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For about 50 years, the scientific community has been grappling with a major problem: there is not enough visible matter in the universe.
All the matter we can see – stars, planets, cosmic dust and everything else – cannot explain why the universe behaves the way it does, and there must be five times as much of it for observations researchers make sense. according to NASA. Scientists call this dark matter because it does not interact with light and is invisible.
In the 1970s, American astronomers Vera Rubin and W. Kent Ford confirmed the existence of dark matter by observing stars orbiting spiral galaxies. They noted that these stars were moving too fast to be held together by the galaxy’s visible matter and its gravity – they should have separated instead. The only explanation was a large amount of invisible matter, binding the galaxy together.
“What you see in a spiral galaxy,” Rubin said at the time, “is not what you get.” His work built on a hypothesis formulated in the 1930s by Swiss astronomer Fritz Zwicky and launched the search for this elusive substance.
Since then, scientists have been trying to observe dark matter directly and have even built large devices to detect it – but so far, without success.
Early in his research, the famous British physicist Stephen Hawking postulated that dark matter could be hidden in black holes – the main subject of his work – formed during the Big Bang.
Archives Bettmann/Getty Images
The late physicist Stephen Hawking hypothesized that dark matter might be hiding in black holes formed during the Big Bang.
Now, a new study by researchers at the Massachusetts Institute of Technology has brought the theory back into the spotlight, revealing what these primordial black holes were made of and potentially discovering a whole new type of exotic black hole.
“It was really a wonderful surprise,” said David Kaiser, one of the study’s authors.
“We were using Stephen Hawking’s famous calculations on black holes, particularly his important results on the radiation emitted by black holes,” Kaiser said. “These exotic black holes emerge from the attempt to solve the dark matter problem – they are a byproduct of the dark matter explanation.”
Scientists have proposed many hypotheses about what dark matter might be, ranging from unknown particles to extra dimensions. But Hawking’s black hole theory has only recently come into use.
“People didn’t really take it seriously until maybe 10 years ago,” said Elba Alonso-Monsalve, study co-author and MIT graduate student. “And that’s because black holes once seemed really elusive: in the early 20th century, people thought they were just a fun math fact, nothing physical.”
We now know that almost every galaxy has a black hole at its center, and researchers’ discovery of Einstein’s gravitational waves created by colliding black holes in 2015 – a historic discovery – clearly showed that they are everywhere .
“Actually, the universe is full of black holes,” Alonso-Monsalve said. “But the dark matter particle was not found, even though people looked everywhere they expected to find it. This is not to say that dark matter is not a particle, nor that it is certainly black holes. It could be a combination of both. But now black holes, as candidates for dark matter, are taken much more seriously.
Other recent studies have confirmed the validity of Hawking’s hypothesis, but the work of Alonso-Monsalve and Kaiser, professor of physics and Germeshausen Professor of the History of Science at MIT, goes even further and examines exactly what happened when the primordial black holes first formed.
The study, published June 6 in the journal Physical Review Letters, reveals that these black holes must have appeared in the first quintillionth of a second of the big bang: “This is very early, and well before the moment when protons and neutrons, the particles that everything is made of, formed,” Alonso-Monsalve said.
In our everyday world, we don’t find separate protons and neutrons, she added, and they act like elementary particles. However, we know that they are not, because they are made up of even smaller particles called quarks, linked together by other particles called gluons.
“You can’t find single, free quarks and gluons in the universe right now because it’s too cold,” Alonso-Monsalve added. “But at the beginning of the Big Bang, when it was very hot, you could find them alone and free. So primordial black holes formed by absorbing free quarks and gluons.
Such a formation would make them fundamentally different from the astrophysical black holes that scientists normally observe in the universe and which are the result of collapsing stars. Additionally, a primordial black hole would be much smaller – only the mass of an asteroid, on average, condensed into the volume of a single atom. But if enough of these primordial black holes didn’t evaporate at the start of the Big Bang and survive to the present day, they could account for all or most of dark matter.
According to the study, when primordial black holes formed, another type of previously unseen black hole must have formed as a sort of byproduct. These would have been even smaller – just the mass of a rhino, condensed into less than the volume of a single proton.
These tiny black holes, because of their small size, could have captured a rare and exotic property of the quark-gluon soup in which they formed, called “colored charge.” This is a charge state exclusive to quarks and gluons, never found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which generally have no charge of any kind. “It is inevitable that these even smaller black holes also formed, as a byproduct (of the formation of primordial black holes),” Alonso-Monsalve said, “but they would not be there today, because they would have already evaporated.”
However, if they were still only about ten millionths of a second from the big bang, when the protons and neutrons formed, they could have left observable signatures by changing the balance between the two types of particles.
“The balance between the number of protons and the number of neutrons produced is very delicate and depends on what other elements existed in the universe at that time. If these color-charged black holes were still there, they could have shifted the balance between protons and neutrons (in favor of one or the other), just enough for us to be able to measure it in the coming years,” she added.
The measurement could come from ground-based telescopes or sensitive instruments on orbiting satellites, Kaiser said. But there could be another way to confirm the existence of these exotic black holes, he adds.
“Creating a population of black holes is a very violent process that would send huge ripples through the surrounding space-time. These would attenuate over the course of cosmic history, but not to zero,” Kaiser said. “The next generation of gravitational detectors could glimpse small-mass black holes – an exotic state of matter that was an unexpected byproduct of the more mundane black holes that could explain dark matter today. »
What does this mean for ongoing experiments that try to detect dark matter, like the LZ Dark Matter experiment in South Dakota?
“The idea that there are new exotic particles remains an interesting hypothesis,” Kaiser said. “There are other types of large-scale experiments, some of which are under construction, that are looking for sophisticated ways to detect gravitational waves. And these could indeed capture some of the spurious signals from the very violent formation process of primordial black holes.
It’s also possible that primordial black holes represent only a fraction of dark matter, Alonso-Monsalve added. “It doesn’t have to be the same,” she said. “There is five times more dark matter than ordinary matter, and ordinary matter is made up of a multitude of different particles. So why should dark matter be only one type of object?
Primordial black holes have regained popularity with the discovery of gravitational waves, but little is known about their formation, according to Nico Cappelluti, an assistant professor in the physics department at the University of Miami. He did not participate in the study.
“This work provides an interesting and viable option for explaining the elusive dark matter,” Cappelluti said.
The study is exciting and suggests a new formation mechanism for the first generation of black holes, said Priyamvada Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University. She also did not participate in the study.
“All the hydrogen and helium we have in our universe today was created in the first three minutes, and if enough of these primordial black holes were present until then, they would have had an impact on this process and these effects might be detectable,” Natarajan said. .
“The fact that this is an observationally testable hypothesis is what I find really exciting, apart from the fact that it suggests that nature has probably been creating black holes since ancient times through multiple pathways .”