Conventional wisdom holds that in the early universe, pools of dark matter—specifically, “cold” dark matter, a type of dark matter composed of sluggish, slow-moving particles—attracted gas that gave rise to the first stars. But every experiment to find cold dark matter has failed, leaving some astronomers wondering if they should instead be searching for alternative forms. If, early on, the universe had been filled with some entirely different type of dark matter, what kinds of galaxies would have followed?
In an April paper uploaded to the preprint server arXiv.org and submitted to the Monthly Notices of the Royal Astronomical Society, a group of theorists simulated how primordial galaxies would look if they formed inside clouds of three different types of alternative dark matter: “warm” dark matter, “fuzzy” dark matter and “interacting” dark matter with acoustic oscillations. By comparing them against cold dark matter galaxy simulations, the researchers discovered odd structural and chemical differences, galactic tweaks that the James Webb Space Telescope (JWST) might be able to see.
“It’s an extremely impressive amount of work,” said Ethan Nadler, a postdoctoral fellow at Carnegie Observatories and the University of Southern California, Los Angeles, who was not involved in the project. “The cool thing for me was seeing different dark matter models all explored simultaneously.”
In the 1970s, via her meticulous work measuring the rotation rates of galaxies, the astronomer Vera Rubin showed that in the absence of “extra” gravity that could come from some mysterious, unseen substance, most galaxies wouldn’t stay glued together. Many suspected the universe’s galaxies were seated within giant dark matter clouds, called “halos,” many times larger than the galaxies themselves. Most astronomers now think a stunning 84 percent of the universe’s matter is composed of this invisible stuff. Since then scientists have worked hard to uncover it—to no avail.
“On larger scales, everything is consistent with [cold dark matter],” says Romeel Dave, an astronomer at the University of Edinburgh in Scotland, who was not involved in the research. “But questions arise when you start to go to very small scales.”
“Small” here is relative, of course: simulations can replicate giant galaxy clusters and other large cosmic structures with astonishing fidelity, but on the smaller scales of individual galaxies, tantalizing inconsistencies challenge the cold dark matter model. For instance, the origin story may not be so simple for satellite dwarf galaxies, tiny galaxies orbiting larger ones, much like the moon orbits Earth. Across decades of searching, observers have generally found fewer satellite dwarf galaxies than predicted by theorists, spawning an astronomical dust-up called the “small-scale crisis.”
Enter alternative dark matter.
“Cosmologists love to invent crises,” Dave says, “but that’s basically the genesis of the idea for this paper, to try to understand the impact of alternative dark matter solutions to the small scale crisis. And it’s something we can test.”
Alternative dark matter was the original solution to the small-scale crisis because it could inhibit the mysterious substance from concentrating to form smaller-scale “fine” structures upon which satellite galaxies and other relatively tiny cosmic objects could coalesce. It turns out that tweaking cold dark matter in this way resolved many of the paradoxes. In all, the group studied three types of alternative dark matter: warm dark matter, fuzzy dark matter and interacting dark matter with acoustic oscillations.
Cold dark matter is torpid. It moves slowly. By virtue of such sluggishness, cold dark matter simply sits around, not doing much at all other than attracting and concentrating matter into galaxies and galaxy clusters. On the other end of the spectrum, hot dark matter moves at relativistic speeds, rendering it incapable of gathering most anything. Warm dark matter sits in the middle of these extremes, potentially exhibiting a wide range of velocities. The team chose warm dark matter parameters that permitted the halos to assemble galaxies with a velocity fast enough to hinder the formation of smaller-scale structures.
Fuzzy dark matter (also known as ultralight dark matter) is similar to cold dark matter in terms of velocity, except the mass of each particle is so minuscule that quantum effects become important—which gives it a remarkable wavelike quality. The extra pressure on the dark matter from the quantum effect also inhibits fine structures in dark matter, resulting in a fuzzy appearance.
Acoustic oscillation dark matter is especially subtle. This form of dark matter is able to interact with “dark radiation,” which acts as force carrier between the dark matter particles, creating an outward pressure on the dark matter. The tug of gravity and push of pressure lead to wavelike density patterns in a fluidic “dark plasma.” These density waves, subgalactic in size, are themselves a form of dark matter. The closest analogy is sound waves traveling in a fluid. The acoustic oscillations also suppress dense features.
Jacob Shen, a graduate student at the California Institute of Technology, Josh Borrow, a postdoc at the Massachusetts Institute of Technology, and their co-authors asked, How do early-universe galaxies born inside alternative dark matter halos start out, and what happens as they grow?
“It’s a pilot study but a very interesting one,” Dave says.
The group settled on a time period starting around a billion years after the big bang, when the universe’s first cosmic structures appeared—an era the group suspected would be its best shot at finding prominent differences. Starting from this point, the researchers simulated the formation of faint, primordial dwarf galaxies.
“The motivation for looking at dwarf galaxies is that they live in the smallest dark matter halos,” Nadler explains. “The changes should be readily reflected in dwarf galaxies that live in those tiny halos.”
To run their simulations, the researchers used a code called AREPO and the models IllustrisTNG and THESAN. The software divides virtual space into tiny, three-dimensional tessellations capable of moving and deforming, a style that allowed the group to intricately track primordial gas and dark matter physics in areas that required it—and to gloss over areas that didn’t.
Around two million CPU hours later, the simulations revealed unique structures and compositions.
As expected, slow-moving cold dark matter had the right properties to allow dense structures to form, while in alternative dark matter scenarios, these fine features were suppressed. Acoustic oscillation dark matter had the most detailed structures, followed by warm dark matter. The model for fuzzy dark matter produced the least detailed, fuzziest dark matter structures.
The simulations also revealed a new discovery: a connection between alternative dark matter types and starbursts, periods of extremely rapid star formation inside a galaxy. Alternative dark matter galaxies entered their starburst periods at later times than galaxies built around cold dark matter. But those later starbursts didn’t result in star-sparse galaxies—all the late-blooming alternate dark matter galaxies eventually caught up with star production. Some even ultimately experienced elevated star formation rates: those formed via fuzzy dark matter displayed late-but-great starbursts, resulting in three to four times more stars than would be expected from cold dark matter.
A delay in galactic starbursts has a knock-on effect, namely, a corresponding delay in the eruption of supernovae that enrich galaxies with heavy elements. So the various alternative dark matter halos constructed petite dwarf galaxies with low abundances of heavy elements. And as time continued to tick in the simulations, the later-time starbursts erupted into cores of metal-rich stars that were skirted with stars containing lower metallicity, compared with cold dark matter.
Although there are other possible ways for cold dark matter to end the small-scale crisis, such as better modeling of various galaxy-sculpting feedback mechanisms, one appeal of studying alternative dark matter models is the possibility of finding new phenomena that can be observationally tested.
In this specific case, JWST just might be up to the task of finding and studying some of the universe’s earliest dwarf galaxies, making comparisons between model-based predictions of their size, shape and chemical composition to reveal which flavor of dark matter actually prevails. The key will be linking JWST to far larger cosmic telescopes called gravitational lenses. Despite their fancy name, these are merely galaxy clusters so hefty that the mass-warped spacetime around them amplifies the light from background objects. Using JWST to look through suitable gravitational lenses luckily aligned with far-distant background galaxies from the early universe, astronomers just might glimpse the glow from small, primordial satellite galaxies—and with it, shed light on dark matter’s still mysterious true form.