How primordial black holes could explain dark matter

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

All the matter we see—stars, planets, cosmic dust, and everything in between—cannot explain why the universe behaves the way it does, and there must be around five times as much of 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 mass of the galaxy and its gravity—they should have flown away instead. The only explanation was a large amount of invisible matter that held the galaxy together.

“What you see in a spiral galaxy,” Rubin said at the time, “you don’t get.” Her work was based on a hypothesis formulated by the Swiss astronomer Fritz Zwicky in the 1930s and started the search for the elusive substance.

Since then, scientists have tried to observe dark matter directly and even built large devices to detect it – but so far without success.

At the beginning of the search, the renowned British physicist Stephen Hawking hypothesized that dark matter could be hiding in black holes – the main subject of his work – created during the big bang.

Now, a new study by scientists at the Massachusetts Institute of Technology has brought the theory back into focus, revealing what these primordial black holes were made of and potentially discovering a whole new type of exotic black hole in the process.

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

“We used 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 arise from trying to solve the dark matter problem – they are a byproduct of the dark matter explanation.”

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

“People didn’t 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 once seemed 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 Einstein’s discovery of gravitational waves produced by colliding black holes in 2015 – a landmark finding – made it clear that they are everywhere.

“In reality, the universe is teeming with black holes,” Alonso-Monsalve said. “But the dark matter particle was not 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 they are definitely black holes. It can be a combination of both. But now black holes are being taken much more seriously as dark matter candidates.

Other recent studies have confirmed the validity of Hawking’s hypothesis, but the work by Alonso-Monsalve and Kaiser, professor of physics and Germeshausen Professor of the History of Science at MIT, goes a step further and examines exactly what happened when the primordial black hole 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: “That’s really early and much earlier than when protons and neutrons, particles were created from of which everything is made,” said Alonso-Monsalve.

In our everyday world, we cannot find protons and neutrons broken down, she added, and they function as elementary particles. But we know they aren’t because they are made up of even smaller particles called quarks that are held together by other particles called gluons.

“You can’t find quarks and gluons alone and free in space 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 the primordial black holes were created by absorbing free quarks and gluons.

Such a formation would make them fundamentally different from the astrophysical black holes that scientists commonly observe in the universe, which are the result of collapsing stars. Also, the primordial black hole would have been much smaller – only the mass of the asteroid condensed to the volume of one atom on average. But if a sufficient number of these primordial black holes did not evaporate in the early big bang and survive to this day, they could represent all or most of the dark matter.

When the primordial black holes were created, another type of previously unseen black hole must have formed as a kind of byproduct, according to the study. These would be even smaller – just the mass of a rhinoceros, compressed into a volume smaller than that of a single proton.

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

This colored charge would make them unique among black holes, which usually have no charge. “It’s inevitable that these even smaller black holes would also form as a by-product (of the primordial black holes),” Alonso-Monsalve said, “but they wouldn’t be here today because they would have evaporated.”

However, if they were still around just ten millionths of a second before the big bang, when protons and neutrons were created, they could leave observable traces by changing the balance between the two types of particles.

“The balance of the number of protons and neutrons is very delicate and depends on what other things were in the universe at the time.” If these black holes with a colored charge were still around, they could shift the balance between protons and neutrons (in favor of one or the other), just enough that we’ll be able to measure it in the next few years,” she added.

The measurements 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 added.

“Creating a population of black holes is a very violent process that would cause huge ripples in the surrounding space-time. These would have attenuated over cosmic history, but not to zero,” Kaiser said. “The next generation of gravity detectors could spot small black holes—an exotic state of matter that was an unexpected byproduct of more mundane 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 exotic new particles remains an interesting hypothesis,” Kaiser said. “There are other kinds of large experiments, some of which are under construction, looking for fantastic ways to detect gravitational waves.” And they could indeed pick up some stray signals from the very violent process of primordial black hole formation.”

There is also the possibility that primordial black holes are just a fraction of dark matter, Alonso-Monsalve added. “It doesn’t actually have to be all the same,” she said. “There is five times more dark matter than ordinary matter, and regular matter is made up of a whole range of different particles. So why should dark matter be the only type of object?

Primordial black holes have regained popularity thanks to the discovery of gravitational waves, yet not much is known about their origin, says Nico Cappelluti, an assistant professor in the Department of Physics at the University of Miami. He was not involved in the study.

“This work is an interesting and viable possibility for explaining the elusive dark matter,” said Cappelluti.

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 join 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 around these primordial black holes by that time, they would have affected this process and these 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 probably creates black holes from the earliest times through several pathways.”