Science

‘Fuzzy’ dark matter could cause stars to form into giant ‘pancakes’

The exotic dark matter model suggests that the first stars may not have formed individually, but as tiny pockets embedded in giant pancake-like sheets. According to the research team, this would lead to the formation of truly giant stars, which the James Webb Space Telescope could possibly detect.

Astronomers have plenty of evidence that the vast majority of all matter in the universe is dark matter, meaning it does not interact with light or ordinary matter. For example, stars revolve around the centers of their galaxies too quickly given the gravity of all the matter we can see. The same thing happens when we observe the movement of galaxies within clusters. And the cosmic web, a large structure of galaxies throughout the universe, came into being and evolved far too quickly given the meager gravitational force provided by all visible objects.

So, most of our Universe is invisible, but we don’t yet know what this dark part consists of. One popular assumption is known as cold dark matter, which means that dark matter is made up of some sort of exotic particle that typically travels much slower than the speed of light. Although this model is extremely successful – it can explain all the strange observations of galaxies and structures – it has some drawbacks.

Related: Dark matter can form tiny cold ‘clumps’. Scientists have found the smallest of them.

First, the cold dark matter model struggles with scales smaller than galaxies. For example, the model predicts much more material at the centers of galaxies than we observe, and predicts many more small satellite galaxies than we can detect.

One idea to get around this is to make cold dark matter a bit “fuzzy”. If dark matter is made up of incredibly tiny particles—say, 10^22 times smaller than an electron—then it would be light enough for its quantum mechanical wave-like nature to show up on a large scale. Thus, instead of these existing particles as point objects, they would be fuzzy and their identity would be scattered over regions up to 1000 light-years across.

New recipe

By making dark matter fuzzy, this wavelike nature of the particle effectively smears it out over long distances, which solves many of the accumulation problems that cold dark matter faces. In other words, this model prevents dark matter from building structures smaller than 1,000 light-years.

Since this model was designed to explain existing observations, in order to do the job of science, we must find some new way to test this idea. This is the motivation for a new paper submitted for publication in The Astrophysical Journal Letters and available as a preprint via arXiv.

In the article, astronomers developed computer simulations of the early universe and the appearance of the first stars. They allowed dark matter to be “fuzzy” and observed how this changed the evolution of normal matter and the development of stars.

Dark matter is needed to form stars and galaxies. Since the universe is constantly expanding, you need a lot of gravity to pull the clump of gas together to get high enough densities to cause fusion and start star formation. And there simply isn’t enough normal matter in the universe for that to happen. But clumps of dark matter in the early universe serve as gravitational incubators, pulling in enough ordinary matter to form stars and galaxies.

Therefore, if you change the properties of dark matter, for example, make it fuzzy, you will change the evolution of stars and galaxies.

lumps in dough

In their simulations, the researchers found that when dark matter becomes fuzzy, it changes how stars form. In ordinary cold dark matter, stars first shine deep inside tiny individual pockets scattered throughout the cosmos. But fuzzy dark matter first forms giant two-dimensional pancake-like sheets.

The pancake then quickly disintegrates into individual pockets, which eventually turn into stars. So, no matter what, you populate the universe with a set of stars, just like in normal cold dark matter scenarios. But the researchers found a key observed difference.

Because 2D pancakes have so much mass and collapse so quickly, first-generation stars are much larger than cold dark matter scenarios would predict. These first stars in fuzzy dark matter models can reach a million times the mass of the Sun, and cold dark matter can at best produce stars several hundred times the size of the Sun.

Due to their huge size, stars do not live long. And in the blink of an eye, the first generation of stars would disappear in a furious storm of supernova explosions. From there, when the pancakes dissipate, normal star formation will begin, and the Universe will become more like ours.

Although the James Webb Space Telescope will not be able to directly observe the first stars that appeared in the universe, it is able to image some of the first galaxies, which may contain the remnants of the primary generation of stars. The researchers predict that if Webb sees no first-generation stars at all, this could be evidence in favor of the team’s scenario, because in their model, all first-generation stars die quickly.

Alternatively, Webb could have detected the remnants of radiation from an intense supernova circle.

However, when it comes to dark matter, it’s impossible to say what the universe might have in store.

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