How primordial black holes might explain the enduring mystery of dark matter | CNN

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For about 50 years, the scientific community has faced a fundamental problem: There is not enough visible matter in the universe.

All the matter we can see—stars, planets, cosmic dust, and everything in between—cannot explain why the universe behaves the way it does, and there must be five times as much around it for the researchers’ observations to make sense. according to NASA. Scientists call it 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 looking at stars orbiting at the edges of spiral galaxies. They noticed that these stars were moving too fast to be held together by the visible matter of the galaxy and its gravity – instead they must have flown away. The only explanation was a vast 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.” Her work built on a hypothesis formulated in the 1930s by Swiss astronomer Fritz Zwicky and launched a search for the elusive substance.

Since then, scientists have been trying to observe dark matter directly and even built large devices to detect it – but so far, with no luck.

At the beginning of the 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.

Now, a new study by researchers at the Massachusetts Institute of Technology has brought the theory back into the spotlight, revealing where these primitive black holes were created from and potentially discovering a whole new type of exotic black hole in the process. .

“It was really a wonderful surprise in that way,” said David Kaiser, one of the study’s authors.

“We were using Stephen Hawking’s famous calculations about black holes, especially his important result about the radiation that black holes emit,” Kaiser said. “These exotic black holes come out of trying to tackle the dark matter problem—they’re a byproduct of the dark matter explanation.”

Scientists have made many guesses about what dark matter might be, ranging from unknown particles to extra dimensions. But Hawking’s theory of black holes has only recently come into play.

“People didn’t really take it seriously until maybe 10 years ago,” said study co-author Elba Alonso-Monsalve, a graduate student at MIT. “And that’s because black holes used to seem really elusive — in the early 20th century, people thought they were just a mathematical fun 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 landmark discovery – made it clear that they are everywhere.

“In fact, the universe is filling up with black holes,” Alonso-Monsalve said. “But the dark matter particle has not been found, even though people have looked in all the places where they expected to find it. This does not mean that dark matter is not a particle, or that it is definitely a black hole. It can 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, a professor of physics and the Germeshausen Professor of the History of Science at MIT, goes a step further and looks at exactly what happened when primary black holes initially 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 really early and much earlier than when protons and neutrons, the particles from which everything is made were formed,” said Alonso-Monsalve.

In our everyday world, we can’t find protons and neutrons separated, she added, and they act like elementary particles. However, we know they are not, because they are made up of even smaller particles called quarks, held together by other particles called gluons.

“You can’t find quarks and gluons alone and free 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, they could be found alone and free. So primordial black holes were 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, which are the result of collapsing stars. Also, 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 a sufficient number of these primordial black holes did not evaporate in the early big bang and survive to the present day, they could make up all or most of the dark matter.

According to the study, during the creation of primordial black holes, another type of black hole unseen before must have formed as a kind of byproduct. These would have been even smaller – only the mass of a rhinoceros, condensed into less than the volume of a single proton.

These tiny black holes, because of their small size, would have been able to capture a rare and exotic property from the quark-gluon soup in which they were created, called “color charge.” It’s a state of charge that’s exclusive to quarks and gluons, never found in ordinary objects, Kaiser said.

This color charge would make them unique among black holes, which usually have no charge of any kind. “It’s inevitable that these even smaller black holes would have formed as well, as a by-product (of the formation of the primordial black holes),” Alonso-Monsalve said, “but they wouldn’t be around today, as had already evaporated”.

However, if they were still around ten-millionths of a second into the big bang, when protons and neutrons were formed, they could have left observable signatures by changing the balance between the two types of particles.

“The balance of how many protons and how many neutrons are made is very delicate and depends on what else was in the universe at the time. If these color-charged black holes were still around, they could have shifted the balance between protons and neutrons (in favor of one or the other), enough that in the next few years, we could measure it,” she added. .

The measurement could come from Earth-based telescopes or sensitive instruments on orbiting satellites, Kaiser said. But there may be another way to confirm the existence of these exotic black holes, he added.

“The creation of a population of black holes is a very violent process that would send huge ripples into the surrounding space-time. They will attenuate over cosmic history, but not to zero,” Kaiser said. “The next generation of gravitational detectors could catch a glimpse of low-mass black holes—an exotic state of matter that was a byproduct of unexpectedly of the most common black holes that could explain dark matter today.”

What does this mean for ongoing experiments trying to detect dark matter, such as 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 kinds of big experiments, some of which are under construction, looking for wonderful ways to detect gravitational waves. And they can indeed pick up some of the stray signals from the very violent process of primordial black hole formation.

There is also the possibility that primordial black holes are just part of dark matter, Alonso-Monsalve added. “It doesn’t have to be the same,” she said. “There is five times as much dark matter as regular matter, and regular matter is made up of a whole bunch of different particles. So why does dark matter have to be a single type of object?”

Primordial black holes have regained popularity with the discovery of gravitational waves, yet not much is known about their formation, according to Nico Cappelluti, an assistant professor in the University of Miami’s physics department. He was not involved in the study.

“This work is an interesting and viable option to explain the elusive dark matter,” Cappelluti said.

The study is exciting and proposes a new formation mechanism for the first generation of black holes, said Priyamvada Natarajan, Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University. She was also not involved in the study.

“All the hydrogen and helium we have in our universe today was created in the first three minutes, and if there were enough of these primordial black holes by then, they would have affected that process, and those effects could be detectable,” Natarajan said. .

“The fact that this is an observationally testable hypothesis is what I find really exciting, in addition to the fact that it suggests that nature likely creates black holes starting from very early times through multiple pathways.”