For about 50 years, the scientific community has faced a substantial problem: there is not enough visible matter in the universe.
All the matter we can see — stars, planets, cosmic dust, and everything else — can’t explain why the universe behaves the way it does, and there must be five times as much of that matter for researchers’ observations to make sense, according to with NASA (United States space agency). Scientists call this matter dark matterbecause 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 outskirts of spiral galaxies. They noticed that these stars were moving too fast to be held together by the galaxy’s visible matter and its gravity — they should be dispersing. 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.” His work was based on a hypothesis formulated in the 1930s by Swiss astronomer Fritz Zwicky and began a search for the elusive substance.
Since then, scientists have tried to observe dark matter directly and have even built large devices to detect it — but so far, without success.
At the beginning of the search, the renowned British physicist Stephen Hawking postulated that dark matter could be hidden in black holes — the main subject of his work — formed during the big bang.
Now, a new study from researchers at the Massachusetts Institute of Technology (MIT) has brought the theory back into the spotlight, revealing what these primordial black holes were made of and potentially discovering a entirely new type of exotic black hole in the process.
“It was really a wonderful surprise in that sense,” 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 that black holes emit,” Kaiser said. “These exotic black holes arise when trying to solve the dark matter problem — they are a byproduct of explaining dark matter.”
The beginning of everything
Scientists have made many guesses about what dark matter could be, ranging from unknown particles to extra dimensions. But Hawking’s theory of black holes has only recently come into play.
“People didn’t take it very seriously until maybe 10 years ago,” said study co-author Elba Alonso-Monsalve, an MIT graduate student. “And that’s because black holes seemed really elusive — in the early 20th century, people thought they were just a fun mathematical fact, not physical at all.”
We now know that almost every galaxy has a black hole at its center, and Einstein’s discovery of gravitational waves created by colliding black holes in 2015 — a landmark discovery — made it clear that they are everywhere.
“In fact, the universe is full of black holes,” said Alonso-Monsalve. “But the dark matter particle was not found, even though people looked everywhere they expected to find it. This doesn’t mean that dark matter isn’t a particle, or that it definitely is black holes. It could be a combination of both. But now, black holes as candidates for dark matter are taken much more seriously.”
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 a step further and investigates exactly what happened when primordial black holes formed for the first time.
The study, published on June 6 in Physical Review Letters magazinereveals that these black holes must have appeared in the first quintillionth of a second of the big Bang: “This is really very early, and long before the time when protons and neutrons, the particles that everything is made of, were formed,” said Alonso-Monsalve.
In our everyday world, we cannot find separate protons and neutrons, she added, and they act like elementary particles. However, we know they are not, because they are made of even smaller particles called quarks, held together by other particles called gluons.
“You can’t find quarks and gluons alone and free in the universe right 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, primordial black holes formed by absorbing free quarks and gluons.”
Such a formation would make them fundamentally different from the astrophysical black holes that scientists typically observe in the universe, which are the result of collapsing stars. Furthermore, a primordial black hole would be much smaller — just the mass of an asteroid, on average, condensed into the volume of a single atom. But if enough of these primordial black holes didn’t evaporate at the start of the big bang and survive to this day, they could represent all or most of dark matter.
A lasting signature
During the formation of primordial black holes, another type of black hole, never seen before, must have formed as a type of byproduct, according to the study. These would have been even smaller—just the mass of a rhinoceros, condensed into less than the volume of a single proton.
These tiny black holes, due to their small size, would have been able to acquire a rare and exotic property from the soup of quarks and gluons in which they formed, called “color charge”. It’s a charge state unique to quarks and gluons, never found in ordinary objects, Kaiser said.
This color charge would make them unique among black holes, which generally have no charge of any kind. “It is inevitable that these even smaller black holes also formed, 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 present just ten millionths of a second after the big bang, when protons and neutrons formed, they could have left observable signatures by altering the balance between the two types of particles.
“The balance of how many protons and how many neutrons were formed is very delicate and depends on what else was in the universe at that time. If these color-charged black holes were still around, they could have shifted the balance between protons and neutrons (in favor of one or the other) just enough so that in the next few years we could measure it,” she added.
The measurement could come from Earth-based telescopes or from sensitive instruments on orbiting satellites, Kaiser said. But there could be another way to confirm the existence of these exotic black holes, he added.
“Producing a population of black holes is a very violent process that would send enormous ripples in the surrounding space-time. These ripples would attenuate over cosmic history, but not to zero,” Kaiser said. “The next generation of gravitational detectors could glimpse low-mass black holes — an exotic state of matter that was an unexpected byproduct of the more common black holes that could explain dark matter today.”
Many forms of dark matter
What does this mean for ongoing experiments that are trying to detect dark matter, like the LZ Dark Matter Experiment in South Dakota?
“The idea that there are new exotic particles remains an interesting hypothesis,” Kaiser said. “There are other types of large experiments, some of which are under construction, looking for sophisticated ways to detect gravitational waves. And these could indeed pick up some of the scattered signals from the very violent process of formation of primordial black holes.”
There’s also the possibility that primordial black holes are just 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 series of different particles. So why should dark matter be a single type of object?”
Primordial black holes regained popularity with the discovery of gravitational waves, but little is still known about their formation, according to Nico Cappelluti, assistant professor in the physics department at the University of Miami. He was not involved in the study.
“This work is an interesting and viable option for explaining the elusive dark matter,” said Cappelluti.
The study is exciting and proposes a new 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 existed by then, they would have impacted this process and these effects may be detectable,” said Natarajan.
“The fact that this is a testable hypothesis through observations is what I find really exciting, in addition to the fact that it suggests that nature likely creates black holes from the earliest times through multiple pathways.”
Oldest black hole ever observed has been detected by astronomers
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