Dark matter doesn’t do much of anything in the current universe. But in the early days of the universe there may have been pockets of dark matter with a density high enough to provide a heat source for newly formed stars. Welcome to the strange and wonderful world of “Dark Stars”.
dark matter dance
The simplest dark matter models are incredibly boring. He just… sits there, pulls. We only know about its existence through the subtle gravitational effect on galaxies and larger structures in the universe. It does not react with light, with ordinary matter, or even with itself. At this very moment, you might be swimming in a vast sea of dark matter particles, and you wouldn’t even know it.
But this simplified picture of dark matter has some problems. When astrophysicists run computer simulations of galaxy formation including dark matter, they find that if dark matter particles are too boring, they don’t quite match up with reality. The nuclei of galaxies become denser than we observe, and typical galaxies have many more satellites than we see.
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So maybe dark matter is a little complicated. It may still not react with light or with ordinary matter, but it may sometimes react with itself. However, this self-interaction cannot be very strong, otherwise the dark matter will either clump into very small balls or will self-destruct long ago.
This “interacting but not much” hypothesis makes it difficult for astronomers to come up with ways to test the scenario. Fortunately, astronomers are very smart people.
youth and darkness
The very early universe, when it was only a few hundred million years old, was very different from today. First, it was more dense, with all the matter of the universe stuffed into a much smaller volume. Second, it was much darker, because stars and galaxies had not yet formed.
At the time, the universe consisted of dark matter (whatever it was) and neutral hydrogen and helium. Slowly, over the course of eons, all of that material began to break down due to gravity, forming ever-larger structures. The first stars began to form dense clumps of no more than a thousand parts the size of the Sun. In the traditional picture of the formation of the first stars, these masses steadily grew to become giants a hundred times the size of the Sun, fueled by nuclear fusion in their core.
But a team of astrophysicists realized the traditional story might be different, they report in a new research paper appearing recently in the preprint journal arXiv. If dark matter interacts with itself, then when dark matter particles collide they release very little energy. Not every collision results in much, but in the early days of the universe, star-forming sites probably had a density high enough to annihilate dark matter to be a major player.
In this scenario, the first stars are not powered by nuclear fusion, but instead by the elimination of dark matter in their cores. The research team calls them “dark stars,” although the stars themselves are still made mostly of natural matter. These stars do not exist in the modern universe, because the density of dark matter is very low, so we cannot see them in the galaxy today.
But the researchers hope that the James Webb Space Telescope, designed specifically to study the early universe and the formation of the first stars, will be able to see these dark stars directly.