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 many 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 its center becomes so dense that it triggers nuclear fusion. This fusion powers the star by pumping enormous amounts of heat and energy into the surrounding plasma and gas.

Dark stars could have formed in the same way in the early universe, when everything was denser, especially dark matter. If the cloud that collapsed to form a star contained enough dark matter, the dark matter would begin to break up and annihilate long before fusion could begin, emitting enough energy to make the dark star glow and prevent it from collapsing further.

The formation of a dark star would be quite simple, and now a team led by Katherine Freese of the University of Texas at Austin has determined what its demise might look like.

In a massive regular star, once the hydrogen and helium are exhausted, the star fuses heavier elements until it eventually runs out of fuel and collapses to form a black hole. The more material you throw into the star, the faster this process takes place.

This is not the case for dark stars. “You can take an ordinary solar-mass star, put 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 can keep feeding it. As long as you keep feeding it with enough dark matter as well, it will never undergo the nuclear evolution that causes it problems,” says George Fuller of 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 a limited time. According to Albert Einstein’s theory, an object’s gravitational field does not increase directly with mass: gravity begets more gravity. Eventually, an object becomes so large that it becomes unstable, and any small disturbance can cause gravity to collapse and collapse into a black hole. The researchers calculated that for dark stars, this should happen at masses between 1,000 and 10 million times that of the sun.

This range of masses makes supermassive black stars an excellent candidate for explaining one of the great mysteries of the early universe: supermassive black holes. Astronomers spotted huge black holes very early in the history of the universe, but it’s not entirely clear how they were able to form so quickly. One of the main hypotheses is that instead of forming from normal stars, they were created from some sort of enormous “seed”.

“If you have a black hole that’s 100 solar masses, how are you going to get to 1 billion solar masses in a few hundred million years? It’s just not possible if you only create black holes from standard stars,” Freese says. “Whereas if you start with seeds that are big enough, it really makes a difference.” The dark stars could be these seeds.

But that’s not the only mystery of the early universe that could be solved by dark stars. The James Webb Space Telescope (JWST) also spotted two other types of unexpected objects, dubbed little red dots and blue monsters, respectively. They are both extremely distant objects and the immediate explanation for each is that they are compact galaxies.

However, like supermassive black holes, these objects are too distant, and therefore too old in the history of the universe, for us to easily explain how they formed – there simply wasn’t enough time. Based on the observations we have, Freese and another group of colleagues calculated that the little red dots and blue monsters could actually be extremely massive individual black stars.

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

Freese and his colleagues found hints of this absorption in early JWST observations of several of these distant objects, but the data is too noisy to say with certainty that they are there. “Right now, of all the candidates we have, there are two things that could fit the spectrum just as well: a supermassive dark star or an entire galaxy of regular stars,” Freese says. “If you see this dip, it’s definitely not a galaxy full of normal stars, it’s a dim star. But for now, all we have is a pathetic little clue.”

We can’t say yet that we’ve definitely detected dark stars, but it’s a step forward. “It’s not irrefutable, unambiguous proof, but it’s a very motivated thing that they’re looking for, and some aspects of what JWST sees point in that kind of direction,” says Dan Hooper of the University of Wisconsin-Madison.

To determine whether these objects are truly dark stars, we will need more observations, ideally at higher sensitivities, but it is not yet clear whether JWST is capable of achieving the level of detail needed for galaxies – or dark stars – so distant.

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

Mysteries of the Universe: Cheshire, England

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