by Strange behavior in a giant cluster of merging galaxies could be explained if dark matter, the most mysterious stuff in the universe, can collide with itself. However, the most widely held cosmological model at present is the cold dark matter (CDM) model — and it implies the absence of dark matter, which is effectively invisible because it does not interact with light, self-interactive.
To get to the bottom of this distance, researchers from the Astrophysics and Cosmology group of Scuola Internazionale Superiore di Studi Avanzati (SISSA) in Italy proceeded to simulate what is happening inside the giant galaxy cluster “El Gordo” (which literally means “the Fat One” in Spanish).It is located about 7 billion light years from Earth.
This simulation revealed the physics of the supercluster of colliding galaxies – which has a mass equal to 3 million billion suns and is officially named ACT-CL J0102-4915 – which could be explained by a theory other than the CDM. This alternative theory is called the self-interacting dark matter model (SIDM).
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As the name suggests, this model shows that matter, whatever dark matter it is, could 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 each other.
‘fat’ dark matter lab
Because dark matter does not interact with light or visible matter, it has informed scientists that it cannot be composed of atoms composed of electrons, protons and neutrons. These are the bits that make up the stars, planets, moons and our bodies. These particles are collectively part of the baron family, so everyday matter is more technically known as “baronical matter”.
Dark matter interacts with gravity, so its effect on the true shape of space can affect visible baryonic matter and light. That is what scientists understand about the presence of dark matter. However, dark matter poses a major problem for physics. Dark matter particles outnumber dark matter particles by at least 5 to 1, and perhaps as much as 9 to 1, meaning that the stuff we see in the cosmos is only a small fraction of its actual content.
“According to the currently accepted standard cosmological model, the baryonic matter density of the universe can only be 10% of its total matter. The other 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 collisionless cold particles, which respond only to gravity.
“However, there are still some observations that have not yet been explained using the standard model.”
El Gordo is made up of two separate sub-clusters of galaxies colliding at several million miles per hour. It seems as far away as it was when the Earth was less than half of the current century. Valdarnini explained that massive and massive structures like El Gordo, discovered in 2012, provide the perfect cosmic laboratories to study possible SIDM models.
“These are the massive galaxy clusters, massive cosmic structures that determine, after collision, the most energetic events from the Big Bang,” said Valdarnini. “El Gordo is one of the largest galaxy clusters we know. Because of its characteristics, El Gordo has been the subject of many theoretical and observational studies.”
A problem for the standard model of cosmology
The standard model of CDM cosmology suggests that when galaxies collide and merge into a cluster, the gas component of such an event should behave differently from the dark matter component, dissipating as part of the initially released energy.
“This is why, after the collision, the peak of the gas mass density will be behind those of dark matter and galaxies,” explained Valdarnini.
The SIDM model suggests that something different would happen during these collisions. In this model, there would be a physical separation between points of maximum dark matter mass density, referred to as the “dark matter core,” from other mass components of the colliding galaxies. El Gordo’s observations seem to indicate that this SIDM signature is accurate.
El Gordo is composed of two giant galactic subclusters named northwest (NW) and southeast (OS), respectively. X-ray images of the colliding supercluster show a single X-ray peak in the SE subcluster and two faint, elongated tails extending beyond this peak.
One odd feature of these emissions is the different peak locations of the different mass components. Unlike what is seen in another massive supercluster of colliding galaxies, known as the Bullet Cluster, El Gordo’s X-ray peak precedes the SE dark matter peak. In addition, the Brightest Cluster Galaxy (BCG) in El Gordo is near the X-ray peak, and appears to be offset from the SE center of mass. There are also strange features in the NW El Gordo cluster. In this field, the peak density of galaxies is spatially offset from the corresponding peak mass.
To explain these features and validate the SIDM model, Valdarnini and team performed a large series of hydrodynamic simulations of El Gordo, which aimed to reproduce the observed characteristics of the giant mantle.
“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 the dark matter is self-interacting,” he continued. “For this reason, these results provide an unambiguous signature of dark matter behavior exhibiting collisional properties at high energetic redshift. [very distant] cluster collision.”
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However, the SISSM researcher admits that there are discrepancies between SIDM models and El Gordo observations as well as simulations, with some measured values higher than the model’s predicted upper limits for such a cluster merger.
“This suggests that current SIDM models should be considered a low-order approximation and that the underlying physical processes describing the interaction of dark matter in a merging cluster are more complex than can be adequately represented by the approach commonly accepted based on the dark matter particle distribution,” concluded Valdarnini. “The study makes a strong 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.
