Contrary to the prevailing assumptions of the standard model, recent research suggests that dark matter may indeed be self-interacting. This groundbreaking study, published in Astronomy & Astrophysics and led by Riccardo Valdarnini of SISSA's Astrophysics and Cosmology group, used numerical simulations to explore the dynamics within “El Gordo” (Spanish for “The Fat One”), a colossal galaxy cluster merger located seven billion light-years away.
The study's findings indicate that the observed physical separation between the maximum density points of dark matter and those of other mass components within this cluster can be explained using the Self-Interacting Dark Matter (SIDM) model, challenging the standard Cold Dark Matter (CDM) model. The implications of these results are significant, suggesting that dark matter particles may exchange energy through collisions, which could lead to a better understanding of various astrophysical phenomena.
El Gordo: A Massive Laboratory for Dark Matter Study
According to the currently accepted cosmological model, only about 10% of the universe's matter is made up of baryonic matter, the type we can see and interact with. The remaining 90% is dark matter, which is thought to be non-baryonic and composed of cold, collisionless particles that only respond to gravity, hence the term “Cold Dark Matter” (CDM). However, there are still numerous observations that the standard model cannot fully explain.
Valdarnini explains, “To answer these questions, several researchers have proposed an alternative model known as SIDM. Proving the collisional properties of dark matter and, more broadly, alternative theories to the standard cosmological model is very challenging. However, massive galaxy clusters like El Gordo provide unique laboratories for testing these ideas.”
With a mass of about (10^{15}) solar masses, El Gordo is one of the largest known galaxy clusters. Due to its unique characteristics, it has been the focus of extensive theoretical and observational studies.
The Collisional Nature of Dark Matter
In the standard model, during a galaxy cluster merger, the behavior of the collisional gas mass component differs from that of galaxies and dark matter. The gas dissipates part of its initial energy, causing the peak of gas mass density to lag behind those of dark matter and galaxies after the collision.
Valdarnini explains that under the SIDM model, dark matter should exhibit a physical separation from other mass components, which would be a signature of self-interacting dark matter. Observations of El Gordo support this hypothesis, showing a separation between the centroids of dark matter and other components, consistent with the SIDM model.
Observations and Findings in El Gordo
El Gordo consists of two massive subclusters, designated as northwestern (NW) and southeastern (SE). X-ray images reveal a single X-ray emission peak in the SE subcluster, with faint tails extending beyond this peak. Interestingly, the peak locations of the various mass components differ from those observed in other colliding clusters like the Bullet Cluster. In El Gordo, the X-ray peak precedes the SE dark matter peak, and the Brightest Cluster Galaxy (BCG) trails the X-ray peak, also showing a spatial offset from the SE mass centroid. In the NW cluster, the galaxy number density peak is also spatially offset from the corresponding mass peak.
To validate the SIDM model, Valdarnini conducted a series of N-body/hydrodynamical simulations to reproduce these observational features. “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,” Valdarnini states.
Implications and Future Directions
These findings provide compelling evidence that dark matter could exhibit collisional properties in high-energy, high-redshift cluster collisions. However, the SIDM cross-section values derived from the simulations are higher than current upper limits, suggesting that existing SIDM models may be oversimplified. This indicates that the physical processes governing dark matter interactions in major cluster mergers are more complex than currently understood.
Valdarnini concludes, “The study makes a compelling case for the possibility of self-interacting dark matter in colliding clusters, offering a promising alternative to the standard collisionless dark matter paradigm. Further research is needed to refine our understanding of these interactions and develop more accurate models.”
The research represents a significant step forward in dark matter studies, potentially reshaping our understanding of the universe's most mysterious component. As scientists continue to explore the behavior of dark matter, studies like this pave the way for new insights and advancements in cosmology.
Source: International School of Advanced Studies (SISSA)