The strange behavior in a massive cluster of merging galaxies could be explained if dark matter, the most mysterious matter in the universe, can collide with itself. However, the most popular model of cosmology is the cold dark matter model (CDM) – and it suggests that dark matter, which is effectively invisible because it does not interact with light, does not interact with itself.
To get to the bottom of this conundrum, researchers from the Astrophysics and Cosmology Group of Italy’s Scuola Internazionale Superiore di Studi Avanzati (SISSA) set out to simulate what is happening in the massive galactic cluster “El Gordo” (which literally means “The Fat”. in Spanish.) Located about 7 billion light years from Earth.
This simulation revealed that the physics of the colliding supercluster of galaxies – which has a mass equivalent to 3 million billion suns and is officially designated ACT-CL J0102-4915 – could be explained by an alternative theory to CDM. This alternative theory is called the Self-Interacting Dark Matter Model (SIDM).
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As its name suggests, this model assumes that whatever dark matter is composed of, the matter can collide and interact with itself. If the universe is accurately described by the SIDM model, it would mean that dark matter particles can exchange energy with themselves.
“Fat” Dark Matter Laboratory
The fact that dark matter does not interact with light or visible matter has suggested to scientists that it cannot be composed of atoms composed of electrons, protons and neutrons. These are the pieces that make up stars, planets, moons, and our bodies. Together, these particles are part of the baryonic family, so everyday matter is more technically called “baryonic matter”.
Dark matter interacts with gravity, so the effect it has on the very fabric of the universe can affect baryonic visible matter and light. Scientists thus conclude on the presence of dark matter. However, dark matter poses a big problem for physics. Dark matter particles vastly outnumber baryon particles by at least 5 to 1 and possibly as much as 9 to 1, meaning that the matter we see in space is only a tiny fraction of its actual content.
“According to the currently accepted standard cosmological model, the current baryonic matter density in the Universe may only account for 10% of its total matter content. The remaining 90% is in the form of dark matter,” team leader and SISSA scientist Riccardo Valdarnini said in a statement. “This matter is generally thought to be non-baryonic and made of cold, collisionless particles that only respond to gravity.
“However, there are still a number of observations that have not yet been explained by the standard model.”
El Gordo consists of two separate dwarf galaxies colliding at several million miles per hour. It is so distant that it can be seen as when the universe was less than half its current age. Valdarnini explained that large and massive structures like El Gordo, which was discovered in 2012, provide perfect space laboratories for studying potential SIDM models.
“They are massive clusters of galaxies, gigantic cosmic structures that, after the collision, determine the most energetic events since the big bang,” Valdarnini said. “El Gordo is one of the largest galaxy clusters known. Due to its peculiarities, El Gordo has been the subject of numerous studies, both theoretical and observational.”
A problem for the standard model of cosmology
The Standard Model of CDM cosmology suggests that when galaxies in a cluster collide and merge, the gas component of such an event should behave differently from the dark matter component and dissipate as part of the initially released energy.
“This is why, after the collision, the gas density peak will lag behind the dark matter and galaxy peaks,” Valdarnini explained.
The SIDM model suggests that something else would happen during these collisions. In this model, there would be a physical separation between the points of maximum dark matter mass density, referred to as “dark matter centroids”, from the other material components of colliding galaxies. The El Gordo observations seem to indicate that precisely this SIDM signature.
El Gordo consists of two massive galactic sub-clusters named Northwest (NW) and Southeast (SE). X-ray images of the entire colliding supercluster show a single X-ray peak in the SE subcluster and two faint, elongated tails extending beyond this peak.
One particular feature of these emissions is the different positions of the peaks of the different mass components. Unlike what is seen in another massive supercluster of colliding galaxies, called the Bullet Cluster, the El Gordo X-ray peak precedes the SE dark matter peak. In addition, the brightest galaxy cluster (BCG) in El Gordo is behind the X-ray peak and also appears to be offset from the SE mass centroid. There are also odd features in the NW cluster of El Gordo. In this region, the galaxy density peak is spatially offset from the corresponding mass peak.
To explain these properties and potentially confirm the SIDM model, Valdarnini and team performed a large set of hydrodynamical simulations of El Gordo aimed at reproducing the observed characteristics of the massive supercluster.
“The most significant result of this simulation study is that the relative separations observed between the various centroids of the mass of the ‘El Gordo’ cluster are naturally explained if the dark matter interacts with itself,” he continued. “Therefore, these findings provide a clear signature of dark matter behavior that exhibits collisional properties at very energetic high redshift.” [very distant] cluster collision.”
However, the SISSM researcher also acknowledges that there are discrepancies between the SIDM models and the El Gordo observations as well as the simulations, with some of the measurements being higher than the model’s predicted upper limits for such cluster mergers.
“This suggests that current SIDM models should only be considered 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. Valdarnini concluded, “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.”
The team’s research was published in April in the journal Astronomy & Astrophysics.