Fast-rotating black holes can narrow the search for dark matter

(Credit: Robin Dienel / Carnegie Institution for Science)We are constantly learning new things about the universe, but the more we know, the clearer it becomes that we are missing the pieces of the puzzle. The possible existence of dark matter is one of the most disturbing missing pieces, but a team at MIT believes that rapidly rotating black holes could help narrow the search for these mysterious particles.

The matter we can see in the universe represents only 15% of what we do think it is there based on the pace of expansion. This missing mass is known as dark matter and there are many ideas about what it is and how we can find it. One of the most popular suspects is a theoretical particle called an ultra-light boson. If there were, ultra-light bosons would be so tiny that they would not interact with almost anything else in the universe – except, perhaps, certain black holes.

Quantum theory predicts that objects on very small scales, such as the ultra-light boson, do not work in the same way as larger ones, which are subject to classical physics. We don’t know how small the ultra-light boson is, but as the name suggests, it’s small. This means that it should have what is known as the Compton wavelength, which is inversely proportional to its mass. Therefore, an ultralight boson has an extremely long wavelength that could overlap with certain black holes. This would cause particles to accumulate around the black hole and slow down its rotational speed. If there is no slowing down, then it narrows the range of masses in which the ultralight boson could exist.

The MIT LIGO Lab team went hunting for black holes that would match the bill to test this hypothesis. LIGO, the gravitational wave observatory with laser interferometer, is able to listen to gravitational waves propagating from distant sources, such as black hole tracks. The team analyzed all 45 black hole tracks identified by LIGO and its accompanying project, Virgo. They made zero to two, known as GW190412 and GW190517.

It was found that both objects were spinning close to maximum speed, which would predict the established physics. This means that the ultra-light boson cannot exist between 1 × 10 ^ -13 and 2 × 10 ^ -11 electron volts. Otherwise, the ultra-light bosons would begin to gather around the black holes and siphon about half of their rotational energy. No slow black holes, no ultra-light bosons.

This does not mean that the ultra-light boson is a fantasy. It just means it doesn’t exist in this meal range. Past experiments have been able to exclude the particle from small pieces of space, but this is a huge piece that researchers could discover in search of dark matter. Of course, other teams will have to confirm the finding. This paper also shows that tools such as LIGO can be useful in the search for exotic particles.

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