New galactic simulations narrow the hunt for dark matter
Our take
The universe holds a lot of secrets, and dark matter is arguably one of the biggest. We know it’s *there* – its gravitational effects are undeniable – but we still have no idea what it *is*. Recent work coming out of the University of Washington is adding a valuable layer to this ongoing puzzle, and it’s a really solid step forward. Astronomers have used incredibly detailed simulations to model how dark matter might influence the streams of stars orbiting our Milky Way galaxy, hoping to better distinguish genuine dark matter signals from potential false positives. This work builds on previous efforts to understand the cosmos; for instance, [Q&A: UW professors explain how we’re misreading the energy crisis] highlights how our understanding of complex systems can be easily skewed, and this research speaks directly to the challenge of avoiding those pitfalls in cosmological modeling. It's a reminder that even with powerful tools, interpretation is key. Furthermore, considering the ongoing impacts of environmental factors closer to home, as explored in [Q&A: As smoke blankets Washington, UW experts share how they navigate wildfire season], underscores the importance of rigorous data analysis and careful consideration of potential confounding variables, principles directly applicable to disentangling dark matter’s influence.
The beauty of this simulation lies in its longevity and scale. Five billion years is an immense amount of time to model, and doing so for galaxies the size of our own is a significant computational feat. The core of the research revolves around the subtle distortions that dark matter should impart on stellar streams – those graceful trails of stars left behind by smaller galaxies that have been absorbed by the Milky Way. By simulating these distortions across multiple galaxies, researchers are creating a baseline against which they can compare real-world observations. The potential to filter out spurious signals, those caused by the complex gravitational interactions of ordinary matter, is huge. It's like refining a search algorithm to weed out irrelevant results – only instead of finding cat videos, we’re trying to find the fundamental building blocks of the universe. This approach offers a more robust method for identifying the telltale signs of dark matter, moving beyond simply *detecting* a signal to confirming its true origin.
What makes this research particularly compelling is its practical implications for the future of dark matter detection. Numerous experiments around the world are actively searching for dark matter particles directly, while others, like those utilizing gravitational lensing and stellar stream analysis, rely on indirect observations. The simulations developed at UW provide a crucial framework for interpreting those observations, helping scientists to avoid misinterpretations that could lead them down blind alleys. It’s a vital piece of the puzzle, providing a theoretical scaffolding to support the experimental hunt. This kind of modeling work, akin to the innovative approaches explored in [Q&A: UW professor Hossein Naghavi uses terahertz waves to help sensors augment human vision], demonstrates how advancements in computational tools and techniques can unlock new avenues for scientific discovery, allowing us to "see" beyond the limits of our current observational capabilities.
Looking ahead, the next frontier will likely involve incorporating even more complex physics into these simulations, accounting for factors like the distribution of gas and dust within galaxies, and the effects of supermassive black holes. The accuracy of these models is paramount, and continuous refinement based on new observational data will be essential. Will these improved simulations ultimately lead to a definitive detection of dark matter particles, or will they reveal that our current understanding of gravity needs to be revised? That’s the question hanging in the air, and the UW’s work provides a powerful tool for navigating the search for answers.
Most of the stars in our Milky Way galaxy sit neatly on a flat plane. But the space around our galaxy is much more chaotic. Rogue bands of stars called “stellar streams” orbit the Milky Way much like planets in our solar system orbit the sun.
Astronomers have long been fascinated by the possibility that stellar streams could indirectly reveal the presence of dark matter, that mysterious theorized substance that doesn’t interact with light or normal matter — except via gravity. However, a new University of Washington study casts doubt on a leading theory linking dark matter and stellar streams, and raises new questions about both galactic phenomena.
“Dark matter makes up most of the mass in the universe and forms the scaffolding that galaxies grow on, but we still don’t know what it is,” said co-author Nora Shipp, a UW assistant professor of astronomy. “The Milky Way is one of the best laboratories we have for figuring that out, and stellar streams are one of the sharpest tools inside it.”
The study was published Aug. 27 in The Astrophysical Journal.
Take a stellar stream tour
Use the visualizer below to explore some of the simulated stellar streams from the study. Click and drag the image to rotate the view. Scroll to zoom. Click or tap the gear icon to access variables like number of streams, rotation rate and more. Use the icon in the lower lefthand corner to go fullscreen.
Credit: Visualization by Arpit Arora and Adrian Price-Whelan. Milky Way image credit: Stefan Payne-Wardenaar.
A stellar stream forms when a group of stars crashes into a galaxy and becomes ensnared by its gravity. As the stars orbit the galaxy, its gravity stretches the cluster into a long, thin filament of stars. Most galaxies host stellar streams, though the Milky Way’s are the most visible to astronomers.
In our galaxy, most stellar streams we can see are irregular — gaps and kinks interrupt an otherwise uniform smear of stars. Many astronomers believe that those irregularities could signal the gravitational tug from small clumps of dark matter, called subhalos. If there are indeed subhalos sprinkled throughout the galaxy, studying the aberrations in stellar streams could teach us about the composition of dark matter.
The new study was an effort to understand the role that the host galaxy — rather than the dark matter clumps within it — plays in shaping stellar streams. Astronomers simulated four Milky Way-sized galaxies without any dark matter clumps, then peppered them with roughly 15,000 stellar streams. After five billion simulated years, the team observed irregularities in nearly every stellar stream.
“In our simulations, the host galaxies alone caused the same kinds of irregularities that we observe in real stellar streams,” said lead author Arpit Arora, a UW postdoctoral scholar in astronomy. “Now that we can predict what the host galaxy does on its own, we can start isolating the part that dark matter is responsible for.”
The cause of the irregularities was the structure of the galaxies themselves. In each simulated galaxy, stars were spread somewhat unevenly across the disc, creating areas of greater and lesser density to mimic the composition of a real galaxy like ours. As the simulated streams of stars passed through denser regions of space, they were bent and torn by the irregular gravitational landscape.
Arora expected the host galaxies to impart some irregularities on the streams, but the sheer number caught him off guard.
“We found that almost all of the streams had some sort of structural variation,” Arora said. “So this idea that streams are naturally thin and smooth wasn’t really necessarily true.”
The simulation generated wiggles, kinks, spurs, branches, gaps and clumps; some streams were totally torn apart by the gravitational froth of their host galaxies. Streams orbiting closer to the galactic core were thrown into dense clumpy regions of space more often, where they acquired more irregularities. Out of the 15,000 streams spread across the four host galaxies, only 70 remained perfectly smooth after five billion years.
The results might seem disheartening, but the UW team believes they chart a clear and exciting course for the future of dark matter research. Arora wants to include dark matter clumps in the next simulation to see whether they produce stellar stream irregularities that are distinct from those caused by the host galaxy alone.
There may also be opportunities to check simulations against new observations: the Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory is expected to find many more stellar streams within our galaxy, which will help astronomers build a taxonomy of stream features and — hopefully — discover fingerprints of dark matter.
“Sadly there’s no magic wand to reveal the structure of dark matter,” said James Davenport, a research assistant professor of astronomy at the UW. “Streams are complex systems, but they’re still the most interesting way to study the dark matter close to home.”
Co-authors from the UW astronomy department include Peter Ferguson, a postdoctoral fellow; Videep Reddy, an undergraduate student; and Jack Kohm and Laurella Marin, graduate students.
A complete list of co-authors is included with the paper.
This research was funded by the Gordon and Betty Moore Foundation.
For more information, contact Arora at arora125@uw.edu.
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