Researchers simulate twin galaxies of the Milky Way to enhance understanding of dark matter

A team of researchers led by USC has generated a set of highly detailed supercomputer-simulated counterparts of our Milky Way galaxy — a breakthrough that may aid scientists in uncovering fresh insights about one of the universe’s most profound enigmas: dark matter, the unseen substance that constitutes approximately 85% of all matter present.

The investigation was driven by cosmologist Vera Gluscevic, an associate professor at the USC Dornsife College of Letters, Arts, and Sciences; along with Ethan Nadler, previously a postdoctoral researcher at USC and Carnegie Observatories, who is now an assistant professor at the University of California, San Diego; and Andrew Benson, a research scientist at Carnegie Observatories.

This simulation initiative is dubbed “COZMIC,” which is an acronym for “Cosmological Zoom-in Simulations with Initial Conditions beyond Cold Dark Matter.”

For decades, scientists have been aware of dark matter’s existence — yet until now, they were unable to explore how galaxies form and develop in a universe where dark and ordinary matter interact. COZMIC has enabled this exploration, according to the research team.

The launch of COZMIC and the findings from the team are outlined in a series of studies published on Monday in The Astrophysical Journal, a journal of the American Astronomical Society.

The core of dark matter

Researchers acknowledge the reality of dark matter due to its influence on the motion and cohesion of galaxies. For instance, galaxies rotate with such velocity that they should disband, yet they remain intact. Something imperceptible binds them together; many researchers conjecture that dark matter is central to this concept — a notion first posited in 1933 by Swiss scientist, Fritz Zwicky. Studies on dark matter have progressed significantly since then.

Investigating dark matter presents challenges as it does not emit light or energy that can be easily observed. Scientists analyze dark matter by observing its effects on the motion and formations of structures like galaxies. Nevertheless, this is somewhat akin to studying an individual’s shadow without being able to scrutinize in depth the actual person casting it.

In the series of studies, the research group took the initiative to implement new physics — not just conventional particle physics and relativity — and programmed a supercomputer to develop intricate cosmological simulations via COZMIC to evaluate various theories concerning the potential behaviors of dark matter.

“Our goal is to ascertain the masses and additional quantum characteristics of these particles, along with understanding how they interact with everything else,” Gluscevic stated. “With COZMIC, we are for the first time capable of simulating galaxies similar to ours under drastically different physical conditions — and test

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those regulations opposing genuine astronomical observations.

Alongside Gluscevic, Nadler, and Benson, the group behind COZMIC includes Hai-Bo Yu from UC Riverside; Daneng Yang, who previously worked at UC Riverside and is now at Purple Mountain Observatory CAS; Xiaolong Du from UCLA; and Rui An, who was formerly affiliated with USC.

Various dark matter hypotheses

“Our simulations indicate that observations of the tiniest galaxies can help differentiate dark matter theories,” Nadler noted.

For the research conducted with COZMIC, the researchers considered these dark matter behavior hypotheses:

• Billiard-ball model: In this initial study, each dark matter particle collides with protons in the universe’s early phases, akin to billiard balls when they first start moving. This interaction smooths out small-scale structures and eradicates satellite galaxies within the Milky Way. This research also encompasses scenarios where dark matter is traveling at elevated velocities, as well as others in which it consists of extremely lightweight particles.
• Mixed-sector model: This second investigation presents a hybrid scenario in which some dark matter particles interact with conventional matter, while others traverse through it.
• Self-interacting model: For this third analysis, the scientists simulated a scenario where dark matter interacts with itself both at the beginning of time and in the present, altering galaxy formation throughout cosmic history.

While processing these simulations, the researchers incorporated new physical principles into the supercomputer to generate a galaxy whose structure reflects the signatures of those interactions between normal and dark matter, expressed Benson.

Gluscevic commented: “While numerous prior simulation suites have investigated the effects of dark matter mass or self-interactions, up until now, none have modeled dark matter interactions with normal matter. Such interactions are neither exotic nor implausible; they likely exist.”

A new era for dark matter

The group asserts that this is a significant advancement in comprehending the true nature of dark matter. They aspire that by comparing their twin galaxies to actual telescope imagery, they can draw nearer to unraveling one of the universe’s greatest enigmas.

“We can finally pose the question, ‘Which iteration of the universe resembles ours the most?’” Gluscevic remarked.

The COZMIC team intends to broaden their research by directly validating the forecasts from their simulations against telescope data, searching for signs of dark matter behavior in actual galaxies.

This subsequent phase may bring scientists closer than ever to grasping what dark matter is and how it influences the cosmos.