Supermassive dark matter stars may be lurking in the early universe

We may have seen the first hints of the existence of strange stars powered by dark matter. These so-called dark stars could explain some of the most mysterious objects in the Universe, as well as give us clues about the true nature of dark matter itself.

Ordinary stars form when a cloud of gas collapses in on itself and the center becomes so dense that it causes nuclear fusion. This fusion powers the star, pumping enormous amounts of heat and energy into the surrounding plasma and gas.

Dark stars may have formed in a similar way in the early Universe, when everything was denser, especially dark matter. If there was enough dark matter in the cloud that collapsed to form a star, the dark matter would begin to collide and annihilate long before fusion began, releasing enough energy to cause the dark star to glow and prevent it from collapsing further.

Formation dark star would be quite simple, and now the team is led by Katherine Freese at the University of Texas at Austin have figured out what its decline might look like.

In a massive regular star, once it runs out of hydrogen and helium, the star continues to fuse heavier elements until it eventually runs out of fuel and collapses, forming a black hole. The more material you throw at a star, the faster this process happens.

This is not the case with dark stars. “You could take a regular solar-mass star, put some dark matter in it, so that the source of energy for that star is not nuclear reactions, but the annihilation of dark matter, and you could keep feeding it. As long as you keep feeding it enough dark matter, it will never go through nuclear evolution, which will cause problems for it,” says George Fuller at the University of California, San Diego, who was part of Freese's team.

But thanks to general relativity, dark matter can only save these strange giants for so long. According to Albert Einstein's theory, an object's gravitational field does not increase directly with mass—gravity begets more gravity. Eventually, the object gets so large that it becomes unstable, and any small disturbance can cause gravity to take over and collapse it. black hole. The researchers calculated that for dark stars this should occur at masses between 1,000 and 10 million solar masses.

This range of masses makes supermassive dark stars an excellent candidate for explaining one of the greatest mysteries of the early Universe: supermassive black holes. Astronomers have noticed huge black holes very early in the history of the Universe, but it's unclear how they could form so quickly. One of the leading hypotheses is that they were formed not from ordinary stars, but from some kind of huge “seed”.

“If you have a 100 solar mass black hole, how the hell are you going to get to 1 billion solar masses in a few hundred million years? It's simply not possible if you only create black holes from standard stars,” Freese says. “And if you start with fairly large seeds, it really makes a difference.” Dark stars could be these seeds.

But this is not the only mystery of the early Universe that dark stars could solve. The James Webb Space Telescope (JWST) also discovered two other unexpected types of objects, dubbed small red dots and blue monsters respectively. Both are extremely distant objects, and the immediate explanation for each is that they are compact galaxies.

However, like supermassive black holes, these objects are located too far away and therefore too early in the history of the Universe for us to easily explain how they formed – there just wasn't enough time. Based on our observations, Freese and another group of colleagues estimate that both the small red dots and the blue monsters may actually be individual, extremely massive dark stars.

If they are dark stars, there should be a signature in their light. This signature is associated with a specific wavelength of light that dark stars, if they exist, should absorb. Regular stars—and galaxies full of them—are too hot to absorb this light.

Freese and her colleagues discovered hints about this takeover in JWST's initial observations of some of these distant objects, but the data is too noisy to say with certainty that they are there. “Right now, among all the candidates we have, there are two things that could match the spectra equally well: a single supermassive dark star or an entire galaxy of ordinary stars,” Freese says. “If you see this dip, it's probably not just one galaxy full of normal stars, it's a dark star. But at the moment, all we have is a small, pathetic hint.”

We cannot yet say that we have definitely discovered dark stars, but this is a step forward. “It's not some deep and clear smoking gun, but it's a really well-motivated thing that they're looking for, and there are some aspects of what JWST is seeing that really point in that direction,” says Dan Hooper at the University of Wisconsin-Madison.

To determine whether these objects are indeed dark stars, we will need more observations, ideally with higher sensitivity, but it is not yet clear whether JWST can achieve the necessary level of detail for galaxies—or dark stars—that far away.

“Confirming the existence of a dark star would be a big discovery,” says Vladimir Takhistov at the High Energy Accelerator Research Organization in Japan. This could open a new window of observation in fundamental physics, he said. That's because dark stars could not only solve the cosmic mysteries of supermassive black holes, little red dots and blue monsters, but we could also use them to probe the nature of dark matter, about which we currently know very little.

This is especially true if they are the seeds of supermassive black holes. Freese, Fuller and their team calculated that the mass at which they collapse to form black holes depends on the mass Dark matter particles annihilate at their core. This means we could use supermassive black holes to measure, or at least constrain, the properties of dark matter. Of course, first we need to confirm that dark stars even exist. “If such things exist, they are rare,” Hooper says. “Rare, but unusual.”

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