Scientists are creating virtual copies of our home galaxy, the Milky Way, using supercomputers. This groundbreaking project aims to finally understand dark matter, the invisible substance that makes up 85% of the universe but remains one of science’s greatest puzzles. By simulating how galaxies form under different dark matter behaviors, researchers can compare these “twin” universes to our own and search for crucial clues.
The Ghostly Glue Holding Galaxies Together
For decades, astronomers have known something strange is out there. Galaxies, like our Milky Way, spin so fast that their stars should fling outwards, yet they stay together. It’s like watching a figure skater spin incredibly fast without losing control. This stability is attributed to an invisible force field – dark matter. While we know it’s real because of its gravitational pull, we can’t see it, feel it, or detect it directly with light or energy. Studying dark matter is like trying to understand a person purely by looking at their shadow; you see its effect, but not the source.
Led by cosmologist Vera Gluscevic at USC Dornsife, along with Ethan Nadler and Andrew Benson, a team developed a new simulation project called COZMIC. This isn’t just about gravity; COZMIC introduces new physics to explore how dark matter particles might interact not only with themselves but also with the “normal” matter we can see and touch – the stuff that makes up stars, planets, and us.
“With COZMIC, for the first time, we’re able to simulate galaxies like our own under radically different physical laws — and test those laws against real astronomical observations,” explains Gluscevic.
Artist's concept showing the structure of the Milky Way galaxy with its central bar and spiral arms
Simulating Different Universes
The COZMIC team ran a series of detailed simulations, each exploring a different idea for how dark matter behaves. Imagine creating parallel universes where the basic rules for dark matter are slightly different.
They tested scenarios including:
- The Billiard-Ball Model: In this version, dark matter particles strongly interact with normal matter in the early universe, like billiard balls colliding. This kind of interaction would smooth out small structures and could dramatically change the number of tiny satellite galaxies orbiting the Milky Way.
- The Mixed-Sector Model: This simulation explores a hybrid universe where some dark matter interacts with normal matter, while others pass right through, unaffected.
- The Self-Interacting Model: This scenario tests if dark matter particles interact with each other, both in the distant past and today. These self-interactions could modify how galaxies form over cosmic history.
By programming these different physics rules into the supercomputer, the simulations produce “twin” Milky Ways whose structure bears the fingerprint of these unique dark matter behaviors. “While many previous simulation suites have explored the effects of dark matter mass or self-interactions,” Gluscevic notes, “until now, none have simulated dark matter interactions with normal matter. Such interactions are not exotic or implausible. They are, in fact, likely to exist.”
Which Twin is Ours?
The power of COZMIC lies in its ability to produce these different possible realities. “Our simulations reveal that observations of the smallest galaxies can be used to distinguish dark matter models,” says Nadler.
The next crucial step is to compare these simulated Milky Ways and their satellite galaxies to actual telescope observations of the real universe. By looking for specific patterns or structures predicted by each simulation, scientists can start to rule out some dark matter theories and potentially identify the behavior of the actual dark matter around us.
“We’re finally able to ask, ‘Which version of the universe looks most like ours?'” Gluscevic says. This work brings us significantly closer to solving the cosmic riddle of dark matter and understanding how this mysterious substance has shaped the vast structure of the universe we see today.
