Supermassive dark matter stars may be lurking in the early universe

We may have seen the first hints of strange stars powered by dark matter. These so-called dark stars could explain several of the most mysterious objects in the universe, while also giving us clues about the true nature of dark matter itself.

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

Dark stars could have formed in a similar way in the early universe, when everything was denser, esp dark matter. If a cloud collapsing to form a star had enough dark matter inside, the dark matter would begin to break up and destroy itself long before fusion could begin, emitting enough energy to make the dark star shine and prevent further collapse.

Forming dark star would be quite simple, and now the team headed by Katherine Freese at the University of Texas at Austin worked out what its demise might look like.

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

Not so for dark stars. “You can take an ordinary solar-mass star, put some dark matter in it so that the source of energy for that star is not nuclear reactions but the destruction of dark matter, and you can keep feeding it. As long as you keep feeding it enough dark matter, it will never go through nuclear evolution to get it into trouble,” he 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 save these strange giants for only so long. According to Albert Einstein’s theory, the gravitational field of an object does not increase uniformly with its mass – gravity begets more gravity. Eventually, the object becomes so large that it becomes unstable, and any small disturbance can cause gravity to take over and collapse it into black hole. The researchers calculated that for dark stars this should occur at masses between 1000 and 10 million times that of the Sun.

That mass range makes supermassive dark stars an excellent candidate for explaining one of the great mysteries of the early universe: supermassive black holes. Astronomers spotted supermassive black holes very early in the universe’s history, but it’s not clear how they could have formed so quickly. One of the leading hypotheses is that instead of forming from normal stars, they are made from some kind of giant “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? That’s just not possible if you’re just making black holes out of standard stars,” Freese says. “However, if you’re starting with fairly large seeds, it really makes a difference.” Dark stars could be those seeds.

But that’s not the only mystery in the early universe that dark stars could solve. The James Webb Space Telescope (JWST) also spotted two more unexpected types of objects, which have been nicknamed small red dots that is, blue monsters. 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 too far away, and thus too early in the history of the universe, for us to easily explain how they came to be – there just hasn’t been enough time. From the observations we have of them, Freese and another group of colleagues calculated that the little red dots and blue monsters might actually be individual, extremely massive dark stars.

If they are dark stars, there should be a signature in their light. This signature has to do with the specific wavelength of light that dark stars, if they exist, should absorb. Ordinary stars – and galaxies full of them – are too hot to absorb this light.

Freese and her colleagues did indications of that absorption in the initial JWST observations of several of these distant objects, but the data is too noisy to say with certainty that it is there. “Right now, all the candidates we have, there are two things that could fit the spectrum equally well: a single supermassive dark star or a whole galaxy of ordinary stars,” Freese says. “If you see this drop, it’s certainly not a galaxy full of normal stars, it’s a dark star. But for now, all we have is a poor little hint.”

We cannot say that we have definitively discovered dark stars, but this is a step forward. “This isn’t some deep, unequivocal thing that smokes, but it’s a really well-motivated thing that they’re looking for, and there are some aspects of what JWST is seeing that point in that direction,” he says. Dan Hooper at the University of Wisconsin-Madison.

To determine whether these objects are indeed dark stars, we will need more observations, ideally at higher sensitivity, but it is not yet clear whether JWST is capable of reaching the necessary level of detail for galaxies – or dark stars – this far away.

“Confirming the existence of a dark star would be a great discovery,” he says Volodymyr Takhistov at the High Energy Accelerator Research Organization in Japan. This could open a new window for observing fundamental physics, he says. 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 the case if they are seeds for supermassive black holes. Freese, Fuller and their team calculated that the mass at which they would collapse to form black holes depends on the mass destruction of dark matter particles in their cores. This means we could use supermassive black holes to measure, or at least constrain, the properties of dark matter. Of course, we must first confirm that dark stars exist at all. “If these things exist, they’re rare,” says Hooper. “Rare, but extraordinary.”

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