Astronomical Observations of Active Galactic Nuclei Help Researchers to Constrain Theory of Dark Matter Particles

Analysis of data from astronomical observations of active galactic nuclei has enabled scientists from Russia, Finland, and the United States to put a constraint on the theory of dark matter particles.

The outcomes of the new study offer further motivation for research teams across the globe making efforts to unravel the mystery behind dark matter: no one knows for sure what it is made of. The study was reported in the Journal of Cosmology and Astroparticle Physics.

In modern particle physics, it is vital to find an answer to the question of what particles make up dark matter. In spite of the huge expectations that dark matter particles would be discovered at the Large Hadron Collider, that did not materialize. Several previous mainstream hypotheses related to the nature of dark matter had to be dismissed. A number of observations suggest that dark matter does exist; however, it seems that something other than the particles in the Standard Model make up the dark matter.

Hence, it is essential for physicists to take further options into account that are highly complex. It is necessary for the Standard Model to be extended. Of the candidates for inclusion are hypothetical particles with proposed masses in the range of 10⁻²⁶ to 10⁺¹⁴ times the mass of the electron. In other words, the heaviest proposed particle has a mass 40 orders of magnitude higher compared to that of the lightest one.

One theoretical model considers dark matter to be constituted of ultralight particles. This provides an explanation for several astronomical observations. Yet, particles such as these would be very light that they would interact with light and other matter very weakly, rendering them extremely hard to analyze. Since it is nearly impossible to find a particle of this type in a lab, the researchers opted to find one through astronomical observations.

We are talking about dark matter particles that are 28 orders of magnitude lighter than the electron. This notion is critically important for the model that we decided to test. The gravitational interaction is what betrays the presence of dark matter. If we explain all the observed dark matter mass in terms of ultralight particles, that would mean there is a tremendous number of them. But with particles as light as these, the question arises: How do we protect them from acquiring effective mass due to quantum corrections? Calculations show that one possible answer would be that these particles interact weakly with photons—that is, with electromagnetic radiation. This offers a much easier way to study them: by observing electromagnetic radiation in space.

Sergey Troitsky, Study Co-Author and Chief Researcher, Institute for Nuclear Research, Russian Academy of Sciences.

In the case of a very high number of particles, rather than treating them as individual particles, they can be considered as a field of specific density permeating the universe. This field consistently oscillates across domains with a size of the order of 100 parsecs, or roughly 325 light-years. The mass of the particles governs the oscillation period. If the model proposed by the authors is precise, this period should be nearly one year.

When such a field is penetrated by polarized radiation, the radiation polarization plane oscillates with the same period. In case periodic variations like these occur indeed, astronomical observations can show them. Moreover, the length of the period, that is, one terrestrial year, is extremely convenient since several astronomical objects are observed over several years, which is sufficient for the variations in polarization to manifest themselves.

The study authors intended to use the data from Earth-based radio telescopes since they return to the same astronomical objects a number of times over a cycle of observations. Telescopes such as these have the ability to observe remote active galactic nuclei—regions of superheated plasma nearer to the centers of galaxies. Highly polarized radiation is emitted by these regions, and by observing them, it will be easy to track the variation in polarization angle over multiple years.

At first it seemed that the signals of individual astronomical objects were exhibiting sinusoidal oscillations. But the problem was that the sine period has to be determined by the dark matter particle mass, which means it must be the same for every object. There were 30 objects in our sample. And it may be that some of them oscillated due to their own internal physics, but anyway, the periods were never the same. This means that the interaction of our ultralight particles with radiation may well be constrained. We are not saying such particles do not exist, but we have demonstrated that they don’t interact with photons, putting a constraint on the available models describing the composition of dark matter.

Sergey Troitsky, Study Co-Author and Chief Researcher, Institute for Nuclear Research, Russian Academy of Sciences.

Just imagine how exciting that was! You spend years studying quasars, when one day theoretical physicists turn up, and the results of our high-precision and high angular resolution polarization measurements are suddenly useful for understanding the nature of dark matter.

Yuri Kovalev, Study Co-Author and Laboratory Director, Moscow Institute of Physics and Technology and Lebedev Physical Institute, Russian Academy of Sciences.

In the future, the researchers aim to look for manifestations of proposed heavier dark matter particles put forward by other theoretical models. This will necessitate working in various spectral ranges and using other observation methods. Troitsky says that the constraints on alternative models are more severe.

“Right now, the whole world is engaged in the search for dark matter particles. This is one of the great mysteries of particle physics. As of today, no model is accepted as favored, better-developed, or more plausible with regard to the available experimental data. We have to test them all. Inconveniently, dark matter is ‘dark’ in the sense that it hardly interacts with anything, particularly with light. Apparently, in some scenarios it could have a slight effect on light waves passing through. But other scenarios predict no interactions at all between our world and dark matter, other than those mediated by gravity. This would make its particles very hard to find,” concluded Troitsky.

The Russian Science Foundation supported this study under the grant No. 18-12-00258.