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For nearly 50 years, the scientific community has been grappling with an important problem: There is not 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 researchers need to have five times as much matter around for their 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 noted that these stars were moving too fast to be held together by the galaxy’s visible matter and gravity, and instead had to break apart and fly away. The only explanation was the vast amount of unseen matter binding the galaxy together.
“What you see in a spiral galaxy is not what you get,” Rubin said at the time. His work built on a hypothesis formulated by Swiss astronomer Fritz Zwicky in the 1930s and initiated a search for elusive matter.
Since then, scientists have been trying to observe dark matter directly and have even built large devices to detect it; but so far they have not achieved any results.
Early in the search, famous British physicist Stephen Hawking suggested that dark matter might be hiding inside black holes formed during the big bang (the main subject of his work).
Now, a new study by researchers at the Massachusetts Institute of Technology has resurrected the theory, revealing what these primordial black holes are made of and potentially discovering an entirely new type of exotic black hole in the process.
“This was a really wonderful surprise,” 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 results regarding the radiation emitted by black holes,” Kaiser said. “These exotic black holes appear when trying to solve the problem of dark matter; they are a byproduct of explaining dark matter.”
First quintillionth of a second
Scientists have made many predictions about what dark matter might be, from unknown particles to extra dimensions. But Hawking’s theory of black holes has only recently come into play.
“People didn’t really take this seriously until maybe 10 years ago,” said MIT graduate student Elba Alonso-Monsalve, one of the study’s authors. “And that’s because black holes once seemed really elusive; in the early 20th century, people thought they were just a mathematical fun fact, not a physical thing.”
We now know that there is a black hole at the center of almost every galaxy, and researchers’ discovery in 2015 of Einstein’s discovery of gravitational waves created by the collision of black holes—a landmark finding—made clear that they are everywhere.
“In fact, the universe is full of black holes,” Alonso-Monsalve said. “But although people looked everywhere they expected to find it, no dark matter particles were found. This doesn’t mean that dark matter isn’t a particle, or that there are definitely black holes. It could be a combination of both. “However, black holes, which are dark matter candidates, are now being 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 investigates exactly what happened when primitive black races existed. First, holes formed.
The study, published June 6 in the journal Physical Review Letters, reveals that these black holes must have emerged within the first quintillionth of a second of the big bang: “This is really very early, and much earlier than the moment when protons and neutrons formed,” Alonso-Monsalve said. “Particles formed from which everything was formed,” he said.
He added that we cannot find fragmented protons and neutrons in our everyday world and that they behave like fundamental particles. But we know they are not, because they consist of even smaller particles called quarks and combine with other particles called gluons.
“You can no longer find quarks and gluons alone and free in the universe because it is too cold,” Alonso-Monsalve added. “But early in the big bang, when it was very hot, they could be found alone and free. In other words, primitive black holes were formed by the absorption of 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 formed by the collapse of stars. Also, a primitive black hole would be much smaller; On average, the mass of an asteroid will be concentrated into the volume of a single atom. However, if enough of these primordial black holes had not evaporated early in the Big Bang and survived to the present day, they could explain all or most of the dark matter.
A long-lasting signature
According to the research, another type of black hole that has not been seen before must have formed as a kind of byproduct during the formation of primitive black holes. These would be even smaller; the mass of just one rhinoceros was concentrated into less than the volume of a single proton.
Because of their small size, these tiny black holes should have been able to pick up a rare and exotic property called “color charge” from the quark-gluon soup in which they formed. This is a special case of charge on quarks and gluons that is never found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which usually have no charge at all. “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 no longer be around today because they would have already evaporated.”
However, if they were still around ten-millionths of a second into the big bang when protons and neutrons 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 formed 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 tip the balance between protons and neutrons (in favor of one or the other), which could change over the next few years.” Enough so that we can measure it within the year,” he added.
The measurement could come from Earth-based telescopes or sensitive instruments on orbiting satellites, Kaiser said. But he added that there may be another way to confirm the existence of these exotic black holes.
“Creating a population of black holes is a very violent process that will send enormous ripples through the surrounding space-time. “These will weaken over the course of cosmic history, but not to zero,” Kaiser said. “A new generation of gravitational detectors can capture a snapshot of low-mass black holes; “The exotic state of matter is an unexpected byproduct of more ordinary black holes that could explain today’s dark matter.”
Many forms 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 exotic new particles remains an interesting hypothesis,” Kaiser said. “There are other kinds of large experiments, some in the works, that are looking for fancy ways to detect gravitational waves. And these might actually pick up some of the stray signals from the extremely violent formation process of primordial black holes.”
Alonso-Monsalve added that there is also the possibility that primordial black holes are just a portion of dark matter. “They don’t actually all have to be the same,” he said. “There is five times more dark matter than regular matter, and regular matter is made up of lots of different particles. So why would dark matter be a uniform object?”
Primordial black holes have regained popularity with the discovery of gravitational waves, but not much is known about their formation, according to Nico Cappelluti, an assistant professor in the University of Miami department of physics. He did not participate in the research.
“This work is an interesting and feasible option to explain elusive dark matter,” Cappelluti said.
Priyamvada Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University, said the study is exciting and suggests a new mechanism for the formation of the first generation of black holes. He was also not involved in the research.
“All the hydrogen and helium that 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 influenced this process and these effects could have been detected,” Natarajan said. .
“The fact that this is an observationally testable hypothesis is something I find really exciting; “Except that this suggests that nature probably formed black holes in more than one way, starting from the earliest times.”
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