Exotic stars could be powered by dark matter
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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 hints about the true nature of dark matter itself.
Normal stars form when a cloud of gas collapses in on itself and the centre gets so dense that it sparks nuclear fusion. That fusion powers the star by pumping huge 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, especially dark matter. If the cloud that collapsed to form a star had enough dark matter inside, the dark matter would begin smashing together and annihilating well before fusion could start, emitting enough energy to make the dark star glow and keep it from collapsing further.
The formation of a dark star would be fairly straightforward, and now a team led by Katherine Freese at at the University of Texas at Austin has worked out what its demise might look like.
In a massive regular star, once hydrogen and helium run out, the star goes on to fuse together 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.
Not so for dark stars. “You can take an ordinary, solar-mass sort of star, put some dark matter into it so the power source for that star is not nuclear reactions but dark matter annihilation, and you can keep feeding it. As long as you keep feeding it with enough dark matter too, it’ll never go through the nuclear evolution that gets it in trouble,” 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, the gravitational field of an object doesn’t grow straightforwardly with mass – gravity begets more gravity. Eventually, an object gets so big that it becomes unstable, and any small perturbation can cause gravity to take over and collapse it into a black hole. The researchers calculated that for dark stars, this should happen at masses between 1000 and 10 million times that of the sun.
That mass range makes supermassive dark stars an excellent contender to explain one of the great mysteries of the early universe: supermassive black holes. Astronomers have spotted enormous black holes extremely early in the universe’s history, but it is unclear how they could have formed so quickly. One of the leading hypotheses is that rather than forming from normal stars, they were made from some sort of enormous “seed”.
“If you have a black hole of 100 solar masses, how the hell are you going to get up to 1 billion solar masses in a few hundred million years? It’s just not possible if you’re only making black holes from standard stars,” says Freese. “Whereas, if you’re starting with pretty big seeds, that really makes a difference.” Dark stars could be those seeds.
But that isn’t the only mystery in the early universe that could be addressed by dark stars. The James Webb Space Telescope (JWST) has also spotted two other unexpected types of objects, which have been nicknamed 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.
Like supermassive black holes, though, these objects are too distant, and therefore too early in the universe’s history, for us to easily explain how they formed – there simply hadn’t been enough time. From the observations we have of them, Freese and another group of colleagues calculated that both little red dots and blue monsters could 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 a particular 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 did find hints of that absorption in initial JWST observations of several of these distant objects, but the data is too noisy to say for sure that it is there. “Right now, all the candidates that we have, there are two things that could fit the spectra equally well: one supermassive dark star or an entire galaxy of regular stars,” says Freese. “If you see this one dip, for sure that is not one galaxy full of normal stars, that is a dark star. But for now all we have is a pathetic little hint.”
We cannot say that we have definitely detected dark stars yet, but this is a step forward. “This isn’t some profound, unambiguous 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 do point in that sort of direction,” says Dan Hooper at the University of Wisconsin-Madison.
To determine whether or not these objects really are dark stars, we will need more observations, ideally at higher sensitivities, but it isn’t yet clear whether JWST is capable of reaching the necessary level of detail for galaxies – or dark stars – this far away.
“Confirming dark star existence would be a major discovery,” says Volodymyr Takhistov at the High Energy Accelerator Research Organization in Japan. It could open a new observational window on fundamental physics, he says. That is because dark stars could not only solve the cosmic mysteries of supermassive black holes, little red dots and blue monsters, but we also could use them to probe the nature of dark matter, about which we currently know very little.
That is particularly the case if they are the seeds for supermassive black holes. Freese, Fuller and their team calculated that the mass at which they would collapse and form black holes is dependent on the mass of the dark matter particles annihilating at their cores. This means we could use supermassive black holes to measure, or at least constrain, the properties of dark matter. Of course, first we have to confirm that dark stars even exist. “If these things are out there, they’re rare,” says Hooper. “Rare, but extraordinary.”
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