Cluster Soup: How Russian Physicists Explain Formation of Primordial Black Holes

Black holes are regions of spacetime that have such a strong gravitational effect that nothing, not even objects moving at the speed of light, including photons, can escape them.

At this point, researchers have only indirect evidence of the existence of black holes, which, as many scientists believe, is just as compelling as direct evidence. If only direct evidence was considered true, we should have admitted that the Sun orbits the Earth, and not the other way around. Long ago, researchers realized that black holes were formed after the collapse of massive stars. Astronomers can observe dozens of these events taking place in space. Researchers believe that the cores of almost every galaxy (and there are about a hundred billion of them) contain a supermassive black hole, heavier than millions or billions of solar masses. For instance, the core of our own galaxy, the Milky Way, contains a supermassive black hole which is as heavy as several million solar masses.

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But the data obtained during observations shows that before black holes, there were so-called primordial black holes, which appeared so early that their formation is hard to explain in standard ways. The thing is, it takes at least a billion years for a supermassive black hole to form. The universe was born 13.8 billion years ago, which means that the world’s oldest black hole must have formed at least a billion years later. However, astronomers have discovered black holes that formed just 700 million years after the universe was born, and these black holes are super massive – about several billion solar masses.

Moreover, the presence of gravitational waves, which were discovered in 2016, also proves the existence of a non-stellar version of the formation of black holes. These waves were generated by a collision of two supermassive black holes, which means that at a certain period of time, these two objects were located close to each other. The probability of two supermassive black holes existing close to each other is very low from the standpoint of the stellar version of their formation.

This means, there must be another explanation for the formation of primordial black holes. A new approach, developed at the National Research Nuclear University MEPhI by a group of researchers lead by Sergei Rubin, a professor at the university’s Department of Elementary Particle Physics, helps to explain the formation of primordial black holes, which does not contradict the theory of the stellar nature of black hole formation.

"Let us imagine that the universe is filled with a hypothetical field," Professor Sergei Rubin explained. "When we introduce the notion of a field, we suggest that it has an energy potential. When we set the parameters of a field, we know how much energy it has. If the parameters of a field change, its energy content changes as well. This means that the potential energy (its potential) depends on the mass of this field. No one knows the general form of this potential. But if we assume that it has two minimums, it might turn out that due to the fluctuations of the early expanding universe, at a certain point this field would "jump over" its maximum and drop to a minimum.

As we know, energy decreases in the presence of friction, which means that most of the space will gravitate towards one minimum, while a smaller region will gravitate towards another, and this smaller region has a very high energy density, which can lead to the formation of a black hole.”

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Unlike many other models of the formation of primordial black holes, the model developed by the team of MEPhI researchers led by Professor Sergei Rubin suggests that black holes are formed in clusters. The calculations show that when one spatial domain has the potential of “jumping over” the maximum, the neighboring spatial domains are very likely to have it as well. Now scientists are researching the evolution of primordial black hole clusters after their formation.

"The most important part is what happens to these clusters afterwards," Professor Rubin explained. "It is clear that the first spatial domain to "jump over" the maximum will have the largest mass. We do not know what kind of mass it is, or how the distribution of black holes by their mass will take place. These things, as well as the subsequent dynamics, depend on the parametres of the model and the initial conditions. Once primordial black holes are born, they start interacting with each other, colliding and merging. Moreover, black holes located on the periphery of the area get involved in the process of the general expansion of space and leave the cluster forever. Thus, black hole clusters start living their own inner life, cooking in the primordial soup of the early universe. In short, this dynamic is complex, and we are now developing a code that will help us analyze all these transformations."

Unfortunately, it is still impossible to test Sergei Rubin’s theory using a particle accelerator – it is simply not possible to create the amount of energy necessary to form a black hole in a laboratory environment. However, new data obtained in the course of primordial black hole observation will help researchers answer the questions of their origin.