For nearly a century, the enigmatic presence of dark matter has stood as one of cosmology’s most profound mysteries. While invisible to our telescopes, its pervasive gravitational influence orchestrates the formation and evolution of galaxies and the grand tapestry of the universe’s large-scale structure. Now, at the Perimeter Institute for Theoretical Physics, researchers James Gurian and Simon May are delving into the fascinating behavior of a theoretical variant known as self-interacting dark matter (SIDM), investigating how its unique particle interactions could dramatically alter the growth and transformation of cosmic structures over eons. Their groundbreaking work, detailed in a recent publication in Physical Review Letters, introduces a novel computational tool that promises to revolutionize our understanding of SIDM’s role in galaxy formation, enabling the exploration of particle interactions previously deemed too complex or computationally prohibitive to model accurately.

The concept of SIDM posits a form of dark matter where its constituent particles can collide with each other, but remain aloof from baryonic matter – the familiar stuff of stars, planets, and ourselves. These collisions are not dissipative in the traditional sense; rather, they conserve energy through elastic self-interactions, a behavior that has profound implications for dark matter halos. These halos, vast invisible reservoirs of dark matter, envelop galaxies and act as gravitational scaffolding, guiding their formation and evolution. "Dark matter forms relatively diffuse clumps which are still much denser than the average density of the universe," explains Gurian, a Perimeter postdoctoral fellow and co-author of the study. "The Milky Way and other galaxies live in these dark matter halos."

The self-interacting nature of SIDM can initiate a process called gravothermal collapse within these halos. This phenomenon, rooted in a counterintuitive aspect of gravity, describes how systems bound by gravitational forces can actually become hotter as they lose energy, a stark contrast to the cooling observed in systems governed by other forces. "You have this self-interacting dark matter which transports energy, and it tends to transport energy outwards in these halos," Gurian elaborates. "This leads to the inner core getting really hot and dense as energy is transported outwards." Over immense timescales, this relentless outward energy transport can drive the central region of the dark matter halo towards a dramatic and potentially catastrophic collapse.

Simulating the intricate structures that emerge from SIDM has long been a significant hurdle for theoretical physicists. Existing simulation methods, while powerful, each possess limitations. Some excel when dark matter is thinly distributed and collisions are infrequent, operating effectively in low-density regimes. Others are tailored for extremely dense dark matter environments where interactions are commonplace, thriving in high-density scenarios. "One approach is an N-body simulation approach that works really well when dark matter is not very dense and collisions are infrequent. The other approach is a fluid approach – and this works when dark matter is very dense and collisions are frequent," Gurian states. "But for the in-between, there wasn’t a good method. You need an intermediate range approach to correctly go between the low-density and high-density parts. That was the origin of this project."

To bridge this critical gap, Gurian, in collaboration with Simon May, a former Perimeter postdoctoral researcher now an ERC Preparative Fellow at Bielefeld University, developed a novel computational code named KISS-SIDM. This software ingeniously bridges the limitations of prior simulation methods, offering enhanced accuracy while demanding significantly less computational power. Furthermore, its public availability empowers the broader research community to explore SIDM phenomena. "Before, if you wanted to check different parameters for self-interacting dark matter, you needed to either use this really simplified fluid model, or go to a cluster, which is computationally expensive. This code is faster, and you can run it on your laptop," Gurian exclaims, highlighting the accessibility and efficiency of their creation.

The growing interest in interacting dark matter models, including SIDM, is not merely academic. It is partly fueled by perplexing observational anomalies observed in galaxies that standard dark matter models struggle to explain. "There has been considerable interest recently in interacting dark matter models, due to possible anomalies detected in observations of galaxies that may require new physics in the dark sector," remarks Neal Dalal, a member of the Perimeter Institute research faculty. "Previously, it was not possible to perform accurate calculations of cosmic structure formation in these sorts of models, but the method developed by James and Simon provides a solution that finally allows us to simulate the evolution of dark matter in models with significant interactions," Dalal emphasizes. "Their paper should enable a broad spectrum of studies that previously were intractable."

The implications of dark matter core collapse are particularly captivating, as this process could leave observable signatures, potentially even offering insights into the formation of black holes. However, the ultimate fate of these collapsing cores remains an open and tantalizing question. "The fundamental question is, what’s the final endpoint of this collapse? That’s what we’d really like to do – study the phase after you form a black hole," Gurian expresses, articulating the future directions of this research. By enabling detailed investigations into these extreme cosmic conditions, the KISS-SIDM code represents a pivotal advancement, paving the way for answering some of the most profound questions that continue to surround dark matter and the very structure of our universe. The ability to simulate these complex interactions with unprecedented ease and accuracy opens a new vista in our quest to comprehend the invisible scaffolding that shapes the cosmos.