Detecting Primordial Black Hole Mergers Might be Within Our Grasp

Imagine a black hole with the mass of the asteroid Ceres. It would be no larger than a bacterium and practically undetectable. But if such black holes are common in the Universe, they would affect the motions of stars and galaxies, just as we observe. Perhaps they are the source of dark matter.

Such tiny black holes could not form from dying stars, but they might have formed within the hot, dense cosmos soon after the Big Bang. For this reason, they are known as primordial black holes. We have no evidence they exist, but since they would be such a great explanation for dark matter, astronomers keep looking.

The one thing we know at this point is that most primordial black holes are ruled out by the data. Large, almost stellar mass black holes would affect the clustering of galaxies in a way we don’t observe. Tiny black holes of mountain mass or smaller would have evaporated long ago, making them useless as a dark matter candidate. But asteroid mass black holes are still possible. They aren’t likely, but they haven’t been formally excluded by the data. So a new study looks at how asteroid mass primordial black holes might be detected through gravitational waves.

To account for dark matter, the smaller the primordial black hole, the more common they must be. For asteroid masses, the cosmos would need to contain a vast sea of them. Since they would cluster within galaxies, they would be common enough within galaxies for some of them to merge on a regular basis. As the study points out, each of these mergers would produce a gravitational chirp similar to the ones we have observed with stellar-mass black holes. They would just have a much higher frequency and be more common.

The frequency of these primordial chirps would be too high for current observatories such as LIGO to observe, but the authors point out that some current dark matter experiments might be able to observe them. One alternative model for dark matter involves a hypothetical particle known as the axion. Axions were originally proposed to solve some issues in high-energy particle physics, and while they have fallen out of popularity in particle physics, they’ve gained some popularity in cosmology. We have made a few attempts to detect axions, but to no success. In their paper, the authors show how axion experiments could be tweaked slightly to observe the chirps of primordial black hole mergers in ideal conditions.

The chances of success are pretty slim. It would be odd for primordial black holes to exist in the only allowed mass range and nowhere else, and the conditions we could observe would be pretty narrow. But it might be worth doing a search on the off chance. The nature of dark matter remains a huge mystery in astronomy, so we don’t have much to lose in trying the occasional long-shot idea.

Reference: Profumo, Stefano, et al. “The Maximal Gravitational Wave Signal from Asteroid-Mass Primordial Black Hole Mergers.” arXiv preprint arXiv:2410.15400 (2024).