Black holes were thought to have been ruled out as candidates for the dark matter present in the galaxy by the use of some interesting techniques for searching for them, such as micro-lensing. This means looking for the bending of light rays from distant sources as they pass such an object. There were none found in the multiple searches.
The searches were concentrated on the supposed location of the dark matter, which was in a halo around the Milky Way. Some measurements of the velocity of stars around the center of the galaxy were use to estimate the total mass of the galaxy and its distribution with distance; in other words, how much is in the disk and how much in the bulge and how much in the halo. Unfortunately, the measurements made indicated that the dark matter was in the halo, so that’s where the searches were conducted. Exactly why there are so few black holes in the halo should be an interesting topic for another time.
Recent results on the velocity of the stars around the center of galaxies in general, not just ours, showed that the velocity curves are proportional to the visible mass. This means that the invisible mass is proportional, in location and amount, to the visible mass. Measurements of distant galaxies can not be three-dimensional, but two-dimensional, so to be precise, the invisible mass is proportional, in distance from the center of the galaxy and the amount, to the visible mass.
The post on black holes being prevalent in the galactic disk of the Milky Way, and by inference, in other galaxies with disks, seems to have been prescient, as they serve to explain this result. Black holes, and their less-massive cousins, neutron stars, are mostly invisible to ordinary astronomical observations. A search for them using gravitational lensing, concentrating on the disk of our galaxy, would help in confirming their prevalence and their contribution to the missing mass. Where should they be searched for?
Black holes form from large stars which undergo supernova explosions, leaving a large part of their mass behind. At this time in our galaxy, large stars occupy the entire disk, meaning the entire vertical column as well. This is because they form from large clouds of gas, which have that vertical distribution. While the large stars are alive and illuminating, they should follow the same distribution. However, because they are more massive than smaller stars, interacting with these smaller stars will change the vertical distribution, much like heavier molecules and multimolecular particles drift downward in the mesosphere. This, of course, is fortunate for life on Earth, as it reduces the loss of water into space from the atmosphere, and helps keep enough here for us to fill our pools.
As a black hole ages, it interacts more and more with other objects in the disk, and since a large majority of them are smaller stars, it will exchange energy, both kinetic and vertical potential energy. This might be thought of as a kind of equipartition of energy, and has the result that the black holes drift downward over time toward the medial plane of the galaxy. Now, the medial plane of the galaxy is not necessarily a plane, but a curved surface, as interaction with dwarf galaxies in orbit around the Milky Way distort it. Yet, the basic idea holds, in that black holes should over time form a tighter distribution than their original large stars, or smaller stars. This is where they should be searched for.
The distribution of black holes is nowhere near as dense as, for example, in a swarm of them that might comprise the central mass object in the galaxy. Yet black holes in a swarm do not interact and do not collide, as their numerical densities are too low and their interaction cross-sections too small. We see no large gamma ray bursts from the center of the galaxy that would be the indicators of black hole collisions. Nor do we even see any binary black holes spiraling in to self-destruction, such as might be formed by a close encounter between black holes in such a swarm. So it is even less likely that there would be any such signature of a high population of black holes in the disk, even in the thinner disk occupied by older black holes. In other words, black holes are innocuous neighbors. In order to account for the missing mass in a galaxy, there might have to be more mass in black holes and neutron stars than in visible stars, but this still would not provide any obvious signatures. Younger galaxies should have less of a black hole count in their disks, and older ones more, and this is something that could be examined, if someone could figure out how to detect black holes in the disks of distant galaxies.
Galactic perils that might face an alien civilization include stellar encounters, where the approach of one star toward the solar system of another might disturb planetary orbits. The extreme case of this is where they strip the solar system of one or more planets, but the less extreme case can be just as disturbing to an alien civilization. If the stellar encounter results in the change of a planetary orbit, even one of the outermost, over time that change will have an effect on the orbit of the planet upon which they reside, perhaps moving it out of the liquid water zone, making it more eccentric, or causing some other perturbation of orbital or planetary parameters. This would require the alien civilization to respond, if they desired to preserve their civilization.
The rate at which solar systems are disrupted by stellar encounters in the Milky Way, especially in the further out spirals, is not large, and if it were increased by an order of magnitude, to account for large numbers of massive black holes and neutron stars, it would still not eliminate the possibility of alien civilizations. It does imply that they have another interesting observational challenge. It is very simple to measure the proper motion of all the stars in our vicinity, so we can see which ones will come close and when the point of closest approach would be. As noted elsewhere, this might be a peril for the alien civilization but it could also be an opportunity, in that interstellar travel would never be easier than when another solar system flies by the point of closest approach. But in the case of black holes and neutron stars, the location of them is not easy, nor would be the determination of their proper motion and thereby the point and time of closest approach. Alien civilizations would have to figure out astronomical equipment able to perform this search and measurement, if they were to properly plan for their long-term future. On Earth, we haven’t really put much attention into this, but perhaps, once the realization of the prevalence of black holes in the galactic disk sinks in, we will.
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