Fast-Spinning Black Holes May Narrow Search for Dark Matter

The matter we can see in the universe only accounts for 15 percent of what we think is out there based on the rate of expansion. This missing mass is known as dark matter, and there are numerous ideas about what it is and how we might find it. One of the more popular suspects is a theoretical particle called an ultralight boson. If they exist, ultralight bosons would be so minuscule that they wouldn’t interact with almost anything else in the universe — except, maybe, certain black holes.

Quantum theory predicts that objects at very small scales like the ultralight boson don’t operate in the same way as larger ones, which obey classical physics. We don’t know how small the ultralight boson is, but as the name implies, it’s tiny. This means it should have what’s known as a Compton wavelength, which is inversely proportional to its mass. Therefore, an ultralight boson has an extremely long wavelength that might overlap with certain black holes. That would cause the particles to accumulate around the black hole and slow its rate of rotation. If there’s no slow-down, then that narrows the range of masses where the ultralight boson could exist.

The team from MIT’s LIGO Laboratory went hunting for black holes that would fit the bill to test this hypothesis. LIGO, the Laser Interferometer Gravitational-wave Observatory, is able to listen for gravitational waves propagating from distant sources like black hole binaries. The team looked at all 45 black hole binaries identified by LIGO and its companion project, Virgo. They zeroed in on two, known as GW190412 and GW190517.

Both of these objects were found to be spinning at close to their maximum speed, which is what established physics would predict. That means the ultralight boson cannot exist between 1×10^-13 and 2×10^-11 electronvolts. Otherwise, ultralight bosons would start collecting around the black holes and siphon off about half of their rotational energy. No sluggish black holes, no ultralight bosons.

This doesn’t mean the ultralight boson is a fantasy. It just means it doesn’t exist in this mass range. Past experiments have been able to rule out the particle in small slivers of space, but this is a huge chunk that researchers might be able to discount in their search for dark matter. Of course, other teams will have to confirm the finding. This work also shows that instruments like LIGO can be helpful in the search for exotic particles.

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