Astrophysicists come up with a lot of whacky ideas, some of which actually turn out to be possibly true (like the Big Bang, black holes, accelerating cosmic expansion, dark matter). Of course, all of these conclusions are provisional, but some are now backed by compelling evidence. Evidence is the real key – often the challenge is figuring out a way to find evidence that can potentially support or refute some hypothesis about the cosmos. Sometimes it’s challenging to figure out even theoretically (let alone practically) how we might prove or disprove a hypothesis. Decades may go buy before we have the ability to run relevant experiments or make the kinds of observations necessary.

Black holes fell into that category. They were predicted by physics long before we could find evidence of their existence. There is a category of black hole, however, that we still have not confirmed through any observation – primordial black holes (PBH). As the name implies, these black holes may have been formed in the early universe, even before the first stars. In the early dense universe, fluctuations in the density of space could have lead to the formation of black holes. These black holes could theoretically be of any size, since they are not dependent on a massive star collapsing to form them. This process could lead to black holes smaller than the smaller stellar remnant black hole.

In fact, it is possible that there are enough small primordial black holes out there to account for the missing dark matter  – matter we can detect through its gravitational effects but that we cannot otherwise see (hence dark). PBHs are considered a black hole candidate, but the evidence for this so far is not encouraging. For example, we might be able to detect black holes through microlensing. If a black hole happens to pass in front of a more distant star (from the perspective of an observer on Earth), then gravitational lensing will cause that star to appear to brighten, until the black hole passes. However, microlensing surveys have not found the number of microlensing events that would be necessary for PBHs to explain dark matter. Dark matter makes up 85% of the matter in the universe, so there would have to be lots of PBHs to be the sole cause of dark matter. It’s still possible that longer observation times would detect larger black holes (brightening events can take years if the black holes are large). But so far there is a negative result.

Observations of galaxies have also not shown the effects of swarms of PBHs, which should have (those > 10 solar masses) congregated in the centers of small galaxies over the age of the universe. This would have disturbed stars near the centers of these galaxies, causing the galaxies to appear fluffier. Observations of dwarf galaxies so far have not seen this effect, however.

A recent paper suggest two ways in which we might observe small PBHs, or at least their effects. These ideas are pretty out there, and are extreme long shots, which I think reflects the desperation for new ideas on how we might confirm the existence of PBHs. One idea is that small PBHs might have been gravitationally captured by planets. If the planet had a molten core, it’s then possible that the PBH would consume the molten core, leaving behind a hollow solid shell. The researchers calculate that for planets with a radius smaller than one tenth that of Earth, they outer solid shell could remain intact and not collapse in on itself. This idea then requires that a later collision knocks the PBH out of the center of this hollowed out small planet.

If this sequence of events occurs, then we could theoretically observe small hollow exoplanets to confirm PBHs. We could know a planet is hollow if we can calculate its size and mass, which we can do for some exoplanets. An object can have a mass much too small for its apparent size, meaning that it could be hollow. Yes, such an object would be unlikely, but the universe is a big place and even very unlikely events happen all the time. Being unlikely, however, means that such objects would be hard to find. That doesn’t matter if we can survey large parts of the universe, but finding exoplanets requires lots of observations. So far we have identified over 5 thousand exoplanets, with thousands of candidates waiting for confirmation. Most of these are larger worlds, which are easier to detect. In any case, it may be a long time before we find a small hollow world, if they are out there.

The second proposed method is also highly speculative. The idea here is that there may be really small PBHs that formed in the early universe, which can theoretically have masses in the range of 10^17 to 10^24 grams. The authors calculate that a PBH with a mass of 10^22 grams, if it passed through a solid object at high speed, would leave behind a tunnel of radius 0.1 micrometers. This tunnel would make a long straight path, which is otherwise not something you would expect to see in a solid object.

Therefore, we can look at solid objects, especially really old solid objects, with light microscopy to see if any such tiny straight tunnels exist. If they do, that could be evidence of tiny PBHs. What is the probability of finding such microscopic tunnels? The authors calculate that the probability of a billion year old boulder containing such a tunnel is 0.000001. So on average you would have to examine a million such boulders to find a single PBH tunnel. This may seem like a daunting task – because it is. The authors argue that at least the procedure is not expensive (I guess they are not counting the people time needed).

Perhaps if there were some way to automate such a search, using robots or equipment designed for the purpose. I feel like if such an experiment were to occur, it would be in the future when technology makes it feasible. The only other possibility is to crowd source it in some way. We would need millions of volunteers.

The authors recognize that these are pretty mad ideas, but they also argue that at this point any idea for finding PBHs, or dark matter, is likely to be out there. Fair enough. But unless we can practically do the experiment, it is likely to remain just a thought experiment and not really get us closer to an answer.