Why does dark matter not create black holes?

Black holes: indicators of dark bosons?

It is getting closer: The rotation of black holes can reveal whether there are dark bosons - hypothetical carrier particles of dark matter. Because their presence would have to measurably slow down the spin of the gravity giants. In an initial search of 45 pairs of black holes, however, researchers found no evidence of this braking effect - this further narrows the possible masses of these exotic bosons.

Scientists are still puzzling over what kind of particles dark matter is made of. Now that no trace has been found of the long-favored Weakly Interacting Massive Particles (WIMP), many searches focus on lighter particles, including sterile neutrinos, but also dark bosons such as axions. These exotic intermediary particles are said to be billions of times lighter than an electron.

The first indications of dark bosons could be deviations in particle collisions in accelerators, signals in dark matter detectors that are difficult to explain and abnormalities in the case of quantum leaps in certain atoms.

Braking effect through ultra-light particles

Now researchers around Kwan Yeung Ng from the LIGO Lab at the Massachusetts Institute of Technology have found and tested another method to check the existence of dark bosons. Your starting point is a prediction derived from quantum theory. According to this, large quantities of the ultralight particles would have to arise in the vicinity of a black hole. Their influence then gradually slows down the rotation of the black hole through the process of so-called super radiances.

Ng compares this to a carousel: “If you jump up and down on a carousel, you can withdraw energy from it - and the bosons do the same with the black hole,” explains the physicist. “If these dark bosons exist, we would therefore expect older black holes to rotate more slowly than they should because of their mass. Because the bosons would have to extract energy from it. "

45 pairs of black holes as test objects

Ng and his team have now investigated whether this is the case using 45 pairs of black holes, the merging of which has been detected by the gravitational wave detectors LIGO and Virgo. The mass of the black holes involved, as well as their spin, can be read from the pattern of the space-time oscillations generated in the process. For each of the black holes, the team checked whether the speed of rotation matched that typical of its mass.

"If there were the dark bosons, they would have swallowed a large part of the angular momentum - they are really like vampires," says Ng's colleague Salvatore Vitale. The physicists estimate that the super-radiancy through these exotic particles can reduce the spin of a black hole by up to half.

No indication of slowed spin

But when the team evaluated their data, there was no evidence of such a braking effect - on the contrary: In two pairs, GW190412 and GW190517, the rotation speeds of the black holes were close to the physical maximum of objects of their mass. This suggests that, at least in the immediate vicinity of these two black holes, there are no significant amounts of dark bosons.

"The black holes observed by LIGO and Virgo speak strongly against the existence of scalar, ultra-light bosons", state Ng and his team. It cannot be ruled out that the accretion of matter has accelerated the black holes and thus conceals the braking effect of the bosons. But the timing of the objects we are now observing is not right: "It would have taken too long to accelerate the black hole to the level we see here," says Ng.

The mass range for dark bosons is getting smaller

According to the researchers, their results suggest that dark bosons of at least a certain mass range are very unlikely. This exclusion range is between 1.3 x 10-13 and 2.7 x 10 -13 Electron volts, as the physicists determined based on the masses of their black holes. This narrows the area in which one should look for the dark bosons further.

However, this does not mean that the dark bosons do not even exist: "There are different types of bosons and we have now checked one of them," emphasizes Vitale. “But there could well be others. We therefore want to expand these analyzes to the data sets that LIGO and Virgo will collect in the next few years. "

Bosons that are significantly lighter, for example, can be detected by the spin of heavier black holes. For example, rotational measurements of the supermassive black holes in the heart of galaxies could produce a boson mass range of up to 10-21 be checked down. (Physical Review Letters, 2021; doi: 10.1103 / PhysRevLett.126.151102)

Source: Massachusetts Institute of Technology (MIT)

May 4, 2021

- Nadja Podbregar