hundred or more solar masses, to have a core collapse situation in the center of some galaxies, and thereby pretend to be a huge single black hole, confounding observations. The formation of a swarm of black holes is not that unlikely, and the concept is certainly worth considering. Galactic cores are more or less invisible because of the dust and gas there, but perhaps there is some clever way of better finding and discerning black holes that reside there.
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