The study suggests that dark matter may eventually have collisional properties

Contrary to the Standard Model, dark matter can actually interact with itself. This was the conclusion of one study published in Astronomy and astrophysics and led by Riccardo Valdarnini of the SISSA Astrophysics and Cosmology Group. The study used numerical simulations to analyze what is happening inside “El Gordo” (literally “The Fat” in Spanish), a giant cluster of clusters seven billion light-years away.

Calculations showed that in this cluster the observed physical separation between the points of maximum density of dark matter and the points of other matter components can be explained using the so-called SIDM (Self-Interacting Dark Matter) model, in contrast to the standard model. one. This research makes a significant contribution to the SIDM model, in which dark matter particles exchange energy through collisions with interesting astrophysical implications.

‘El Gordo’: A gigantic cosmic structure to study dark matter

“According to the currently accepted standard cosmological model, the current density of baryonic matter in the universe can only make up 10% of its total matter content. The remaining 90% is in the form of dark matter,” explains Valdarnini, author of the research.

“It is generally believed that this matter is not baryonic and consists of cold, collisionless particles that only respond to gravity. Hence the name ‘Cold Dark Matter’ (CDM). However, there are still a number of observations that have not yet been explained using the Standard Model ,” says the researcher. “To answer these questions, several authors propose an alternative model called SIDM.”

Proving the collisional properties of dark matter and more generally alternative theories to the standard cosmological model is very complicated. He explains: “However, there are unique laboratories that may prove very useful for this purpose, many light-years away. They are massive clusters of galaxies, gigantic cosmic structures that, after collision, determine the most energetic events since the Big Bang.

“Weighing about 10¹⁵ mass of the Sun, El Gordo is one of the largest galaxy clusters known. Thanks to its peculiarities, El Gordo has been the subject of many studies, both theoretical and observational.”

Dark matter can be collisional

According to the standard paradigm, during cluster mergers the behavior of the collisional component of gas matter will differ from the behavior of the other two components – galaxies and dark matter. In this scenario, the gas dissipates some of its initial energy.

“This is why, after the collision, the gas density peak will lag behind the dark matter and galaxy peaks,” explains Valdarnini. However, a special phenomenon should be observed with the SIDM model, namely the physical separation of dark matter’s centroids – its points of maximum density – from other matter components with features that represent the true “Signature of SIDM models”. According to observations, this is exactly what is happening inside “El Gordo.”

Sightings of El Gordo

“Let’s start with observation,” explains Valdarnini. El Gordo consists of two massive subclusters, which are labeled northwest (NW) and southeast (SE). An X-ray image of the “El Gordo” cluster shows a single X-ray emission peak in the SE subcluster and two faint tails extending beyond the X-ray peak. A notable feature is the peak location of the various mass components.

Unlike what can be seen in the Bullet Cluster, another important example of a colliding cluster, the X-ray peak precedes the SE dark matter peak. In addition, the brightest cluster galaxy (BCG) is not only behind the X-ray peak, but also appears to be spatially offset from the SE mass centroid. Another remarkable aspect can be seen in the NW cluster, where the galaxy number density peak is spatially offset from the corresponding mass peak.

Research results suggest that collisional dark matter is the explanation for the phenomena observed in “El Gordo”

In order to explain his findings and validate the SIDM models, Valdarnini used a large set of so-called N-body/hydrodynamic simulations. So he conducted a systematic study aimed at reproducing the observational features of “El Gordo”.

“The most significant result of this simulation study is that the relative separations observed between the different centroids of the mass of the ‘El Gordo’ cluster are naturally explained if the dark matter interacts with itself,” says Valdarnini.

“Therefore, these findings provide a clear signature of dark matter behavior that exhibits collisional properties in the very energetic collision of high-redshift clusters. However, there are discrepancies because the SIDM cross-section values ​​obtained from these simulations are higher than the current upper limits, which are of the order of unity at cluster scales.

“This suggests that current SIDM models should only be considered as a low-order approximation and that the underlying physical processes that describe the interaction of dark matter in large clusters are more complex than can be adequately represented by the commonly assumed approach based on the scattering of dark matter particles.

“The study makes a compelling case for the possibility of self-interacting dark matter between colliding clusters as an alternative to the standard collisionless dark matter paradigm.”