Astronomers say they have found evidence that stars were formed by the annihilation of dark matter particles. If this is true, it could help solve the cosmic mystery of why supermassive black holes appeared so early. In ancient times, and we’re talking a very long time ago, some time after the Big Bang, the universe may have been littered with strange star monsters. These stars were massive enough to swallow entire solar systems and were powered by dark matter rather than nuclear fusion like “normal” stars. More precisely, at the moment of self-annihilation, particles of this mysterious substance turned into fuel for the so-called “dark star”.
But when Katherine Freese, a theoretical astrophysicist at the University of Texas at Austin, first presented the theory at a conference in 2007, it didn’t cause much excitement. “I heard some graduate students call us crazy,” she says. However, the concept of a dark star stuck with Freeze. Over the past 16 years, she and her colleagues have been able to expand our understanding of these fascinating virtual objects. The problem was that it was still impossible to find evidence of the existence of these objects. Until recently, Freeze and his colleagues were discussing possible evidence of rare galaxies discovered by new telescopes. “Some of these objects may not be galaxies at all, but individual stars, dark stars,” says team member Gillian Poulin, then at Colgate University in New York. However, many astronomers still have doubts about this theory. “This is a very controversial concept,” says team leader Cosmin Illy of Colgate University. But if dark stars exist, they not only provide evidence for the existence of a special type of dark matter. It could also help solve one of science’s biggest problems. Cosmology – The mysterious origin of supermassive black holes that drive the evolution of galaxies. ” However, many astronomers still have doubts about this theory. “This is a very controversial concept,” says team leader Cosmin Illy of Colgate University. But if dark stars do exist, they not only provide evidence for the existence of a special type of dark matter. It could also help solve one of science’s biggest problems. Cosmology – The mysterious origin of supermassive black holes that drive the evolution of galaxies. ” Our universe is full of dark matter. We can’t see it because it doesn’t interact with light, but we know it’s there. It manifests itself through the curvature of space-time, creating a lensing effect that distorts the way we see other objects within our field of vision. “There’s a lot of evidence for the gravitational properties of dark matter,” says Pearl Sandick of the University of Utah. “It exists and we know where it is.”
Thanks to these observations, we now know that dark matter must make up about 27 percent of the universe. About 68 percent comes from the equally unexplained dark energy that drives the expansion of the universe, and only a small amount comes from the ordinary matter that makes up everything around us, including chairs, tables, and humans. It’s 5 percent. dark matter candidate Astrophysicists don’t know exactly what dark matter is, although they have a few ideas. At the end of the 20th century, a new option was proposed: weakly interacting large particles (WIMPs). WIMPs are 1,000 times more massive than protons, so they cannot interact with normal matter. But they can affect each other, which is very “violent”. Two such particles annihilate the moment they come into contact, producing a burst of energy in the form of gamma rays. Thanks to this theory, it is possible to predict how dark matter behaved in the early universe. Immediately after the Big Bang, about 13.8 billion years ago, the universe was filled with particles with no complex structure. Dark matter moves slightly more slowly than normal matter, so it clumps together more quickly under the influence of gravity.
During the first 200 million years of our universe’s existence, huge clumps of dark matter, called mini-halos, formed. Eventually, they absorbed the ordinary matter from which stars and galaxies are formed. Freeze hypothesized that if these tiny halos were filled with his WIMPs, an interesting process similar to the birth of normal stars could occur. “We realized there was a process that the astronomical community had overlooked,” she says. He was a completely new type of star. Star formation occurs through the condensation of collapsing clouds of dust and gas made up of ordinary matter. Gravity compresses the hydrogen and helium atoms, eventually reaching a threshold above which fusion begins and the star’s core is formed. In a star, the inward gravitational force is perfectly balanced by the outward force produced by nuclear fusion. However, if the density of the dark matter in some halo is high enough, the annihilation of the WIMP could generate enough energy that the outward force prevents normal matter from reaching a critical density. In this case, fusion would never start. Instead, a completely different class of objects will form and stabilize at a much larger size, perhaps with a diameter about the same as that of Saturn’s orbit and a million times more mass. More than the mass of our sun. “There is a heat source that prevents the cloud from collapsing further,” Freeze said. “It’s powered by dark matter.” The proportion of dark matter in such stars is small, only 0.1 percent. But this is enough for trillions and millions of annihilations to occur every second, causing stars to glow with incredibly bright white or blue light. In this sense, the term “dark star” is misleading. In the most extreme events in the early universe, dark stars became truly massive, sometimes outshining entire galaxies. “They could be a billion times brighter than the sun,” Freeze says.
Freeze has published many articles on dark stars, the first of which was published in 2008 with colleagues Douglas Spolier and Paolo Gondolo. Dan Hooper of the University of Chicago recalls that its publication sparked “intense debate.” “I would go to conferences and hear people yelling at each other,” he says. “I didn’t know what was right and what was wrong.” However, this idea received some support. “I think this is a great idea that is really possible,” Sandik says. This isn’t the only exotic star discovered either (see “The Strangest Stars” at the end of the article). There is good reason to believe that these hypothetical stars actually exist. Not only are dark stars an entirely new type of star, they may also provide a solution to a great cosmological mystery: the origin of supermassive black holes. These are massive black holes located at the centers of galaxies, so dense that not even light can escape them.
