Evidence of dark matter interacting with itself found in El Gordo merger

This image was obtained as a result of computer simulation of the distribution of matter in the Universe. The orange areas contain galaxies; blue structures are gas and dark matter. We can map dark matter, but we don't know what it is.

Standard model particle physics explains well the interactions between the basic elements of matter. But she's not perfect. She has a hard time explaining dark matter. Dark matter makes up most of the matter in the universe, but we don't know what it is.

The Standard Model states that whatever dark matter is, it cannot interact with itself. A new study may change this view.

Physicists have proposed many different dark matter candidates, including dark photons, weakly interacting massive particles (WIMPs), primordial black holes, and more. Each of them is intriguing in its own way, but none of them have yet been confirmed. And each of them is supposed to fit into the Standard Model.

A new study published in the journal Astronomy and Astrophysics suggests that we may be looking in the wrong places. It assumes that another model, called model of self-interacting dark mattercan explain dark matter, while the Standard Model and its accompanying theory lambda cold dark matter (ΛCDM) simply can't do it.

The article is called “N-body exploration/hydrodynamic modeling of the merging El Gordo cluster: a compelling case for dark matter interacting with itself?“The lead author is Riccardo Valdarnini from the Astrophysics and Cosmology Group at SISSA (Scuola Internazionale Superiore di Studi Avanzati).

El Gordo is an extremely massive and extremely distant galaxy cluster located more than seven billion light-years from Earth. It consists of two galactic subclusters that collide with each other at speeds of several million kilometers per hour. It is at the center of the confrontation between proponents of various theories of dark matter and the Lambda-CDM model.

IN work 2021 El Gordo is said to challenge the Lambda-CDM model because it appeared so early in cosmic history, is so massive, and has such a high impact velocity. “Such a rapid collision between individual rare massive clusters is unexpected for Lambda-CDM cosmology at such a high z,” write the authors of the work.

In a later work, published in 2021, produced a lower estimate of El Gordo's mass that is consistent with Lambda-CDM. “This extreme mass of El Gordo has prompted a number of discussions about whether the presence of the cluster contradicts the Lambda-CDM paradigm,” the authors write. “The new mass is compatible with the current Lambda-CDM cosmology.”

The key part of Lambda CDM is that dark matter is both cold and collision-resistant. In this model, dark matter particles cannot collide with each other; they can only interact through gravity and possibly weak forces. This study challenges this notion.

Proving that dark matter can interact with itself through particle collisions is complex and difficult. El Gordo is a good place to explore the idea of Self-Interacting Dark Matter (SIDM). “There are unique natural laboratories located many light years away that could be very useful for this purpose,” says lead author of the study Valdarnini. “These are massive clusters of galaxies, giant cosmic structures that, when they collide, determine the most energetic events since the Big Bang.” El Gordo is one of them.

Galaxy clusters like El Gordo can be divided into three components: galaxies, dark matter and gas mass. The standard model states that a colliding gas loses some of its original energy during the collision. “This is why after the collision the peak gas mass density will lag behind the density of dark matter and galaxies,” explains Valdarnini.

But SIDM tells a different story. It says that the points where dark matter reaches its maximum density, called centroids, should be physically separated from other components of mass. The features of this department are a sign of the existence of SIDM.

Observations of El Gordo show that it consists of two large subclusters – northwestern (NW) and southeastern (SE), which merge into one.

This Hubble Space Telescope image shows El Gordo's two main components, the NW and SE subclusters.

X-ray images show different peak locations for components of different masses. The X-ray image below shows a single X-ray peak in the SE subcluster and two faint tails extending beyond the X-ray peak. The X-ray peak precedes the dark matter peak. The brightest galaxy in the cluster (BCG) is also offset from the SE mass centroid. In astronomer's terminology, a BCG is the brightest galaxy in any particular cluster and is usually extremely massive and the center of mass.

“Another remarkable aspect can be seen in the NW cluster, where the peak in galaxy number density is spatially offset from the corresponding peak in mass,” Valdarnini explained.

This combined X-ray and infrared image shows the X-rays from "Chandra"and blue - places where dark matter has been discovered. — This combined X-ray and infrared image shows X-rays from Chandra in pink and where dark matter was detected in blue.

But observations alone are not enough. In the new work, published in the journal Astronomy and Astrophysics, Valdarnini used a large number of N-body/hydrodynamic simulations to study the physical properties of El Gordo. Systematic modeling aims to match observations. Each simulation has slightly different parameters, and when the simulation matches the observations, those parameters are likely to provide some explanation for the observations.

Valdarnini explains this clearly in his article. “…The goal of this work is to determine whether it is possible to construct merger models for the El Gordo cluster that can consistently reproduce the observed X-ray morphology, as well as many of its physical properties.”

A critical part of this work and its modeling concerns the distances between the centers of mass at El Gordo. If the simulation is able to obtain such results, this is evidence in favor of SIDM.

“The most important result of this study is that the relative distances observed between the different centers of mass of the El Gordo cluster are naturally explained if dark matter interacts with itself,” says Valdarnini.

This figure from the study shows some of the simulation results. The red contours show the X-ray surface brightness, and the white contours show the mass density. The green crosses are the mass centroids, and the red crosses are the X-ray surface brightness centroids. Each row refers to a different simulation run with different parameters, and each panel represents a different viewing angle. Of particular interest is the middle top panel. It especially recreates El Gordo's twin tails well.

“For this reason, these findings provide a clear signature of the behavior of dark matter, which exhibits collisional properties in very energetic collisions of high-speed clusters,” he continues.

This is a classic “tip of the iceberg” scenario. While these results support the self-interacting dark matter model, they are not yet conclusive, as Valdarnini makes clear when citing inconsistencies in the results.

Valdarnini's work shows that while the results are a rough idea of how dark matter might behave during cluster mergers, there is much more to it. The “underlying physical processes” are extremely complex.

“The study provides compelling evidence for the possibility of dark matter self-interaction between colliding clusters as an alternative to the standard collisionless dark matter paradigm,” he concludes.

For most of the eight billion people alive today, dark matter is of little importance in everyday life. But if we want to entertain hopes and dreams of human civilization lasting centuries, millennia or even longer, going into space and traveling to other stars, it is imperative that we understand everything we can about nature. The history of human progress parallels our growing understanding of nature.

Understanding dark matter is critical to understanding nature. If we want civilization to continue, a better understanding of all things natural is the best way forward.

For now, let's return to our daily life within the Standard Model.