Dark matter continues to resist our best efforts to contain it. While dark matter remains a predominant theory of cosmology, and there is much evidence to support a universe filled with cold dark matter, any search for particles of dark matter yields nothing. A new study continues this tradition and rules out a number of candidates for dark matter.
What we know about interactions with dark matter. Photo credit: Perimeter Institute
When dark matter particles exist, we know that they cannot interact strongly with light. They have to interact gravitationally and can also interact via the strong and weak nuclear forces. We also know that it cannot be a matter of highly massive particles. If they were, they would break down into lighter particles over time, and we see little evidence of this. This leaves three broad candidates: tiny black holes, sterile neutrinos, or some kind of light boson. This latest work focuses on the third option.
A table of supersymmetrical particles. Photo credit: Claire David / CERN
Known elementary particles of matter can be divided into one of two categories: fermions and bosons. So electrons, quarks and neutrinos are fermions, while photons and gluons are bosons. Within the Standard Model of particle physics, there are no bosons that are suitable for dark matter. However, some alternative models predict particles that could be dark matter. Supersymmetry models, for example, predict that every known fermion must have a corresponding boson and vice versa. Thus, the electron would have a counterpart boson known as a selectron, the photon would have a counterpart fermion known as a photino, and so on. Another possibility is axions, which were proposed in 1977 to study subtle aspects of quark interaction.
Both axions and supersymmetry particles could be low-mass bosons and would satisfy the needs of dark matter. But if both exist, they have not yet been found. Still, these light bosons would gravitationally interact with regular matter, hence this latest study.
Bosons can slow down a black hole like children jumping on a carousel. Photo credit: Jose-Luis Olivares, MIT
If dark matter was made up of bright bosons, these particles would spread throughout the universe, including near black holes. A black hole would gravitationally trap nearby bosons and thus increase its mass. When a black hole rotates, trapping dark matter particles would also tend to slow its rotation. You can imagine children in a playground that has a carousel. If children jump onto the carousel while it is spinning, the carousel will slow down slightly due to the extra mass. The same goes for black holes.
In other words, dark matter bosons would limit the speed of rotation of the black holes. The team realized that heavier bosons would constrain black holes more and lighter bosons would constrain them less. So they looked at the LIGO and Virgo data from black hole fusions, which shows the rate of rotation of black holes before they merge. It turns out that some of these black holes were spinning so fast that the existence of ultra-light bosons made of dark matter is ruled out. Based on this study, dark matter cannot be axions or bright supersymmetry particles.
So a search for dark matter did not show us what dark matter is, but what it is not. It is extremely frustrating and potentially exciting because we are quickly running out of options on dark matter.
Reference: Ng, Ken KY et al. “Limitations for ultra-light scalar bosons in black hole spin measurements with the LIGO-Virgo GWTC-2.” Physical Review Letters 126.15 (2021): 151102.
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