Perturbations by tiny black holes

While the sources and properties of dark matter in our universe remain frustratingly elusive, an old idea has been circulating again among cosmologists and physicists. What if very tiny black holes, formed just after the Big Bang, could account for this mysterious dark matter?

David I. Kaiser, Professor of Physics and Germeshausen Professor of the History of Science, and Peter Fisher, Thomas A. Frank Professor of Physics, suggest that primordial black holes (PBHs), each about the mass of an asteroid but the size of a hydrogen atom, could exist and make up all the dark matter in the universe. If this is the case, these PBHs might be detected in our inner Solar System, they explain. “PBHs in the asteroid-mass range would be massive enough to perturb the orbits of planets and spacecraft and hot enough to emit X-ray photons via Hawking radiation,” say Kaiser and Fisher.

The researchers note that state-of-the-art numerical models of the movement of Solar System objects, along with recent high-precision tracking data of objects, make it possible to turn the inner Solar System into an effective PBH detector. A PBH transiting the Solar System would perturb the motion of planets and other large objects whose motions are tracked to high accuracy, effectively leaving a gravitational footprint in its wake.

Fisher and Kaiser also propose building custom-designed beacon spacecraft that would serve as additional perturbation detectors and contain a compact particle detector to identify particles emitted via Hawking radiation from any passing PBH.

Changing tactics

In the past 50 years, more than $1 billion has been invested worldwide in the search for elementary particles that could play the role of dark matter. Yet despite remarkable improvements in technology and experimental design over this time, the quest has come up short.

“Because experimental dark matter searches traditionally fall under the domain of particle physics funding programs, our proposed experimental search for primordial black holes is unusual in its detection strategies, ambitious in its multi-messenger observables, and therefore unlikely to receive financial support from traditional particle physics funding sources,” say Fisher and Kaiser. “The time is right to move PBHs from theoretical curiosity to experimental research subject.”

Cosmic questions

With the help of the Bose funding, Kaiser and Fisher say their project can settle the question of whether asteroid-sized PBHs are a compelling dark matter candidate—and they could make the first-ever direct observation of a PBH candidate.

PBHs are “fascinating astrophysical objects in their own right,” they say, “which could provide insight into major open questions in cosmic history as well as semiclassical quantum gravity.”