Tiny Primordial Black Holes Could Explain Dark Matter

Black holes are some of the most vexing objects in the universe, being so gravitationally intense that not even light can escape them, making them difficult to study.

Now, a team of researchers has taken that vexation to the next level, suggesting that super-small black holes from the early universe could be responsible for dark matter, a huge percentage of the universe’s content that scientists cannot see.

Dark matter is the catch-all term for about 27% of the universe’s mass which is not visible to any instruments humans have yet devised. Instead, dark matter’s presence is inferred through its gravitational effects on other objects—in galaxy clusters, for example. There are many candidates for dark matter, including dark photons, axions, and Weakly Interacting Massive Particles (or WIMPs). But another longstanding candidate is the primordial black hole, or a very small black hole from the early universe, which zips through space and is difficult to see because nothing significant orbits it.The team’s research, published earlier this month in Physical Review D, states that the primordial black hole abundance “would be large enough for at least one object to cross through the inner solar system per decade.” Thus, the team concluded, these flyby events would be detectable as gravitational waves.

The team’s finding is timely; earlier this month, a different team declared that dark matter’s signatures could be hiding in gravitational wave data collected by the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

The idea of certain black holes being “primordial” refers to the idea that they were born in the earliest moments of the universe, as random fluctuations caused globs of matter to collapse on themselves, forming the relatively small and lightless entities. The black holes we can observe range from stellar-mass (about the size of our Sun and similar stars) to many billion times that size. So an asteroid-sized black hole is very small on a relative scale, and yet could be smaller—even the size of an atom.

Sarah Geller, a theoretical physicist at the University of California at Santa Cruz and co-author of the paper, told LiveScience that “we are not making any of the following claims — that primordial black holes definitely exist, that they make up most or all of the dark matter; or that they are definitely here in our solar system.” Rather, the team is saying if all the aforesaid is true, it would mean that one such object would travel through the inner solar system every one to 10 years.

With new gravitational wave detections being made regularly—and LISA, a next-generation gravitational wave observatory in space currently being assembled—we’re in an exciting time for primordial black holes.