The mystery of supermassive black holes
When we look at the distant early Universe, we see galaxies less than a billion years old with a supermassive black hole at their center, one billion times the mass of the Sun. How these black holes reached such size remains a mystery. “The appearance of such a massive black hole in the very early days of the universe is a serious problem,” said Muhammad Latif of the United Arab Emirates University. They could have formed from the merger of smaller black holes, but as it turns out, there wasn’t enough time for that to happen. Dark stars could provide an answer to this question, as they could be the elusive “seed” of a supermassive black hole. Most of them lived early and died young, living only a few million years. A small dark star with about 100 times the mass of the Sun could be reborn as a normal star after the WIMP dies. “Fusion could start as soon as the dark matter fuel runs out,” Freeze says. The distant red spot discovered by JWST could be an exotic dark star. NASA/ESA However, all faint stars turn into black holes when they completely run out of fuel. In the case of supermassive dark stars, the resulting black hole is similarly massive, up to a million solar masses, given its enormous mass. “We have effectively solved the main problem of the existence of supermassive black holes,” Freeze says. “If we start with our faint stars, we get ‘seeds’ containing millions of solar masses, which can merge to create black holes containing billions of solar masses.” However, in the 2010s, the Dark Star theory reached a dead end. Other dark matter candidates emerged, and the WIMP hypothesis gradually became less clear. Since there has never been any evidence for the existence of WIMPs, many physicists have decided to use a lighter theoretical particle called an axion to explain dark matter. However, axions were too light to generate enough energy to form dark stars. In light of this, another group of scientists, including Bernard Carr of Queen Mary University of London, believe that dark matter is not made up of mysterious particles at all, but rather that it formed in the early universe and is similar to primordial black holes. We believe that it is made up of so-called black holes.
However, Freeze did not give up on exploring the future of dark stars. And now things are looking up. In 2022, she and her colleagues calculated that other possible dark matter particles could also form dark stars. “It doesn’t have to be a WIMP,” she says. Alternatively, dark matter is still considered a particle, but one that interacts with itself. Freeze says the concept of self-interacting dark matter is currently the most popular. So early last year, the most far-sighted “observer of the sky” noticed something odd. Traces of a dark star Launched in December 2021, the James Webb Space Telescope (JWST) will peer deep into the history of the universe thanks to a giant gold-plated mirror that uses infrared light. He had already observed galaxies that appeared in the first few hundred million years of the universe’s existence, much earlier than galaxies observed with other telescopes. These results showed that there were more bright objects in the early Universe than expected. “There is no question that the JWST results indicate that something strange is going on,” Kerr said. Galactic evolution was not expected to begin until a billion years after the Big Bang, but JWST appears to have discovered many galaxies that formed much earlier than the Big Bang. Before”. Last June, astronomers working on the JWST Advanced Extragalactic Exploration (JADES) reported the discovery of the earliest “handful” of new candidate galaxies, one of them just 320 million years after the Big Bang. It was reported that it was formed later. However, Freeze and his colleagues recently published a preprint (not yet peer-reviewed study) suggesting that the galaxy may not be a galaxy at all. They believe that the objects that appear as red spots in JWST are themselves dark stars. Scientists explain this by the fact that the three objects have a round shape, similar to stars, and are completely different from galaxies. Additionally, these objects fit a pattern of dark stars, and the amount of light they emit is as expected. According to experts, two of them have a mass a million times the mass of the Sun, and one has half that.
These can be very small galaxies, which can look like individual point sources due to insufficient resolution. “We are still leaning towards simpler options,” says Sandro Tacchella of the University of Cambridge, part of the JADES team. “Currently, there is no clear evidence to support the dark star model,” Marcia Rieke of the University of Arizona, also involved in the JADES project, is similarly cautious. She found that two of her three candidates actually look like small galaxies, and the third could just be a compact galaxy. “You need pretty strong evidence to claim that something is a dark star,” she says. Soon, JADES will obtain the second half of the data, including detailed spectra of these objects, measurements of the light they emit in different wavelength ranges. It may be possible to detect signs of an ionized form of helium known as helium II. If the mysterious object is indeed a faint star, astronomers would find that helium II absorbs light at certain wavelengths. If these were galaxies, Helium II would start emitting light at that wavelength. “The signature of the dark star is helium II,” says Pauline. There may be other dark star candidates. Ilie says he is working on a new paper presenting the latest observations of JWST. “There is no shortage of candidates,” he says. Currently, his JWST is the only one capable of discovering dark stars, but his NASA Rome Space Telescope, which debuts in 2027, may also be able to do so. For most astronomers, the idea of the existence of dark stars remains special. “Most people are conservative and prefer not to talk about strange things like dark stars,” says Kerr. Meanwhile, our universe is full of incredible objects, from black holes to magnetars. “We’re always surrounded by strange things,” says Kerr. So why can’t faint stars also exist?
the strangest stars Dark stars aren’t the only exotic stellar phenomena we can (or can’t) detect in the universe. star janus Last July, astronomers reported the discovery of a star that appeared to be split in half, with one half made of helium and the other half hydrogen. She appears to have two faces, like the ancient Roman god Janus. Last July, astronomers reported the discovery of a star that appeared to be split in half, with one half made of helium and the other half hydrogen. She appears to have two faces, like the ancient Roman god Janus. Perhaps the star was simply caught at a not-so-lucky moment, during the transition from burning helium to hydrogen. Many white dwarfs are thought to undergo this process. hybrid star These objects, also known as Landau-Sorn-Zytkoff objects, consist of neutron stars inside regular stars. It’s like a nesting doll. It was probably formed by the collision and merging of two stars. During their short lifetimes, they are visually indistinguishable from other large stars. Although they still remain hypothetical objects, such stars can only be discovered using gravitational wave measurements and observations of the elements inside the star. boson star At what point does a black hole stop being a black hole?If it is a bosonic star. Stars are usually made up of fermions, which are particles that make up matter. But bosons, the force-carrying particles of nature, can come together in nearly unlimited numbers to create transparent objects that do little except exert huge gravitational effects on everything around them. Such an object would be almost indistinguishable from a black hole. That’s why astronomers have not yet found an easy way to detect them.