How scientists search for dark matter: a theoretical perspective from Dmitry Gorbunov on the universe.

— Dmitry, you are known as a specialist in theoretical cosmology. It sounds as beautiful as it is complex. What led you to this field?

— Let me start from afar. There are observations that do not fit within the standard model of particle physics. For instance, we are made of matter, yet there is no antimatter around us — how did this happen? And there are many such indications that the physics we know is incomplete. To explain them, we need to generalize the standard model — to propose a solution that, on one hand, shows why what we had before was inadequate, and on the other, does not contradict the scientific results we obtain in reality.

I started precisely from this point. For me, cosmology is a field where I can use my imagination to explain something or to test something.

— If we take a step back and go back to school — you probably weren't thinking about particle physics then?

— I liked physics, but I certainly didn’t know which area I would ultimately pursue. The only thing I understood was that theoretical research came more naturally to me than experimental work. In school, there were practical lab sessions, and I didn't perform very well in them — everything worked out much better on paper.

However, I also enjoyed other subjects. History, in particular — I even considered going into history studies. So I can't say that I just picked up a rattle and immediately decided I would be a physicist.

So I can't say that I just picked up a rattle and immediately decided I would be a physicist.

— Did you dream of space?

— No. I didn't want to be an astronaut, and to be honest, I still don't.

— I meant more in a romantic sense.

— I work on theoretical models. Here, too, there is a great opportunity for imagination, but that imagination is limited by experiments and the mathematical tools we use.

— Since we are talking about your work, can you describe what the daily life of theorists is like? It’s hard to imagine.

— Let me explain with an example. Take Coulomb's law, which is taught in schools (Coulomb's law describes the magnitude of the force acting between two electrically charged particles. — editor's note). It is tested over large distances, and at smaller distances, we verify that it works under all conditions.

But what if we conduct an experiment, and it shows that Coulomb's law does not work or does not work as expected? There are two options. You can conduct a similar experiment to find the cause of the deviation, or you can propose a new law. Or a modification of the old one — an addition to Coulomb's law that would not contradict the fact that all previous experiments agreed with Coulomb's law up until this point. This piece — is mine. But it’s not enough to just come up with something; you also need to understand how to test the predictions. Ideally, using existing tools or at least those that will emerge within the next 20 years.

Often, we start investigating and find out that what we’re facing is not some new physics, but problems with the experiment itself. Or underappreciated uncertainties. But theoretical problems also occur. We have fundamental interactions: gravitational, electromagnetic, weak, and strong. If we try to describe the last type as a conditional force between two objects, at some point, such a description will cease to work. The force, conditionally, turns infinite. To avoid this, we can change the scale of work since the interaction function is related to the distance function between the two bodies. But if at large distances we have, for example, protons and neutrons, then at small distances they start to "disintegrate" into constituent parts. And there, at those distances, everything works quite differently. We do not yet have a fully developed formalism for describing such strong interactions.

— And how do you solve this problem?

— Through certain quantities that we calculate as a result of computer simulations. This approach is currently developing actively and quite successfully.

Imagine that in front of you is a complex surface — let’s say a map. There are mountains and ravines, and you want to find the lowest point on the map. You can try to measure this with a ruler, or you can constrain the surface, toss in some balls, and shake it a bit. Eventually, the balls will fall into the area of minimum and stay there. Numerical calculations for strong interactions work on a similar principle, only the computer allows you to shake it properly — not too hard, because the ball could escape, and not too weakly, because the ball might not move.

— I would like to talk about one of the directions of your research. As far as I remember, you are looking for a signal from decaying dark matter? There’s a lot of theory surrounding it; it’s a hot topic even for those far from science. You even visited Kapitsa on the show "The Obvious — The Incredible," where you discussed dark matter with him 13 years ago…

— And just like back then, we still do not know what dark matter is made of. I must say that we have not made any progress here.

— But you have quite an interesting theory about sterile neutrinos. Your hypothesis is that it could be a particle of dark matter?

— Yes, that’s correct.

— How marginal is this theory?

— Marginal — is that when it's on the edge of madness?

— Let me rephrase. How likely do you see it, within all assumptions and corrections, that sterile neutrinos could indeed solve the dark matter problem? Because there are many different options. Microscopic black holes are also a viable solution.

— One could say this: for a considerable time, the most likely candidates for the role of dark matter particles have been the so-called weakly interacting massive particles.

When we say that matter is dark, it means that it does not interact with electromagnetic radiation. If it did, we would easily see it with a telescope and would not call it dark. Perhaps it participates in strong interactions, and we could find it that way? But this option turns out to be not quite suitable for us. Strong interactions are very strange; they can be described like this: the closer particles are to each other, the weaker they interact, and the more they want to fly apart, the stronger they attract. Dark matter does not behave like that.

The last option remains — weak interactions. They can, in principle, fit with dark matter if we assume that there are some particles that manifest themselves solely in weak interactions. Moreover, if we imagine that such objects existed in the early Universe in a plasma along with other particles, and calculate how many should have survived to the present moment, our estimates would be quite accurate. With an order of magnitude precision.

Since we haven’t invented anything new and have just calculated, and everything matched, perhaps there indeed is a particle that participates in the weak interaction we know? In this case, the most understandable candidate for us would be neutrinos.

— But with neutrinos, it seems, something is already understood?

— Yes, we know how to search for them. For example, there are telescopes that are hidden very deep and shielded from interference. They catch disturbances from weak interactions. Alternatively, there’s another approach — when we expect to see secondary particles created due to a weak process. Then we look for signs of protons-antiprotons, electrons-positrons in cosmic rays: some component that has no astrophysical source.

But it’s not that simple. We have another indication that particle physics is incomplete, which is neutrino oscillation. It turns out that objects participating in weak processes have the ability to transition: one neutrino can transform into another. In the standard model, there are three types of generations — electrons, muons, and tau leptons. Each of them has neutrino pairs that are produced in weak processes along with their charged companions.

Objects participating in weak processes have the ability to transition: one neutrino can transform into another!

So if you see an electron, you would expect to find only electron neutrinos with it. In reality, however, it turns out that if you set up a detector, there’s a certain probability that among the electron neutrinos you will also find muon neutrinos. From the perspective of the standard model, such a transition from one neutrino to another is impossible. But in nature — it is quite feasible.

— And what does this mean?

— We define neutrinos as massless particles. But if we assume they have some mass, then everything falls into place, and the transition becomes possible. In that case,