Dark matter: How primordial black holes can explain a long-standing mystery

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For about 50 years, the scientific community has been grappling with a big problem: there isn’t enough visible matter in the universe.

All the matter we can see — stars, planets, cosmic dust, and everything in between — can’t explain why the universe behaves the way it does, and there would have to be five times as much matter around us for researchers’ observations to make sense. According to NASA. Scientists call this substance dark matter, because it does not interact with light and is invisible.

In the 1970s, American astronomers Vera Rubin and… Kent Ford discovered the existence of dark matter by looking at stars orbiting the edge of spiral galaxies. They noted that these stars were moving too fast to be held together by the galaxy’s visible matter and gravity, and should have been blown away instead. The only explanation was that there was a large amount of invisible matter holding the galaxy together.

“What you see in a spiral galaxy” Robin said At the time, “that’s not what you get.” Her work relied on a hypothesis formulated by Swiss astronomer Fritz Zwicky in the 1930s, and began the search for the elusive matter.

Since then, scientists have been trying to directly observe dark matter and even build it Great appliances To discover it, but so far, we have had no luck.

Early in the research, the famous British physicist Stephen Hawking hypothesized that dark matter could be hiding in black holes – the main subject of his work – formed during the big bang.

Now, a new study by researchers from MIT has brought the theory back into the spotlight, revealing what these primordial black holes are made of, and potentially discovering a whole new type of exotic black hole in the process.

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

“We were taking advantage of Stephen Hawking’s famous calculations about black holes, especially his important result about the radiation that black holes emit,” Kaiser said. “These strange black holes arise from trying to address the dark matter problem. They are a byproduct of the dark matter explanation.”

Scientists have made many guesses about what dark matter is, ranging from unknown particles to extra dimensions. But Hawking’s theory about black holes only recently came into effect.

“People didn’t take it seriously until maybe 10 years ago,” said study co-author Elba Alonso Monsalve, a graduate student at MIT. “This is because black holes once seemed really far-fetched. In the early 1900s, people thought they were just a fun mathematical fact, not just a physical fact.”

We now know that almost every galaxy contains a black hole at its center, Einstein researchers discovered Gravitational waves Created by the collision of black holes in 2015 – a landmark discovery – it showed that they are everywhere.

“In fact, the universe is full of black holes,” Alonso Monsalve said. “But no dark matter particle was found, even though people looked in all the places they expected to find it. This does not mean that dark matter is not a particle, or that it is black holes for sure. It could be a combination of both. But now, “Black holes are being taken seriously as candidates for dark matter.”

last Modern studies Hawking’s hypothesis is confirmed to be true, but the work of Alonso Monsalvi and Kaiser, a professor of physics and professor of the history of science at the Germshausen Institute at MIT, goes a step further and looks into exactly what happened when primordial black holes first formed. .

the Stadypublished June 6 in the journal Physical Review Letters, reveals that these black holes must have appeared in the first five-millionth of a second of the big bang: “This is very early, much earlier than the moment when protons and neutrons appeared,” Alonso Monsalve said. “The molecules that make up everything were formed.”

She added that in our daily world, we cannot find disintegrated protons and neutrons, which are elementary particles. However, we know that it is not, because it is made up of smaller particles called quarks, held together by other particles called gluons.

“You cannot find quarks and gluons alone and free in the universe now, because it is too cold,” Alonso Monsalvi added. “But early in the Big Bang, when it was very hot, they could have been found alone and free. So primordial black holes were formed by the absorption of free quarks and gluons.

Such a configuration would make it radically different from the astrophysical black holes that scientists usually observe in the universe, which are the result of the collapse of stars. The primordial black hole would also be much smaller, with only the asteroid’s mass, on average, condensed to the size of one atom. But if enough of these primordial black holes did not evaporate at the beginning of the big bang and survive to this day, they could be responsible for all or most of the dark matter.

While the primordial black holes were forming, another type of invisible black hole must have formed as a sort of byproduct, according to the study. This would have been smaller, just a block A unicorncondensed to less than the size of one proton.

These tiny black holes, because of their small size, were able to capture a rare and strange property of the quark-gluon soup in which they formed, called “color charge.” It is a state of charge that is limited to quarks and gluons, and is never found in ordinary objects, Kaiser said.

This color charge would make it unique among black holes, which usually have no charge of any kind. “It is inevitable that these smaller black holes would have also formed, as a byproduct (of the formation of primordial black holes), but they would no longer exist today, because they would have already evaporated,” Alonso Monsalve said.

However, if it was still about ten millionths of a second away from the big bang, when the protons and neutrons formed, it could have left observable fingerprints by changing the balance between the two types of particles.

“The balance between the number of protons and the number of neutrons created is very delicate, and depends on what other matter was present in the universe at the time,” she added. “If these black holes with colored charges were still around, it could change the balance between protons and neutrons.” (For one or the other), which is just enough to measure that in the next few years.”

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

“The formation of a cluster of black holes is an extremely violent process that would send enormous ripples into the surrounding space-time. These rates will diminish over the course of cosmic history, but not to zero,” Kaiser said. “The next generation of gravity detectors could take a look.” “A glimpse of small-mass black holes — an exotic state of matter that was an unexpected byproduct of more ordinary black holes that could explain today’s dark matter.”

What does this mean for ongoing experiments trying to detect dark matter, e.g LZ dark matter experiment In South Dakota?

“The idea of ​​strange new particles remains an interesting hypothesis,” Kaiser said. There are other types of large experiments, some under construction, looking for innovative ways to detect gravitational waves. These may actually pick up some stray signals from the extremely violent formation process of primordial black holes.

Alonso Monsalvi added that there is also the possibility that primordial black holes are just a small portion of dark matter. “It doesn’t really have to be the same,” she said. “There is five times more dark matter than normal matter, and normal matter is made up of a whole bunch of different particles. So why does dark matter have to be one type of object?”

Primordial black holes regained popularity with the discovery of gravitational waves, yet little is known about their formation, according to Nico Capellotti, an assistant professor in the Department of Physics at the University of Miami. He did not participate in the study.

“This work is an interesting and viable option for explaining elusive dark matter,” Capellotti said.

The study is exciting and suggests a new mechanism for the formation of 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 in our universe today was created in the first three minutes, and if enough of these primordial black holes had existed even then, they would have affected that process and those effects might have been detectable,” Natarajan said. .

“The fact that this is an observationally testable hypothesis is what I find really exciting, apart from the fact that this suggests that nature likely made black holes starting from the earliest times via multiple pathways.”