Let’s talk about normal-sized black holes, with mass a few times that of the sun. We don’t have any of these close by that we know about. So, their existence has to remain slightly tentative. They are fairly simple to imagine. Suppose you have a neutron star, which is simply enough matter to overcome the Coulomb repulsion of nuclei and push them together, making solid neutronic matter. Light can escape from a neutron star, if it is small enough. But as you add more and more neutrons to it, the gravitational attraction at the surface becomes greater and greater, until it holds in light. Then it is a just barely legitimate black hole.
Exactly just how many neutrons this takes is known pretty well, as we know the mass and size of neutrons, and we can assume they are only slightly compressible. A few solar masses is about the right number. You can add more neutrons, but it doesn’t become more invisible. It’s already mostly invisible. If there is anything near it, and falling into it, this stuff might emit some radiation. But if the black hole has been around for a while, and has gobbled up most of the gas that was near, it would be just black, but not in the sense of a blackbody. Blackbodies have nice radiation spectra, but a black hole has none.
So how would you know if there was one nearby, say 40 light years away? You only get neutron stars, which is where we started, by a supernova collapse of a burnt out star, and the supernova would have gotten rid of all the planets and asteroids and anything else in orbit around the star. So there wouldn’t even be any planets to detect. Of course, a rogue planet might get caught by the black hole, but that’s a unique situation and not too likely. Black holes are remarkably small, of the size of Earth, so they are not going to be occluding stars and showing up like a thick globule of gas would. Since they are not near any other star, the fraction of light diminished by a transit would be too small to see.
If the black hole had a binary companion, the orbit of the companion would give away the presence of the black hole, but we have that supernova problem here, meaning the binary companion couldn’t be too close. So, the black hole is simply not detectable except in a few special circumstances. This means it is not clear why there would be any effect at all, such as on the decisions of an alien civilization to perform star travel. They would have no interest in going to one of them, unless there is some way to get energy out of it that we don’t see at all, but there is also no likelihood that it would disturb their space travel, unless they were so unlucky as to try to fly right by it. Getting close to it would probably reveal some tiny amount of mass still falling in and emitting radiation as it did, so a slight change in course would avert any close encounter. Thus, having one of them nearby an alien civilization wouldn’t have any effect at all.
Even if they existed in substantial numbers, they seem inconsequential co-inhabitants of the galaxy. There is the thing called ‘missing mass’ for many galaxies, meaning their rotation curves do not jibe with their observed mass, meaning some is missing from the tabulation. This is often thought of as something exotic, but a bit of it could be undiscovered black holes.
Black holes the same mass as ordinary stars wouldn’t migrate to any other place in the galaxy, but they would just stay in the same place as the ordinary stars inhabit. There could be some in the disk, near us, not doing anything. There could be some in the bulge, just acting like a star but with a tiny radius.
What about other black holes? Can there be small ones? The equations of general relativity allow this to happen, and it has caused some stir, but general relativity is currently uncoupled with QCD, and so the existence of small black holes is a mystery. Perhaps it’s not so much of a mystery if you assume something about the compressibility of neutronic matter or matter in general. The point is, what would you make one of these small black holes out of? Known particles are too similar to neutrons in the ratio of gravitational force to density, and they can’t help make a small black hole. Somehow matter has to be compressed to get the Schwarzschild radius smaller than the radius of the mass itself. Only gravity can do that, and it is so weak you need star mass to do that. So there is not much of a prospect for small black holes.
How about large ones? If you assume that neutrons are simply not compressible and there are no phase changes with increasing pressure, then you can make the black hole as large as you want. If neutrons are compressible somewhat, it just makes it easier. If, on the other hand, there is a phase change for neutronic matter at some threshold pressure, into something else, and that something else was not a carrier of the gravitational force, adding neutrons to an existing black hole would be self-defeating. This does make the wild assumption that gravitation has some weird properties, but nothing is known about it, so, let’s assume it.
In this hypothesis, at the core, where the phase change happens, you would be reducing gravitational attraction, even as you increase it on the outermost shell of neutronic matter. If this goes on, then some other properties of neutronic matter come into play, as you have another example, albeit an unusual one, of a kind of Rayleigh-Taylor instability, and the internal core of what-have-you matter might burst out, perhaps in polar jets or something else. In other words, you might destroy a black hole by trying to make it bigger. If this upper limit on mass of black holes exists, then the question is what size is it? And then we have all the observations of large black holes at the centers of galaxies, both near and far. But since black holes are not visible per se, what these observations are is of matter being swallowed up by the black hole. If there was a cluster of the normal sized ones, in kinetic equilibrium such as a globular cluster is, would the effects be similar? So, probably there are large black holes, but not totally certainly. Either way, there doesn’t seem to be any effect on the willingness of an alien civilization to do star travel, as long as they are wise enough to avoid black holes.
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