Evidence of dark matter interacting with itself in the El Gordo meltdown

The Standard Model of particle physics does a good job of explaining the interactions between the basic building blocks of matter. But it’s not perfect. It struggles to explain dark matter. Dark matter makes up most of the matter in the Universe, but we don’t know what it is.

The Standard Model says that whatever dark matter is, it cannot interact with itself. New research may have turned that on its head.

Physicists propose many different candidates for dark matter, including dark photons, weakly interacting massive particles (WIMPs), primordial black holes, and more. Each is intriguing in its own way, but there is no confirmation about any of them. And each of these is a proposed part of the Standard Model.

New research in the journal Astronomy and Astrophysics suggests we may be barking up the wrong tree. It suggests that another model, called the Self-Interacting Dark Matter model, can explain dark matter, while the Standard Model and Cold Lambda Dark Matter (Lambda CDM) simply cannot.

The paper is “An N-body/hydrodynamical simulation study of the merged El Gordo array: A compelling case for self-interacting dark matter?” The lead author is Riccardo Valdarnini of the Astrophysics and Cosmology group of SISSA (Scuola Internazionale Superiore di Studi Avanzati).

El Gordo is an extremely massive, extremely distant galaxy cluster, more than seven billion light-years from Earth. It consists of two sub-clusters of galaxies colliding with each other at several million kilometers per hour. It is at the center of a back-and-forth over dark matter and CDM Lambda.

A 2021 paper claimed that El Gordo poses a challenge to the Lambda-CDM model because it appeared so early in cosmic history, is extremely massive, and has such a high collision velocity. “Such a rapid collision between individually rare massive clusters is unexpected in the cold Lambda dark matter cosmology at such high z,” the authors of that paper wrote.

A later paper from 2021 arrived at a lower mass estimate for El Gordo, one that was consistent with the Lambda CDM. “Such an extreme measure of El Gordo has prompted a number of discussions about whether or not the group’s presence is in tension with the Lambda CDM paradigm,” those authors wrote. “The new mass is consistent with the current Lambda CDM cosmology.”

A key part of Lambda CDM is that dark matter is cold and collisionless. In that model, it is impossible for dark matter particles to collide with each other; they can only interact through gravity and possibly the weak force. This study challenges this notion.

Proving that dark matter can interact with itself through collisions is difficult and complicated. El Gordo is a good place to study the idea of ​​self-interacting dark matter (SIDM). “However, there is uniqueness
labs that could be very useful for this purpose, many light years away,” said lead author Valdarnini. “These are the massive clusters of galaxies, the giant cosmic structures that, after the collision, define the most energetic events since the Big Bang.” El Gordo is one of them.

Galactic clusters like El Gordo can be divided into three components: galaxies, dark matter and the gas mass. The Standard Model states that the colliding gas loses some of its initial energy during the collision. “This is why, after the collision, the peak of the gas mass density will lag behind those of dark matter and galaxies,” Valdarnini explained.

But SIDM says something different. He says that the points where dark matter reaches its maximum density, called centroids, must be physically separated from the other components of the mass. The features of that partition are the SIDM signature.

Observations of El Gordo show that it consists of two large sub-groups, the northwest (NW) and southeast (SE), which merge into one.

The X-ray images show different peak locations for the different mass components. The X-ray image below shows a single peak of X-ray emission 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 cluster galaxy (BCG) is also displaced from the center of mass SE. BCGs are the brightest galaxies in a given cluster, are extremely massive, and are the centers of mass in clusters.

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

But these observations alone are not enough. In the new paper in Astronomy and Astrophysics, Valdarnini used a large number of N-body/hydrodynamic simulations to study the physical properties of El Gordo. Systematic simulations aim to match observations. Each simulation has slightly different parameters, and when a simulation matches the observations, these parameters are likely to provide an explanation of the observations.

Valdarnini explains it clearly in the paper. “… the goal of this paper 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.”

The critical part of this work and its simulations concerns the separations between the centers of mass at El Gordo. If the simulations can produce this, it is evidence in favor of SIDM.

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

“Therefore, these findings provide a clear signature of a behavior of dark matter exhibiting collisional properties in a high-energy, high-redshift cluster collision,” he continued.

It’s a classic “tip of the iceberg scenario.” While these results favor the Self Interacting Dark Matter model, they’re far from conclusive, as Valdarnini makes clear when he talks about the discrepancies in the results.

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

“The study makes a compelling case for the possibility of dark matter self-interaction between collision clusters as an alternative to the standard collisionless dark matter paradigm,” he concludes.

For most of the eight billion human beings alive today, dark matter has little consequence in everyday life. But if we are to nurture hopes and enjoy the eye-dreams of human civilization lasting centuries, millennia, or even longer, expanding into space and traveling to other stars, it is critical that we understand everything we can for nature. The history of human progress parallels our growing understanding of nature.

Understanding dark matter is critical to understanding nature. If civilization is to continue, a better understanding of everything about nature is the best way forward.

Now, back to our daily lives under the Standard Model.