Dark Matter

Evidence of Dark Matter Interacting With Itself in El Gordo Merger

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

The Standard Model says that whatever dark matter is, it can’t 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 one is intriguing in its own way, but there’s no confirmation regarding any of them. And each one 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 its Lambda Cold Dark Matter (Lambda CDM) simply can’t.

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

El Gordo is an extremely massive, extremely distant galaxy cluster more than seven billion light-years away from Earth. It’s comprised of two galaxy sub-clusters that are colliding with one another at several million kilometres per hour. It’s at the center of a back-and-forth over dark matter and the Lambda CDM.

A 2021 paper claimed that El Gordo presents a challenge for the Lambda-CDM model because it appeared so early in cosmic history, is extremely massive, and has such a high collisional velocity. “Such a fast collision between individually rare massive clusters is unexpected in Lambda cold 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 Lambda CDM. “Such an extreme mass of El Gordo has stimulated a number of discussions on whether or not the presence of the cluster is in tension with the Lambda CDM paradigm,” those authors wrote. “The new mass is compatible with the current Lambda CDM cosmology.”

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

Proving that dark matter can interact with itself via collisions is difficult and complicated. El Gordo is a good place to study the Self-Interacting Dark Matter (SIDM) idea. “There are, however, unique
laboratories that can prove very useful for this purpose, many light years away from us,” said lead author Valdarnini. “These are the massive galaxy clusters, gigantic cosmic structures that, upon collision, 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: the galaxies, the dark matter, and the gas mass. The Standard Model says that the colliding gas loses some of its initial energy during the collision. “This is why, after the collision, the peak of gas mass density will lag behind those of dark matter and galaxies,” Valdarnini explained.

But the SIDM says something different. It says that the points where the dark matter reaches its maximum density, called centroids, should be physically separated from the other mass components. The peculiarities of that separation are a signature of SIDM.

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

This Hubble Space Telescope image shows El Gordo’s two main components, the NW and SE sub-clusters. Image Credit: NASA, ESA, and J. Jee (University of California, Davis)

X-ray images show different peak locations for the different mass components. The X-ray image below shows a single X-ray emission peak in the SE subcluster and two faint tails elongated beyond the X-ray peak. The X-ray peak precedes the dark matter peak. The Brightest Cluster Galaxy (BCG) is also offset from the SE mass centroid. BCGs are the brightest galaxies in a given cluster, are extremely massive, and are centers of mass in clusters.

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

This combined X-ray and infrared image shows X-rays from Chandra in pink, and the blue shows where dark matter is found. Image Credit: X-ray: NASA/CXC/Rutgers/J. Hughes et al.; Infrared: NASA/ESA/CSA, J.M. Diego (IFCA), B.Frye (Univ. of Arizona), P.Kamieneski, T.Carleton & R.Windhorst (ASU)

But those observations alone aren’t enough. In the new paper in Astronomy and Astrophysics, Valdarnini employed a large number of N-body/hydrodynamical simulations to study El Gordo’s physical properties. The systematic simulations aim to match the observations. Each simulation has slightly different parameters, and when a simulation matches observations, those parameters are likely to offer some explanation of the observations.

Valdarnini explains it clearly in the paper. “… the aim 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 in El Gordo. If simulations can produce that, it’s evidence in favour of SIDM.

“The most significant result of this simulation study is that the relative separations observed between the different mass centroids of the “El Gordo” cluster are naturally explained if the dark matter is self-interacting,” states Valdarnini.

This figure from the research shows some of the simulation results. The red contours show X-ray surface brightness, and the white shows mass density. Green crosses are mass centroids, and red crosses are X-ray surface brightness centroids. Each row is from a separate simulation run with different parameters, and each panel represents a different viewing angle. The middle top panel is of particular interest. It recreates El Gordo’s twin tails particularly well. Image Credit: Valdarnini et al. 2024.

“For this reason, these findings provide an unambiguous signature of a dark matter behaviour that exhibits collisional properties in a very energetic high-redshift cluster collision,” he continued.

It’s a classic “tip of the iceberg scenario.” While these results are in favour of the Self Interacting Dark Matter model, they’re nowhere near conclusive, as Valdarnini makes clear when he talks about inconsistencies in the results.

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

“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,” he concludes.

For most of the eight billion human beings alive today, dark matter is of little consequence in daily life. But if we want to entertain hopes and enjoy daydreams of human civilization lasting for centuries, millennia, or even longer, expanding into space and travelling to other stars, it’s critical 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 persist, a better understanding of everything about nature is the best way forward.

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

Evan Gough

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