How do you search for a substance that doesn't give off any kind of light, but has a gravitational influence that shapes galaxies? That's the challenge researchers face as they try to find and explain the mysterious substance called dark matter. They're wrestling with an invisible "something" that appears to make up much of all matter in the Universe.
Cosmologist Gluscevic and astronomer Ethan Nadler have come up with a way to test ideas about dark matter by creating supercomputer simulations of the Milky Way Galaxy. Their project is called "Cosmological Zoom-in Simulations with Initial Conditions Beyond Cold Dark Matter," or COZMIC, for short. They applied what they term "new physics" to the simulated Milky Way clones and used a supercomputer to make different models of the Milky Way than the one we are familiar with today.
“We want to measure the masses and other quantum properties of these particles, and we want to measure how they interact with everything else,” Gluscevic said. “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.”
The Challenge of Dark Matter
The discovery of dark matter in the early part of the 20th century is one of those developments that changed the way astronomers looked at galaxy formation and evolution. It's not something that they can point a telescope at and say, "Oh look, there's the dark matter." It just can't be seen. It can't be touched and it doesn't reflect or give off any kind of radiation. All it seems to do is exert a gravitational influence on things around it. That's the only way scientists know that it's there.
Dark matter's gravitational tug on galaxies affects their shapes—their morphologies—and their motions through space. Astronomers noticed that something was constraining the galaxies in ways they couldn't explain. Scientist Fritz Zwicky suggested something dark and invisible was doing the job. Later on, astronomer Vera Rubin and her team measured the effects on different galaxies, and that work led to the confirmation of dark matter's existence.
Today, astronomers can observe the effects of dark matter not only on galaxies, but on clusters of them, and indeed, on the large-scale structure of the Universe. Still, astronomers don't know what it is. There are several theories about it and about its role in the Universe. You may have heard of the term "cold dark matter", which is a hypothesized kind of dark matter that played a role in the cosmic evolution. Scientists suggest that this matter could be made up of axions (very light particles), massive compact halo objects (MACHOs, which probably don't exist but they haven't yet been disproven), and weakly interacting massive particles (WIMPS), which haven't been found or proven... yet.
How The Milky Way Clones Experimented with Dark Matter
So, we know that dark matter is doing something to galaxies, but we don't know what it is. The COZMIC project allows astronomers and cosmologists to study how galaxies form and evolve in a Universe with a lot of dark matter. The Milky Way clones essentially let astronomers twiddle with the parameters to see what kinds of galaxies get shaped in universes with different dark matter configurations. “We want to measure the masses and other quantum properties of these particles, and we want to measure how they interact with everything else,” Gluscevic said. “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.”
Since dark matter has been around since the beginning of the Universe, it's also important to test its roles throughout cosmic history. In the COZMIC studies, the team studied different types of dark matter behavior by tweaking the laws of physics just a bit. In one scenario, called the Billiard-ball model, every dark matter particle collides with protons in the early Universe. It's like the start of a giant game of cosmic billiard balls. Among other things, it changes the Milky Way by erasing the existence of satellite galaxies. There are other effects as well that reflect changing speeds of dark matter particles and the existence of low-mass particles.
In a second scenario, COZMIC tested the possibility of dark matter particles interacting with normal (baryonic) matter (like planets, atoms, etc.) while other particles simply passed right through. A third model had the dark matter particles interacting with themselves throughout cosmic history, which influenced galaxy formation in different ways than we know of today.
Simulations and the Future of Dark Matter Understanding
The COZMIC project has given astronomers new pathways toward understanding dark matter, according to Gluscevic. “While many previous simulation suites have explored the effects of dark matter mass or self-interactions," she explained, "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.”
The next steps will be to take these simulations and compare them to images from existing and future observatories. Those comparisons would give new insight into how dark matter factored into creating the Universe we know today. “We’re finally able to ask, ‘Which version of the universe looks most like ours?’” Gluscevic said.
For More Information
Astronomers Explore Different Physics on Simulated Clones of the Milky Way
COZMIC. I. Cosmological Zoom-in Simulations with Initial Conditions Beyond Cold Dark Matter
COZMIC. II. Cosmological Zoom-in Simulations with Fractional non-CDM Initial Conditions