In the kitchen cupboard sits a jar of sugar. If you shake it, the grains will spread almost evenly — no need to place each one with tweezers. In much the same way, tiny black holes could have appeared in the first moments after the Big Bang. Their mass is like that of an asteroid, their size smaller than an atom. And they might make up all of dark matter: that invisible mass that keeps galaxies from flying apart.
For a long time, it was thought that such a scenario requires an incredible coincidence: conditions must be tuned with jewel-like precision. The author of this work tested that suspicion with a cold calculation. He took three different measures of "fine-tuning"—like three rulers with different gradations, and applied them to the same set of models. Among them were models that produce primordial black holes and popular dark matter particles, such as WIMPs (heavy particles being sought in underground detectors).
The result resembles sorting fruit into crates. The models fell into three classes of naturalness, and the class was determined not by the nature of the candidate but by the internal structure of the formulas. One class—"natural"—included black holes born on folds of space (domain walls) and particles that were never in thermal equilibrium. Another class—"medium"—grouped together black holes from the early matter-dominated era and particles with a "helper" for annihilation (the process where a particle and antiparticle turn into radiation). Their tuning turned out to be the same, reducing to a simple ratio of two epochs in the universe's history. Only one mechanism, ultra-slow inflation, fell into the "highly tuned" class, because in it a small addition to the field potential is amplified twice by exponentials.
The main conclusion is simple: primordial black holes are not doomed to fine-tuning. They occupy the same shelf of naturalness as recognized particle candidates. Nature doesn't have to choose the most complicated path — sometimes the answer lies on the surface, like sugar in a jar.
A primordial black hole with the mass of an asteroid ($10^{20}$ g) has a radius of about 0.01 femtometer — a hundred billion times smaller than a proton. If it flew through your body, you wouldn't feel a thing.
In Dan Simmons' novel "Hyperion," there are the Time Tombs surrounded by anti-entropic fields — somewhat like primordial black holes, distorting the very notion of causality.