We say the Standard Model is incomplete because we know the universe’s mass-energy allotment consists of mostly dark energy, then a lot of dark matter that gravitationally throws its mass around while refusing to be seen, and finally a tiny amount of regular material like we’re made of. The Standard Model of Particle Physics does a good job explaining all that visible, detectible stuff we call baryonic matter. But it offers no solutions for dark matter. 

The Standard Model is also based on observations and realizing that if we see X and Y, there is probably also a Z, but not having solid physics to explain why X and Y and Z were ever needed in the first place. As we’ve said before, this is like the point when Kepler could explain planetary positions with his three laws of orbital motion, but he didn’t have gravity to explain why the equations worked. The theory was good; it was just missing the underlying physics. Particle physics is currently at that stage. The Standard Model is good, but we know it is missing things.

Dark matter – 27% of the makeup of the universe – it’s a missing thing.

And because it only interacts with gravity (or when it randomly collides with something), scientists are looking at every collision and every gravity field for signs of dark matter.

In addition to thinking dark matter could be sterile neutrinos, particle physicists think a hypothetical particle called an ultralight boson is also a possibility. In general, bosons are particles that carry force or other properties. The Higgs boson carries mass. The photon carries the electromagnetic force. Gluons and W and Z particles carry the strong and weak forces. It’s unclear what ultralight bosons might carry, but if they are out there, they should be interacting. One possible kind of ultralight boson is the axion, and if it is out there, it should be interacting with spinning black holes.

As the theory goes, a supermassive black hole should pull in clouds of ultralight bosons because let’s face it, they’ll pull in anything that gets too close. Since the bosons don’t have the same motion as the spinning black hole, when their slow-motion selves get pulled in, the black hole’s spin should slow. This is like what happens when someone hops onto a spinning playground carousel: its rotation spins as it gains their not-moving-fast-enough mass. 

In looking at supermassive black holes, scientists have realized that fast-rotating systems put limits on what mass the theoretical ultralight bosons might have. If we find fast-spinning, old black holes, that means there probably aren’t a large mass of ultralight bosons out there slowing things down as they hop on or off the spinning black hole like kids on a carousel.

And looking at the black holes GW190412 and GW190517, scientists have determined these two old supermassive black holes are rotating pretty much as fast as a supermassive black hole can rotate, which is twice as fast as would be expected if ultralight bosons exist. This means that ultralight bosons, if they are out there, aren’t the ultralight masses we imagined. They could exist just at an even smaller mass than imagined, but…

The more we look, the more we aren’t finding things, and it is starting to get kind of annoying. But this is why scientists do science; we want to figure out these problems with the universe, and some problems are just going to take us a while to sort.

More Information

MIT press release

“Constraints on Ultralight Scalar Bosons within Black Hole Spin Measurements from the LIGO-Virgo GWTC-2,” Ken K. Y. Ng, Salvatore Vitale, Otto A. Hannuksela, and Tjonnie G. F. Li, 2021 April 14, Physical Review Letters