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For about fifty years, 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 — stars, planets, cosmic dust and everything in between — can’t explain why the universe behaves the way it does, and there needs to be five times as much of it around for researchers’ observations to make sense. according to NASA. Scientists call this 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 the edge of spiral galaxies. They noted that these stars were moving too fast to be held together by the visible matter and gravity of the galaxy; instead they should have flown apart. The only explanation was a large amount of invisible matter holding 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 sparked a search for the elusive substance.
Since then, scientists have been trying to observe dark matter directly and have even built large devices to detect it – but so far without success.
Early in the search, famed British physicist Stephen Hawking suggested that dark matter could hide 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 put the theory back in the spotlight, revealing what these pristine black holes were made of and potentially discovering an entirely new type of exotic black hole.
“It was a really wonderful surprise that way,” said David Kaiser, one of the study’s authors.
“We used Stephen Hawking’s famous calculations on black holes, especially his important result on the radiation that black holes emit,” Kaiser said. “These exotic black holes arise from attempts to tackle the dark matter problem – they are a byproduct of explaining dark matter.”
The first quintillionth of a second
Scientists have made many guesses about what dark matter might be, ranging from unknown particles to extra dimensions. But Hawking’s black hole theory has only recently come into play.
“People didn’t really take it seriously until a decade ago,” says co-author Elba Alonso-Monsalve, a graduate student at MIT. “And that’s because black holes once seemed truly 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 in 2015 of Einstein’s gravitational waves created by colliding black holes – a milestone – made it clear that they exist everywhere.
“Basically the universe is teeming with black holes,” Alonso-Monsalve said. “But the dark matter particle has not been found, even though people looked everywhere they expected to find it. This does not mean that dark matter is not a particle, or that they are definitely black holes. It could be a combination of both. But now black holes are being 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 history of science at MIT, goes one step further and examines exactly what happened when the original black holes 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 very early, and much earlier than when protons and neutrons became the particles from which everything is made,” said Alonso-Monsalve.
In our everyday world, we cannot find disintegrated protons and neutrons, she added, and they act as elementary particles. However, we know that this is not the case because they are made up of even smaller particles called quarks and are linked together by other particles called gluons.
“You can’t find quarks and gluons alone and free in the universe now, because it’s too cold,” Alonso-Monsalve added. “But early in the Big Bang, when it was very hot, they could be found alone and free. The original black holes were thus created 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. Furthermore, a primordial black hole would be much smaller – on average only the mass of an asteroid, condensed to the volume of a single atom. But if a sufficient number of these primordial black holes did not evaporate during the early Big Bang and survived to this day, they could be responsible for all or most of the dark matter.
A lasting signature
During the creation of the original black holes, another type of previously invisible black hole must have formed as a kind of byproduct, according to the research. These would have been even smaller: just the mass of a rhinoceros, condensed to less than the volume of a single proton.
These tiny black holes, because of their small size, could have acquired a rare and exotic property from the quark-gluon soup in which they formed, a so-called ‘color charge’. It’s a charge state exclusive to quarks and gluons and never found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which typically have no charge whatsoever. “It is inevitable that these even smaller black holes would have also formed, as a byproduct (of the formation of primordial black holes),” Alonso-Monsalve said, “but they would not be there today because they would already have would have evaporated.”
However, if they were only about ten millionths of a second into the Big Bang, when protons and neutrons were created, they could have left observable signatures by changing the balance between the two particle types.
“The balance between how many protons and how many neutrons were created is very delicate and depends on what else existed in the universe at the time. If these color-charged black holes still existed, they could have shifted the balance between protons and neutrons (in favor of one or the other) just enough to be able to measure that in the next few years,” she added.
The measurement could come from telescopes on Earth or sensitive instruments on satellites in orbit, 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 enormous ripples in the surrounding space-time. They would get weaker over cosmic history, but not to zero,” Kaiser said. “The next generation of gravity detectors could glimpse the low-mass black holes – an exotic state of matter that was an unexpected byproduct of the more commonplace 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 exotic new particles exist remains an interesting hypothesis,” says Kaiser. “There are other types of large experiments, some of which are under construction, looking for cool ways to detect gravitational waves. And they can indeed 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 all the same,” she said. “There is five times more dark matter than regular matter, and regular matter is made up of a whole range of different particles. So why should dark matter be a single type of object?”
Primordial black holes have become popular again with the discovery of gravitational waves, but not much is known about their formation, said Nico Cappelluti, an assistant professor at the University of Miami’s physics department. He was not involved in the investigation.
“This work represents an interesting, 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, says Priyamvada Natarajan, Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University. She was also not involved in the investigation.
“All the hydrogen and helium that we have in our universe today was created in the first three minutes, and if there had been enough of these primordial black holes around until then, they would have influenced that process and effects can be observable,” Natarajan said. .
“The fact that this is an observationally testable hypothesis is really exciting to me, apart from the fact that it suggests that nature is likely creating black holes via multiple routes from the earliest times.”
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