Dark matter not really do a lot of everything in the current universe. But in the early days of the cosmos, there may have been dark matter with a density so high that they provided a heat source for newly formed stars. Welcome to the strange and wonderful world of ‘dark stars’.
A dance of dark matter
The simplest models of dark matter are incredibly boring. It’s just… sitting there, pulled. We only know of its existence from its subtle gravitational influence on galaxies and larger structures in the universe. It does not interact with light, with ordinary matter, or even with itself. Right now you might be swimming in a vast sea of dark matter particles, and you wouldn’t even know it.
But this simplistic picture of dark matter has some issues† When astrophysicists run computer simulations of the formation of galaxies, including dark matter, they find that if the dark matter particles are too dull, it won’t quite match reality† The cores of galaxies become much denser than we observe, and typical galaxies have far too many satellites than we see.
So maybe dark matter is just a little complicated. Maybe it still doesn’t interact with light or ordinary matter, but maybe it does sometimes interacts with itself† That self-interaction cannot be too strong; otherwise, however, the dark matter would have clumped together into tiny balls or had long since destroyed itself.
This “interaction, but not very much” hypothesis makes it challenging for astronomers to think of ways to: test the scenario† Fortunately, astronomers are very smart people.
Dark Matter: The Young and the Darkness
The extremely early Universe, when it was only a few hundred million years old, was very different from today. First, it was a lot denser, with all the material of the cosmos crammed into a much smaller volume. Second, it was a lot darker because no stars and galaxies had formed yet.
At the time, the universe was made up of dark matter (whatever it is) and neutral hydrogen and helium. Slowly, over the course of eons, all that material began to collapse under gravity and form larger and larger structures. The first protostars started out as dense clumps no larger than a thousandth the size of the sun. In the traditional view of the formation of the first stars, those clumps steadily grew into behemoths a hundred times the size of the Sun, powered by nuclear fusion at their cores.
But a team of astrophysicists have realized the traditional story may be different, as they reported in a new article recently published in the preprint magazine arXiv† When dark matter interacts with itself, the dark matter particles release a small amount of energy when they collide. Each collision doesn’t do much, but in the early days of the universe, the sites of star formation may have been dense enough for the destruction of dark matter to play a major role.
In this scenario, the first stars are not powered by nuclear fusion, but instead by destroying dark matter in their cores. The research team calls them “dark stars,” although the stars themselves are still mostly made up of normal matter. These stars do not exist in the modern universe because the density of dark matter is too low, so we cannot see them in the galaxy today.
But the researchers hope that the James Webb Space Telescope, specially designed to study the early Universe and the formation of the first stars, may be able to see these dark stars directly.
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