September 25, 2020

Mysterious ‘excess’ of gravitational lensing in galaxy clusters questions dark matter hypotheses


Like giant magnifying glasses in space, the gravitational lenses they are formed when a very massive object, such as a galaxy or a cluster, bends the light of other more distant ones located behind, which allows to study them. The enormous gravitational pull of the matter that forms it, in particular the dark one, is what distorts that distant light.

The mystery of dark matter, the invisible part of the cosmos

The mystery of dark matter, the invisible part of the cosmos

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Own galaxy clusters are large-scale gravitational lenses, but the galaxies the interior also act as lenses on a smaller scale.

The latter are those that have now been analyzed by an international team of scientists, led from the National Institute of Astrophysics (INAF) in Italy, who have found that galactic clusters contain many more of these tiny gravitational lenses than expected by standard cosmology. The results are published in the journal Science.

The authors have analyzed eleven clusters, comparing the observations of minor gravitational lenses, captured by the space telescope. Hubble (NASA / ESA) and the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, which indicated the computational simulations on how dark matter should be distributed in these galaxy clusters.

“Thus we have discovered that the ability of real galaxy clusters to create strong small-scale lens effects is ten times greater than in simulations,” the lead author explains to SINC, Massimo Meneghetti, of the INAF.

“This indicates that the matter in galaxies has a spatial distribution that is more compact than expected by theory,” he adds. In other words, the mass density reached in the center of galaxies is so high that many of them can produce strong lensing effects, something that does not appear in numerical simulations with the same frequency ”.

Two possible explanations

Given these results, the researchers conclude that there must be an unknown problem with standard simulation methods, or else that cosmologists have made incorrect assumptions about the nature of dark matter.

“It is difficult to know which of the two options is more likely,” Meneghetti acknowledges. On the one hand, the physical processes that determine how ordinary matter behaves in galaxies and interacts with dark matter are very complex. This is why they are difficult to simulate – they have high computational costs and may not be fully understood. Some important ingredient could be missing ”.

“But on the other hand,” he continues, “the currently accepted assumption that dark matter is made of massive particles without collisions, interacting only through gravity, has already been challenged by other discrepancies between the theory and what is observed in the vicinity of our own Milky Way. Furthermore, these particles have not yet been detected experimentally, and many other candidates have not yet been ruled out by observations.

To try to find the answers, the researchers will continue to run simulations with different dark matter models, evaluating whether other dark matter candidates can explain their observations, as well as extending their analysis to a much larger number of gravitationally lensed galaxy clusters.

“In the coming years, for example, the Euclid mission of ESA, in which several members of our team are very involved, will begin to observe 15,000 square degrees of the sky, offering observations of many thousands of galaxy clusters ”, Meneghetti advances, who hopes to obtain new data that will solve the problem. mystery of small gravitational lenses.

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