Researchers led by the University of Southern California have produced a set of supercomputer-simulated twins of our Milky Way galaxy, which could offer new insights into dark matter, the invisible material that accounts for around 85% of all matter in the universe. The study was published in The Astrophysical Journal, a publication of the American Astronomical Society.
This artist concept illustrates the new view of the Milky Way. The galaxy’s two major arms can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms are less distinct and located between the major arms. Image Credit: NASA/JPL-Caltech
Ethan Nadler, a former postdoc at USC and Carnegie Observatories who is currently an assistant professor at the University of California, San Diego; Andrew Benson, a staff scientist at Carnegie Observatories; and cosmologist Vera Gluscevic, an associate professor at the USC Dornsife College of Letters, Arts, and Sciences, led the study.
“COZMIC,” which stands for “Cosmological Zoom-in Simulations with Initial Conditions beyond Cold Dark Matter,” is the name they gave to their simulation project.
Although dark matter has been known for decades, scientists have not yet been able to investigate how galaxies form and change in a world where dark and regular matter interact. According to the team, COZMIC has enabled that.
The Heart of Dark Matter
Scientists know that dark matter exists because it influences how galaxies move and bind together. For example, galaxies spin so quickly that they should collide, yet they don't. Something unseen keeps them together; many scientists believe dark matter is at the heart of this, a theory initially proposed in 1933 by a Swiss researcher, Fritz Zwicky. Since then, research into dark matter has advanced.
Dark matter is difficult to investigate since it produces no detectable light or energy. Scientists investigate dark matter by observing how it influences the movements and structures of galaxies. However, this is similar to observing someone's shadow without being able to observe the real person who casts the shadow.
For the suite of investigations, the study team used novel physics, not simply normal particle physics and relativity, and programmed a supercomputer to run highly precise cosmological simulations using COZMIC to explore various hypotheses about what dark matter may be doing.
We want to measure the masses and other quantum properties of these particles, and we want to measure how they interact with everything else. 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.
Vera Gluscevic, Cosmologist, Dornsife College, University of Southern California
In addition to Gluscevic, Nadler, and Benson, the COZMIC team includes Hai-Bo Yu of UC Riverside, Daneng Yang, previously of UC Riverside and currently at Purple Mountain Observatory CAS, Xiaolong Du of UCLA, and Rui An, formerly of USC.
Several Dark Matter Scenarios
Our simulations reveal that observations of the smallest galaxies can be used to distinguish dark matter models.
Ethan Nadler, Study Lead Researcher, University of California, San Diego
For the COZMIC research, the scientists considered for the following dark matter behavior scenarios:
- Billiard-ball model: In this initial investigation, every dark matter particle collides with protons early in the universe, much like billiard balls do when they are first started in motion. This interaction flattens small-scale features and destroys satellite galaxies in the Milky Way. This study also considers situations in which dark matter flows at fast speeds and is made up of extremely low-mass particles
- Mixed-sector model: This second research presents a hybrid scenario in which some dark matter particles interact with conventional matter while others pass by it
- Self-interacting model: For the third study, the scientists created a scenario in which dark matter interacts with itself at the beginning of time and influences galaxy formation throughout cosmic history
According to Benson, the scientists used the supercomputer to conduct these simulations and input new physics to create a galaxy whose structure shows the signs of those interactions between normal and dark matter.
Gluscevic added, “While many previous simulation suites have explored the effects of dark matter mass or self-interactions, 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.”
A New Day for Dark Matter
According to the scientists, it is a significant step toward understanding the true nature of dark matter. They want to learn more about one of space's greatest mysteries by comparing their twin galaxies to actual telescope images.
“We’re finally able to ask, ‘Which version of the universe looks most like ours?” noted Gluscevic.
The COZMIC team intends to advance their research by using telescope data to directly verify the simulation predictions in the hopes of finding evidence of dark matter activity in actual galaxies.
Scientists may have a better grasp of dark matter and how it affects the universe with this next phase.