Scientists have questioned the role of supermassive black holes in the composition of dark matter

Primordial black holes could have appeared in the first moments of the universe's existence and, according to some hypotheses, become one of the sources of dark matter. However, new calculations have shown that even the most massive of such objects are not able to explain a significant part of the invisible matter. At the same time, the study does not exclude their existence, but sets stricter restrictions on their possible role. About why scientists have revised this hypothesis, see the Izvestia article.

How primordial black holes were associated with dark matter

Unlike ordinary black holes, which are formed as a result of the collapse of massive stars, primordial ones could have formed directly from dense clusters of matter in the early universe. Their existence is still a hypothesis, but physicists consider such objects as one of the possible explanations for the nature of dark matter.

Of particular interest are the so—called monstrously large primary black holes - their mass can reach 100 billion times the mass of the Sun or more. This significantly exceeds the mass of even the largest supermassive black holes found in the centers of galaxies. Dark matter, in turn, makes up about 85% of all matter in the universe, but its nature is still unknown.

An international team of physicists led by researchers from the Institute of Theoretical Physics in Mainz has tested whether such objects can make up a significant proportion of dark matter. The results of the work are published in the journal Physical Review Letters (PRL).

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How scientists tested the hypothesis of supermassive black holes

The researchers considered a scenario in which primary black holes are randomly distributed in space. In this case, some regions of the universe should contain more similar objects, while others should contain fewer. These statistical differences would have influenced the ratio of dark matter and radiation in the early universe.

At first, the excess of black holes in a certain area would be compensated by a small lack of radiation. But as the universe expanded, the radiation became less dense, and the initial balance was disrupted. As a result, there would be inhomogeneities, the so—called isocrivis perturbations, which could affect the further evolution of the cosmos.

The team calculated the strength of such fluctuations and compared the values obtained with the limits set by observations of microwave background radiation. This signal represents the radiation of the early Universe that has been preserved to this day and allows us to study its state on the largest scales.

Why does the cosmic microwave background contradict the hypothesis

Calculations have shown that a significant population of monstrously large primary black holes would lead to too strong inhomogeneities in the distribution of dark matter relative to radiation. Such changes should have left characteristic traces in the microwave background.

However, the observations do not reveal the expected signs. Therefore, scientists have concluded that primordial black holes with a mass of 100 billion solar masses cannot make up a significant part of dark matter: their alleged abundance contradicts data on the early Universe.

The results also cast doubt on some assumptions that primordial black holes could have become the "embryos" of unusually massive galaxies, which were discovered by the James Webb space Telescope. If such objects made up a significant population, their effect on the cosmic microwave background radiation would be more noticeable.

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What does this mean for the search for dark matter?

The authors emphasize that the work does not exclude the existence of primary black holes of such a mass. The researchers considered a simplified model in which all objects have the same mass. Populations with a wider mass distribution have yet to be studied, so it's too early to draw definitive conclusions about all possible scenarios.

In the future, the limitations may become more precise thanks to new experiments on the study of cosmic microwave background radiation, including LiteBIRD and the Simons Observatory. In addition, scientists expect to expand the analysis to even heavier black holes, studying the gravitational waves that could arise during their formation.

Thus, the study did not reveal the nature of dark matter, but allowed us to narrow down the range of possible explanations. Primary black holes remain one of the objects of interest to physicists, but their supermassive variety, according to new calculations, is unlikely to be the main source of invisible matter.