“We have to think outside of the box because what has been done to find primordial black holes previously hasn’t worked,” said Dejan Stojkovic, a physicist at the University at Buffalo, in a recent media release. For decades, the search for dark matter has led physicists to the farthest reaches of the cosmos, yet the answer may lie much closer perhaps even beneath our feet or within the asteroids that orbit the Sun.
Primordial black holes (PBHs), theoretical remnants from the universe’s early seconds, have been long-time favorites to explain dark matter. Unlike their stellar-mass relatives, PBHs may range in mass from 1017 to 1024 grams about the mass of a mountain condensed into a space less than an atom. These objects, according to recent research, might have developed in the very first quintillionth of a second after the Big Bang, when clumps of the early quark-gluon plasma collapsed because of their own gravity.
The most recent theoretical innovation suggests two clever paths for finding PBHs: through the search for the evacuated shells they could create in planets with liquid interiors, and by searching through ancient rocks and asteroids for tiny tubes a form of cosmic forensic analysis.
The theory is based on general relativity and the physics of material strength. If a PBH becomes gravitationally trapped in a planet or asteroid with a liquid core and solid mantle, it might slowly devour the denser core material. If the outer shell is sufficient, the PBH could leave behind a stable, hollow shell a natural, low-density cavity that can be found using current astronomical methods. Estimates are that such a shell would survive in planets up to ten times the size of Earth, if the shell is made of dense material such as granite or iron.
This interesting size range contains asteroids like Lutetia and Vesta, each 100 and 525 kilometers in diameter. These bodies can also be examined for internal composition and density with a blend of rotational lightcurve analysis, thermophysical modeling, and flyby missions. The asteroid Itokawa, say, has shown large-scale internal density heterogeneities through well-resolved measurements of its YORP-induced spin acceleration, proving that density heterogeneities are accessible to observations with existing technology. The DART mission to Dimorphos went further to highlight the power of high-resolution imaging and impact testing for investigating asteroid interiors, demonstrating that bulk density and porosity may be deduced from orbital motion and ejecta analysis even in the absence of direct mass measurements.
PBHs can also leave more subtle signs. A high-speed PBH moving through solid rock would leave a perfectly cylindrical, submicron tunnel roughly 0.1 microns in radius for a 1022-gram PBH. The tunnel would not damage human tissue, but in brittle materials such as granite, it would be left as a microscopic cavity. These are just large enough to be detected using optical or electron microscopy, providing the potential for a low-cost, in-hand search for PBH tracks in ancient geologic samples or even the stones of old buildings as per the authors of the study.
The chances of encountering such a tunnel in a billion-year-old rock are infinitesimally small something like one in a million. And yet, as Stojkovic puts it, Does it cost much to do this? No, it doesn’t. The scientific reward, the first-ever direct proof of a primordial black hole, would be revolutionary.
The quest for PBHs thereby spans the cosmic and the material. It encourages astronomers on the one side to reinterpret asteroid density statistics, capitalizing on improvements in remote sensing, space vehicle tracking, and thermophysics modeling to identify anomalously empty objects. It enables laboratory researchers on the other to examine rocks for the characteristic marks of a cosmic intruder an experiment as simple as a microscope and a fragment of ancient rock.
As the study of dark matter advances, the possibility that the most elusive objects in the universe could leave their signature in the most mundane locations provides a challenge and an invitation to experiment, to observe, and to redefine the limits of astrophysical detection.
