Title: Searching for small primordial black holes in planets, asteroids and here on Earth
Authors: De-Chang Dai and Dejan Stojkovic
First Author’s Institution: Department of Physics, National Dong Hwa University, Hualien, Taiwan, Republic of China
Status: published in Physics of the Dark Universe [closed access]
Dark matter is the mysterious substance that accounts for the majority of mass in the universe. We don’t know exactly what dark matter is, so despite its ubiquity, we’re still searching for it. One dark matter candidate is primordial black holes (PBHs). When we think of black holes, we typically envision them being formed from the gravitational collapse of a massive star that has exhausted its fuel. PBHs, on the other hand, formed from over-dense regions in the early universe, long before stars began forming. Researchers have yet to observe PBHs, but have been coming up with a variety of innovative ways to search for them (see previous Astrobites here and here for examples). The authors of today’s paper present an intriguingly simple approach to this search effort.
Figure 1: (A) A PBH (black) in a planet/asteroid with solid shell (dark gray) and liquid core (light gray). (B) A PBH (black) within a hollow planet/asteroid after absorbing the liquid core. Adapted from Figure 2 of today’s paper.
Building on recent work that has shown that PBHs can reside in the interiors of stars, the authors suggest that PBHs can also exist within planets or asteroids. If the planet/asteroid has a liquid core with a solid outer shell, then the captured PBH can absorb the liquid and create a hollow object if the outer shell is strong enough to support itself, as pictured in Figure 1. Through calculations, the authors show that a hollow object can have a maximum radius of about one-tenth the size of earth’s radius. We already know of the existence of several planets and asteroids of this size that have or have had liquid cores, and studying them and finding more like them could shed light on PBHs.
Figure 2: A PBH (black) interacting with a completely solid object (dark gray) will produce a straight tunnel through the object. The PBH’s trajectory is represented by the dashed arrow. Adapted from Figure 3 of today’s paper.
But what about planets/asteroids that are completely solid? In this case, an interaction with a PBH will instead create a straight tunnel passing through the object, as shown in Figure 2. This phenomenon not only holds for solid planets and asteroids, but also extends to solid objects existing here on earth, which means a slab of any solid material could act as a PBH detector. If a straight tunnel were to suddenly appear in the material, it could be evidence of an interaction with a PBH. A black hole with a mass of 10-10M☉ should leave a tunnel with a 0.1 μm radius, which would be easily detectable with an optical microscope. The authors note that the frequency of PBH crossing events is extremely small (10-16 events per year for a 1 m2 cross-section), and therefore not conducive to real-time observations. As an alternative, the authors suggest looking for tunnels in old rocks and structures that have existed for hundreds, thousands, or even billions of years. A billion-year-old rock with a cross-sectional area of 10 m2 should have accumulated 10-6 crossings. Although this is still a very small number of PBH crossings, searching for PBH tunnels in this way is a simple, low-cost operation that could have a significant impact on our understanding of PBHs.
Astrobite edited by Kylee Carden
Featured image credit: NASA
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