Russian astrophysicists have found the source of the smallest cosmic particles – neutrinos. It turned out that high-energy neutrinos form near black holes in the centers of distant active galaxies. This work of Russian scientists could be a breakthrough in physics, because it relates to the most wanted in the history of physics elementary particle – neutrino. Without it, there would be nothing in the Universe, except light – neither matter, nor people. Freely overcoming the enormous distances of light-years long, neutrinos can deliver valuable information about all processes from the farthest places of space.

Neutrino is a tiny particle that conquered the Universe. Once it was considered only a “particle without properties”, a wandering cosmic phantom. Now observatories of the world make every effort to study its characteristics. It is component of dark matter, the source of the energy for the expansion of the Universe, the cause of the gravitational instability of the Big Bang era.

Now we are on the threshold of a new era in cosmology – the neutrino era. Discoveries in the interaction of these particles are awarded the Nobel Prize, and the field of knowledge about them is even planned to be separated into a separate section of the science of celestial objects – neutrino astrophysics. These particles freely pierce through the Sun, our planet, and everyone of us! Its extremely small mass helps this “elusive” particle: approaching massive bodies, its speed does not decrease one iota, and it pierces through giant celestial objects easier than a beam of light through the glass. Look around: everything surrounds you now, including you, is being pierced through by hundreds of trillions of neutrinos. But it is impossible to feel neutrino fluxes. This is what is called the interaction intensity: the longer the free path of a particle (i.e., the distance that the particle can pass without displacements, collisions, etc.), the weaker its interaction with the matter. In the case of neutrinos, this distance is measured in astronomical units (the average distance from Earth to the Sun, taken as a unit of measurement). This means that in order to catch a ghost particle, one sometimes has to wait incredibly long until one of them bothers to hit one of the atoms of some molecule. So, astrophysicists go to great lengths not to miss a chance, but to increase the likelihood of it happening. So, in order to sift out other background processes and not to mix up a cosmic ray particle with a neutrino, the facilities for registration of the latter are placed deep underground (the Japanese Super-Kamiokande detector – 1 km deep beneath the surface; the Canadian SNO detector – 2 km) or even better – deep in the ice of Antarctica (IceCube).

Neutrino, despite all the unremarkability of its physical characteristics, is the most common particle in the Universe. There are so many of them that the “non-neutrino” matter is only about 3-10% of the Universe!

This discovery also sheds light on many processes of the Big Bang. For a long time, it was unclear how the matter that formed all the celestial objects was distributed. In the beginning, it was a hot homogeneous substance – plasma. But what made it get so redistributed in places where galaxies were later formed? And the answer again is the neutrinos. The fact is that after 1 second after the Big Bang, the plasma ceased to be an obstacle for these particles – they went beyond it, ceasing to participate in the intraplasmic reactions. Then these particles, full of energy, moved at the speed of light and, interconverting, easily flew in and out of the “neutrino clouds”. But over time (approximately 300 years), neutrinos have wasted their energy, and their speed no longer allowed them to leave the “neutrino clusters” so easily. Thus, the densest clusters of neutrinos were formed. By this time, the plasma had already cooled down and become less dense. Here the gravitational force of the neutrino clusters triggered, what caused disruption of the homogeneous substance. Thus, clusters of matter were distributed within the “neutrino clouds”, later forming entire systems of celestial objects. This is how galaxies appeared in outer space, placed in “neutrino cells”.

All this makes the so-called “phantom particle” incredibly interesting and important to study. If we manage to “make friends” with it, we will be able to learn space and the processes in its depths better. Unlike electromagnetic waves, radiation, etc., neutrinos come to us from the very center of events – the core of stars, for example, such as the Sun, where they participate in thermonuclear reactions. Freely overcoming the enormous distances of light-years long, neutrinos can deliver valuable information about all processes from the farthest places of space.

Neutrino traps, many tens of millions of dollars facilities, are placed all over the planet. But the neutrinos get trapped very rarely, as almost all matter is transparent for them, and they pass through the Universe without showing themselves itself in any way. But from time to time the detector facilities manage to catch the signal. But this particle still has a lot of mysteries. One of them was revealed by scientists from the Lebedev Physical Institute, the Moscow Institute of Physics and Technology and the Institute for Nuclear Research of the Russian Academy of Sciences.

Researchers have found the answer to the question “where and how do the high-energy neutrinos form?”. Scientists from all over the world have been struggling with this for a long time. The interest is obvious. The fact is that such super energetic neutrinos form with the help of protons only under one condition: the protons must be accelerated almost to the speed of light. This is very difficult, because the mass of proton is about 2000 times greater than that of an electron. There are giant accelerator facilities built on Earth for such acceleration.

And how do they accelerate in space? Theorists have long ago explained the cause: this is a quasar, an extremely luminous active galactic nucleus powered by a supermassive black hole. When matter approaches a black hole, protons can accelerate to almost the speed of light and eject into space. Some of them will turn into energetic neutrinos. Theorists have given one tip to search for neutrinos – gamma rays emitting along with neutrinos.

Science has been searching for the connection between neutrinos and gamma radiation for many years. And in 2018, it succeeded: a gamma ray burst and the emittance of neutrinos from one quasar were observed. This unique result has aroused great interest for many years, the most prestigious journals published the related articles. But many credible scientists met this sensation skeptically. They said that the only event on a single quasar in 10 years should be confirmed with new evidences. What if it was a coincidence? Time went on, and there were no new cases.

This was the situation by the time Russian scientists began their research. A new hypothesis was put forward: look for a connection of neutrinos not with gamma rays, but with radio emission from quasars. To prove it, the scientists used data from IceCube, the American neutrino observatory buried 2 kilometers deep in the ice of Antarctica. The scientists compared the data of neutrino emission with the data on the positions and bursts of radio emission from quasars obtained by radio telescopes around the world and the Russian RATAN-600 located in the North Caucasus in the Karachay–Cherkess Republic. In total, they analyzed about fifty high-energy neutrinos registered by IceCube.

The results proved that ultrahigh-energy neutrinos form in the center of a quasar with massive black holes, accompanied with an accretion disk and very hot gas emissions.

And they matched. When a neutrino is detected, a burst of radio emission from the quasar is observed. Everything coincided. The probability that the coincidence is accidental is only 0.2%. Now this result requires a detailed theoretical analysis. The study is published in the Astrophysical Journal.