Strange behavior in a huge cluster of merging galaxies could be explained if dark matter, the universe’s most mysterious stuff, can collide with itself. However, the most favored model of cosmology at the moment is the Cold Dark Matter (CDM) model – and it suggests that dark matter, which is effectively invisible because it does not interact with light, does not self-interact.
To get to the bottom of this mystery, researchers from the astrophysics and cosmology group of Italy’s Scuola Internazionale Superiore di Studi Avanzati (SISSA) have started simulating what’s happening in the massive galactic cluster ‘El Gordo’ (which literally means ‘the Fat One’). ” in Spanish). It is located approximately 7 billion light-years from Earth.
This simulation revealed that the physics of the supercluster of colliding galaxies – which has a mass equivalent to 3 million billion suns and is officially called ACT-CL J0102-4915 – could be explained by an alternative theory to the CDM. This alternative theory is called the self-interacting dark matter (SIDM) model.
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As the name suggests, this model posits that the substance that makes up dark matter can collide and interact with itself. If the universe is accurately described by a SIDM model, this would mean that dark matter particles can exchange energy with themselves.
A ‘fat’ laboratory for dark matter
The fact that dark matter does not interact with light nor with visible matter has pointed out to scientists that it cannot be made up of atoms made up of electrons, protons and neutrons. These are the pieces that make up stars, planets, moons and our bodies. These particles are collectively part of the baryon family, so everyday matter is technically called “baryonic matter.”
Dark matter interacts with gravity, so the effect it has on the fabric of space can affect baryonic visible matter and light. That’s how scientists infer the presence of dark matter. However, dark matter poses a major problem for physics. Dark matter particles outnumber baryonic particles by a wide margin, by at least 5 to 1, and possibly even 9 to 1, meaning that the things we see in the cosmos are only a small part of the actual contents.
“According to the currently accepted standard cosmological model, the current density of baryonic matter in the universe may account for only 10% of the total matter content. The remaining 90% consists of dark matter,” says team leader and SISSA scientist Riccardo Valdarnini. said in a statement. ‘This matter is generally thought to be non-baryonic and composed of cold, collisionless particles that respond only to gravity.
“However, there are still some observations that have not yet been explained using the standard model.”
El Gordo consists of two separate subclusters of galaxies colliding at millions of kilometers per hour. It is so far away that it is seen as it was when the universe was less than half its current age. Valdarnini explained that huge and massive structures like El Gordo, which was discovered in 2012, provide the perfect cosmic laboratories to study potential SIDM models.
“These are the massive clusters of galaxies, gigantic cosmic structures that when collided determine the most energetic events since the Big Bang,” Valdarnini said. “El Gordo is one of the largest clusters of galaxies known. Because of its peculiarities, El Gordo has been the subject of numerous studies, both theoretical and observational.”
A problem for the standard model of cosmology
The CDM standard model for 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, disappearing as part of the initial energy released.
“This is why the peak of gas mass density after the collision lags behind that of dark matter and galaxies,” Valdarnini explains.
The SIDM model suggests that something different would happen during these collisions. In this model, there would be a physical separation between points with the maximum mass density of dark matter, also called ‘dark matter centroids’, and other mass components of the colliding galaxies. Observations from El Gordo seem to suggest that this is the exact SIDM signature.
El Gordo consists of two massive galactic subclusters called northwestern (NW) and southeastern (SE) respectively. X-rays of the entire colliding supercluster show a single X-ray peak in the SE subcluster and two faint, elongated tails extending beyond this peak.
A strange feature of these emissions is the varying peak locations of different mass components. Unlike what is seen in another massive supercluster of colliding galaxies called the Bullet Cluster, El Gordo’s X-ray peak precedes the SE peak of dark matter. Furthermore, the Brightest Cluster Galaxy (BCG) in El Gordo lags behind the X-ray peak, and it also appears to be offset from SE’s center of mass. There are also strange features in the NW cluster of El Gordo. In this region, the peak density of galaxies is spatially shifted from the corresponding mass peak.
To explain these features and potentially validate a SIDM model, Valdarnini and team ran a large number of hydrodynamic simulations of El Gordo, aiming to reproduce the observed features of the massive supercluster.
“The main 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 as the dark matter interacts with itself,” he continued. “For this reason, these findings provide an unambiguous signature of dark matter behavior that exhibits collisional properties at very energetic high redshift.” [very distant] cluster collision.”
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However, the SISSM researcher also acknowledges that there are inconsistencies between SIDM models and El Gordo observations and simulations, with some measured values being higher than the model’s predicted upper limits for such cluster mergers.
“This suggests that current SIDM models should be regarded as only a low-order approximation and that the underlying physical processes describing dark matter interactions in large cluster mergers are more complex than can be adequately captured by the commonly adopted approach based on the scattering of dark matter particles,” Valdarnini concluded. “The study provides a compelling argument 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.