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For the past 50 years or so, the scientific community has been grappling with a substantial problem: There isn’t enough visible matter in the universe.
All the matter we can see – the stars, the planets, the cosmic dust and everything in between – cannot account for why the universe behaves the way it does, and there must be five times more than enough to make sense of the researchers’ comments, 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 observing stars orbiting the edges of spiral galaxies. They noted that these stars were moving too fast to be held together by the visible matter of the galaxy and its gravity — they should be flying apart. The only explanation was an unprecedented amount of matter that held 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 by the Swiss astronomer Fritz Zwicky in the 1930s and she began a search for the missing substance.
Since then, scientists have been trying to observe dark matter directly and even build large devices to detect it – but so far, no luck.
Early in the search, the famous British physicist Stephen Hawking suggested that dark matter could be hidden in black holes – the main subject of his work – created during the big bang.
Now, a new study by researchers at the Massachusetts Institute of Technology has brought the theory back into the limelight, revealing what these primordial black holes were made of and possibly discovering a new type of exotic black hole in the process.
“It was a big 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 emitted by black holes,” Kaiser said. “These exotic black holes arise from trying to tackle the dark matter problem – they are a byproduct of explaining dark matter.”
The first fifth of a second
Scientists have made many guesses as to what dark matter might be, from unknown particles to extra dimensions. But only recently has Hawking’s black hole theory come into play.
“People didn’t take it seriously until maybe 10 years ago,” said study co-author Elba Alonso-Monsalve, an MIT graduate student. “And that’s why black holes seemed unrelated – 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 the discovery of Einstein’s gravitational waves created by black hole collisions in 2015 – a landmark finding – made it clear that they were everywhere.
“In reality, the universe is full of 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 is not to say that dark matter is not a particle, or that it is definitely black holes. It could be a combination of both. But now, black holes are taken much more seriously as candidates for dark matter.”
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 one step further and looks into exactly what happened when there was a primordial black . holes formed first.
The study, published on June 6 in the journal Physical Review Letters, reveals that these black holes must have appeared in the first fifth of a second of the big bang: “That is very early, and much earlier than the moment when there are protons and neutrons. , the particles from which everything is created,” said Alonso-Monsalve.
In our everyday life, we cannot find protons and neutrons broken apart, she said, and they act as fundamental particles. However, we know that they are not, as they are made up of even smaller particles called quarks, held together by other particles called gluons.
“You can’t get a quark and a gene alone and free in the universe now, because it’s too cold,” Alonso-Monsalve added. “But early in the big bang, when it was really hot, they could be found alone and free. So the primordial black holes are formed by absorbing quarks and free electrons.”
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 size of a single atom. But if a sufficient number of these primordial black holes did not escape early in the big bang and survived to the present day, they could account for all or the darkest matter.
Long lasting signature
During the formation of primordial black holes, another type of black hole that has never been seen before must have formed as a byproduct, according to the study. These would be even smaller – only a rhinoceros mass, condensed into less than the volume of a single proton.
Because of their small size, these tiny black holes would be able to acquire rare and exotic properties from the quark-gluon soup in which they formed, known as “colorants”. It’s a charge state exclusive to quarks and electrons, never found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which normally have no charge of any kind. “It is inevitable that these smaller black holes would also have been created, as a byproduct (of the formation of primordial black holes),” said Alonso-Monsalve, “but they would no longer be around today, as they would have already evaporated. “
However, if they were still only about ten million seconds into the big bang, when protons and neutrons were formed, they could have left detectable signatures by changing the balance between the two types of particles.
“The balance between how many protons and how many neutrons were made is very delicate, and it depends on what else was in the universe at the time. If these charged black holes were still around, they could shift the balance between protons and neutrons (in favor of one or the other), just enough that we can measure that in the next few years, ” she said.
The measurement could come from Earth-based telescopes or sensitive instruments on orbiting satellites, Kaiser said. But there could be another way to confirm these exotic black holes, he said.
“Making a population of black holes is a very violent process that would cause huge ripples in the surrounding space-time. Those would be attenuated over cosmic history, but not to zero,” Kaiser said. “The next generation of gravitational detectors could catch a glimpse of tiny black holes – an exotic state of matter that was an unexpected byproduct of the more accurate black holes that could explain dark matter today.”
Many types of dark matter
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 is still an interesting hypothesis,” Kaiser said. “There are other kinds of large experiments, some of which are under construction, looking for great ways to detect gravitational waves. And in fact they might pick up some of the stray signals from the very violent formation process of primordial black holes.”
There is also the possibility that primordial black holes are only a fraction of dark matter, Alonso-Monsalve added. “It doesn’t really have to be the same,” she said. “There is five times more dark matter than regular matter, and regular matter is made up of a whole number of different particles. So why should dark matter be one type of object?”
The discovery of gravitational waves has rekindled the popularity of primordial black holes, but little is known about their formation, according to 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, viable option to explain the dark and inscrutable 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 was not involved in the study either.
“All the hydrogen and helium in our Universe today was created in the first three minutes, and if many of these primordial black holes had been around until then, they would have affected that process and those effects could be noticeable,” said Natarajan .
“The fact that this is an observable hypothesis is really exciting to me, rather than the fact that it suggests that nature is likely to make black holes starting from the earliest times through multiple pathways.”
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