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For about 50 years, science has been grappling with a significant 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 there would have to be five times as much of it for researchers’ observations to make sense, according to NASA. Scientists call this dark matter because it doesn’t 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 at the edge of spiral galaxies. They found that these stars were moving too fast to be held together by the galaxy’s visible matter and gravity – they should have been flying apart instead. The only explanation for this was a large amount of invisible matter holding the galaxy together.
“What you see in a spiral galaxy is not what you get,” Rubin said at the time. Her work was based on a hypothesis formulated by the Swiss astronomer Fritz Zwicky in the 1930s that sparked the 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.
At the beginning of his search, the renowned British physicist Stephen Hawking postulated that dark matter could be hidden in the black holes created by the Big Bang – the main subject of his work.
A new study by researchers at the Massachusetts Institute of Technology has brought this theory back into the spotlight, revealing what those original black holes were made of and may have led to the discovery of an entirely new type of exotic black hole in the process.
“In that respect, it was really a wonderful surprise,” said David Kaiser, one of the study’s authors.
“We used Stephen Hawking’s famous calculations on black holes, particularly his important result about the radiation that black holes emit,” Kaiser said. “These exotic black holes are created in an attempt to address the problem of dark matter – they are a byproduct of the dark matter explanation.”
The first trillionth of a second
Scientists have made many guesses about what dark matter might be, from unknown particles to extra dimensions, but Hawking’s black hole theory has only recently come into play.
“Until about 10 years ago, people didn’t really take the subject seriously,” says Elba Alonso-Monsalve, co-author of the study and an MIT graduate student. “That’s because black holes once seemed really elusive – in the early 20th century, people thought they were just a mathematical joke, not something physical.”
Today we know that almost every galaxy has a black hole at its center, and researchers’ groundbreaking discovery of gravitational waves created by colliding black holes by Einstein in 2015 made it clear that they are everywhere.
“The universe is actually teeming with black holes,” said Alonso-Monsalve. “But the dark matter particle has not been found, despite looking in all the places one would expect to find it. That doesn’t mean that dark matter isn’t a particle, or that it’s 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, a physics professor and Germeshausen Professor of the History of Science at MIT, goes a step further and examines exactly what happened when the first primordial black holes formed.
The study, published on June 6 in the journal Physical Review Letters, shows that these black holes must have appeared in the first trillionth of a second of the Big Bang: “That’s really early and much earlier than the moment when protons and neutrons – the particles that make up everything – were created,” said Alonso-Monsalve.
We cannot find split protons and neutrons in our everyday world, she added, and they behave like elementary particles. But we know they are not because they are made of even smaller particles called quarks, which are bound together by other particles called gluons.
“In today’s universe, you can’t find quarks and gluons alone and free because it’s too cold,” Alonso-Monsalve added. “But at the beginning of the Big Bang, when it was very hot, you could find them alone and free. So the original 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 usually observe in the universe, which are the result of collapsing stars. In addition, a primordial black hole would be much smaller – on average only the mass of an asteroid compressed into the volume of a single atom. However, if a sufficient number of these primordial black holes did not evaporate in the early Big Bang and survived to the present day, they could make up all or most of the dark matter.
A long-lasting signature
According to the study, during the formation of the original black holes, another type of previously unknown black hole must have formed as a byproduct. These would have been even smaller – just the size of a rhinoceros, compressed to less than the volume of a single proton.
Because of their small size, these tiny black holes could have acquired a rare and exotic property from the quark-gluon soup in which they formed: a so-called “color charge.” It is a charge state exclusive to quarks and gluons and not found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which normally have no charge at all. “It is inevitable that these even smaller black holes would also have formed as a byproduct (of the formation of primordial black holes),” said Alonso-Monsalve, “but they would no longer exist today because they would have already evaporated.”
However, if they had still been nearby ten millionths of a second after the Big Bang, when protons and neutrons formed, they could have left behind recognizable signatures by changing the balance between the two types of particles.
“The balance between the number of protons and the number of neutrons is very delicate and depends on what other things existed in the universe at the time. If these color-charged black holes were still there, they could have shifted the balance between protons and neutrons (in favor of one or the other) just enough that we could measure that 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.
“The formation of a population of black holes is a very violent process that would cause enormous ripples in the surrounding space-time. These would weaken over the course of cosmic history, but not to zero,” Kaiser said. “The next generation of gravity detectors could catch a glimpse of the low-mass black holes – an exotic state of matter that was an unexpected byproduct of the more mundane black holes that could explain dark matter today.”
Many forms of dark matter
What does this mean for ongoing experiments 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, that are looking for unusual ways to detect gravitational waves. And these might actually pick up some of the scattered signals from the very violent formation process of primordial black holes.”
There’s also the possibility that primordial black holes make up only a fraction of dark matter, Alonso-Monsalve added. “It doesn’t really all have to be the same,” she said. “There’s five times more dark matter than normal matter, and normal matter is made up of a whole bunch of different particles, so why should dark matter be a single type of object?”
Primordial black holes have regained popularity with the discovery of gravitational waves, but not much is known about how they form, says Nico Cappelluti, an assistant professor in the physics department at the University of Miami. He was not involved in the study.
“This work is an interesting, viable option to explain the elusive dark matter,” said Cappelluti.
The study is exciting and suggests a novel formation mechanism for the first generation of black holes, said Priyamvada Natarajan, Joseph S. and Sophia S. Fruton Professor of Astronomy and Physics at Yale University. She was also not involved in the study.
“All the hydrogen and helium we have in our universe today was created in the first three minutes, and if enough of these primordial black holes had been around by then, they would have influenced this process and these effects could be detectable,” 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 has probably been creating black holes in multiple ways since ancient times.”
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