Astronomers have observed three types of black holes in the universe: stellar-mass black holes formed when a massive star collapses, intermediate-mass black holes found in some star clusters, and supermassive black holes lurking at the center of galaxies. But there is a fourth type that remains hypothetical and unobserved. These are thought to have formed from tiny fluctuations in the hot and dense early cosmos, known as primordial black holes. Since they did not form from stars or mergers, they could have much less mass. And with a low mass, primordial black holes would be tiny. Their event horizon would be smaller than an apple, perhaps as small as a grain of sand. So you can understand why they are hard to find.
If they exist, these dust grain singularities would be a perfect candidate for dark matter. This is not a new idea. Observations of dark matter have ruled out stellar-mass and even planetary-mass black holes, but they have not entirely ruled out primordial black holes. So they are a possible explanation for dark matter, but how would we prove it? A new study on arXiv tries to find out.
Observational constraints for primordial black holes over different mass ranges. Image credit: M. Cirelli (2016)
The authors begin by noting that if dark matter is indeed made up of primordial black holes, they must cluster around normal matter like dark matter. The Milky Way must be surrounded by a halo of tiny black holes, and primordial black holes must be scattered throughout our solar system. The gravitational pull of these tiny black holes should therefore affect the motion of planets, asteroids, and comets in a detectable way. Previous searches have yielded no result, but the authors wanted to know if the effect would be significant enough to observe with our current technology.
So they ran several computer simulations to calculate the size of the effect. Since the gravitational pull of a single black hole would be tiny, the team looked at how close encounters would shift the orbits of bodies in the solar system. We describe orbital motion through ephemeris tables, so they used simulations to determine how the ephemeris would change over time. They found that even if we conducted ephemeris observations over a decade, the effect of primordial black holes would be an order of magnitude smaller than the observational limits. In other words, even if primordial black holes exist, their effect is far too small to be observed in our solar system.
While the result is somewhat disappointing, it contradicts some studies that argue that current observations rule out primordial black holes as dark matter. Although they represent an unlikely solution to this cosmic mystery, they are still in play.
Reference: Thoss, Valentin, and Andreas Burkert. “Primordial Black Holes in the Solar System.” arXiv preprint arXiv:2409.04518 (2024).
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