'Fuzzy' dark matter might make stars form in giant 'pancakes'

A model of exotic dark matter suggests that the first stars may have formed not as individuals, but as tiny pockets embedded in gigantic, pancake-like sheets. This would have led to the formation of truly gigantic stars that the James Webb Space Telescope may be able to detect, a research team says.

Astronomers have a wealth of evidence to suggest that the vast majority of all matter in the universe is dark matter, meaning it does not interact with light or normal matter. For example, stars whip around the centers of their galaxies far too quickly given the gravity of all the matter we can see. The same thing happens when we observe the motions of galaxies within clusters. And the cosmic web, the large-structure arrangement of galaxies throughout the universe, appeared and developed far too quickly given the meager amount of gravity provided by all the visible objects.

So a large portion of our universe is invisible, but we do not yet know what that dark portion is made of. One popular suggestion is known as cold dark matter, which means that the dark matter is made of some kind of exotic particle that generally travels much slower than the speed of light. While this model is enormously successful — it can explain all the strange observations of galaxies and structures — it does have some shortcomings.

Related: Dark matter can form tiny, cold 'clumps.' Scientists have found the smallest ones yet.

For one, the cold dark matter model struggles at scales smaller than galaxies. For example, the model predicts far more material in the centers of galaxies than we observe and predicts far more small satellite galaxies than we can detect.

One idea to get around this is to make cold dark matter a little "fuzzy." If the dark matter is made of an incredibly tiny particle — say, 10^22 times smaller than an electron — then it would be light enough that its quantum mechanical wave-like nature would appear at large scales. So instead of these particles existing as point-like objects, they would be fuzzy, and their identities would be spread out over regions as large as 1,000 light-years.

A new recipe 

By making dark matter fuzzy, this wave nature of the particle effectively smears it out over large distances, which solves many of the build-up problems faced by cold dark matter. In other words, this model prevents dark matter from building structures smaller than 1,000 light-years.

Because this model has been designed to explain existing observations, to do the job of science, we must go out and find some new way to test the idea. That's the motivation behind a new paper submitted for publication to The Astrophysical Journal Letters and available as a preprint via arXiv.

In the paper, the astronomers developed computer simulations of the early universe and the appearance of the first stars. They allowed dark matter to be "fuzzy" and watched how that changed the evolution of normal matter and the development of stars.

Stars and galaxies need dark matter to form. Because the universe is constantly expanding, you need a lot of gravity to pull a clump of gas together to get high enough densities to trigger fusion and the beginning of star formation. And there simply isn't enough normal matter in the universe to make that happen. But clumps of dark matter in the early universe serve as gravitational incubators, attracting enough normal matter to form stars and galaxies.

So if you change the properties of dark matter, like by making it fuzzy, you change how stars and galaxies evolve.

Lumps in the batter 

In their simulations, the researchers found that when dark matter becomes fuzzy, it changes the narrative of how stars form. In regular cold dark matter, stars first shine buried deep within tiny individual pockets scattered throughout the cosmos. But with fuzzy dark matter, gigantic two-dimensional sheets resembling pancakes form first.

The pancake then quickly fragments into individual pockets that eventually develop into stars. So, no matter what, you populate a universe with a collection of stars, just like in normal cold dark matter scenarios. But the researchers found a key observable difference.

Because the two-dimensional pancakes have so much mass and they collapse so quickly, the first generation of stars are much bigger than cold dark matter scenarios predict. These first stars in fuzzy dark matter models can reach up to a million times the mass of the sun, where cold dark matter can produce, at best, stars a few hundred times bigger than the sun.

Because of their enormous sizes, the stars would not live long. And in a blink, the first generation of stars would disappear in a furious storm of supernova explosions. From there, with the pancakes dissipated, normal star formation would begin and the universe would start to look more like our own. 

Although the James Webb Space Telescope won't be able to directly observe the first stars to appear in the universe, it is capable of imaging some of the first galaxies, which might contain a few remnants of the primordial generation of stars. The researchers predict that if Webb sees no first-generation stars at all, that might be evidence for the team's scenario, because in their model, all of the first-generation stars die quickly.

Alternatively, Webb might be able to detect the remnants of the radiation from the intense round of supernovas.

When it comes to dark matter, though, it's impossible to tell what the universe might cook up.

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