There are a lot of times when I read theory papers and think, “That’s a lot of math and nothing I can test with a telescope.” Going into this next story, I was fully prepared to discard it as something completely untestable, but, it turns out, a team of theorists figured out how we may be able to detect tiny black holes left over from the universe’s formation. 

For roughly the first 400,000 years of our universe, everything was more or less a hot soup of stuff like atomic nuclei, free electrons, and light that was constantly interacting as our small universe expanded and cooled. Then, one day, it cooled enough that the electrons and atomic nuclei could come together as neutral matter and the light could fly away as the Cosmic Microwave Background (CMB). That neutral stuff? That stuff became stars and galaxies through gravitational collapse. 

This wasn’t a completely smooth distribution of material, and places, where there was a bit more material than the surroundings, formed galaxies. And according to some theories, the rare spots that contained 50% more material than their surroundings collapsed down into tiny black holes — objects the mass of the moon with event horizon the size of a grain of sand. 

Now, if Stephen Hawking was correct and black holes evaporate, then all these tiny black holes should have puffed out of existence billions and billions of years ago. If they didn’t however, they could still be out there, floating among the stars.

Detecting a grain of sand in the vastness of space seems like it should be an impossible feat, but a remarkable instrument on the 8.2-m Subaru Telescope in Hawaii may be able to do it. The Hyper Suprime-Cam has a massive field of view that is so high resolution that it can observe one hundred million stars in the Andromeda Galaxy simultaneously. And it can do it fast, taking new images every few minutes. 

Theorists at The Kavli Institute for the Physics and Mathematics of the Universe in Japan believe that if there are primordial black holes out there, they should periodically pass in front of these stars and change how we see their light. Mass can act as a lens, bending light, and like a magnifying glass focusing sunlight on a blade of glass, mass can make distant things appear brighter if you are at just the right place. 

If a primordial black hole passes exactly between us and one of those one hundred million stars, we will be in the right place, and we will see that star get brighter and fainter in a special way that is unique to these gravitational lenses. And they may have already seen one. 

In a new paper in Physical Review Letters, the team discusses an event that occurred during their first set of observations that is consistent with one of those moon-massed, grain-of-sand sized primordial black holes. If more of these detections are made and they are consistent with a population of primordial black holes, well these folks will likely get a Nobel Prize. We’ll have a new intriguing candidate for dark matter, and there are also new possibilities for multiple universes existing. 

But that is a story for another day and another show. Expect to see this covered on Astronomy Cast at some point in the not too distant future.

More Information

Kavli IPMU press release 

“Exploring Primordial Black Holes from the Multiverse with Optical Telescopes,” Alexander Kusenko et al., 2020 October 30, Physical Review Letters