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For nearly 50 years, the scientific community has been grappling with a major problem: there isn’t enough visible matter in the universe.
All the matter we see — stars, planets, space dust, and everything in between — can’t explain why the universe behaves the way it does: there would have to be five times as much matter around for researchers’ observations to make sense. According to NASA:Scientists call it dark matter because it does not interact with light and is invisible to the naked eye.
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 realized that these stars were moving too fast to be gravitationally bound to the visible matter of the galaxy; instead, they would move away. The only explanation was that a large amount of invisible matter was holding the galaxy together.
“What you see in a spiral galaxy” Rubin says At the time, she thought, “That’s not what we’re getting.” Her research is based on a hypothesis put forward in the 1930s by Swiss astronomer Fritz Zwicky, who began the search for this elusive substance.
Since then, scientists have attempted to directly observe dark matter, Large Devices I’ve tried to detect it but so far have been unsuccessful.
Early in the search, renowned British physicist Stephen Hawking hypothesized that dark matter could be hiding inside black holes (a major focus of Hawking’s research) that formed during the Big Bang.
Bettman Archive/Getty Images
The late physicist Stephen Hawking hypothesized that dark matter could be hiding inside black holes that formed during the Big Bang.
Now, a new study from researchers at the Massachusetts Institute of Technology is bringing this theory back into the spotlight, shedding light on what these primordial black holes were made of and potentially discovering entirely new types of exotic black holes in the process.
“It was a really nice surprise in that sense,” said David Kaiser, one of the study’s authors.
“We leveraged Stephen Hawking’s famous calculations on black holes, particularly the key results on the radiation they emit,” Kaiser said. “These exotic black holes came about in trying to address the problem of dark matter, and are a by-product of explaining dark matter.”
Scientists have speculated on many possibilities for what dark matter might be, from unknown particles to extra dimensions, but Hawking’s black hole theory has only recently come to prominence.
“Up until about 10 years ago, people didn’t take them very seriously,” says study co-author Elba Alonso-Monsalve, a graduate student at MIT. “That’s because black holes used to be really elusive. In the early 20th century, people thought that black holes were just a fun mathematical fact, not something physical.”
We now know that there is a black hole at the center of almost every galaxy, and researchers have predicted Einstein’s Gravitational waves A groundbreaking discovery in 2015 produced by colliding black holes revealed that black holes exist everywhere.
“Indeed, the universe is full of black holes,” Alonso-Monsalve says, “but we haven’t found any dark matter particles, despite looking in all the places where people expected to find them. This doesn’t mean that dark matter isn’t a particle, or that it’s definitely a black hole — it could be a combination of both. But black holes as candidates for dark matter are now being taken much more seriously.”
other Recent Research While Hawking’s hypothesis has been confirmed, Alonso-Monsalve and Kaiser, professors of physics and the Germeshausen Professor of the History of Science at MIT, have taken their research a step further, examining exactly what happened when primordial black holes first formed.
of studyA study published June 6 in Physical Review Letters reveals that these black holes must have appeared within the first ten trillionths of a second of the Big Bang. “That’s really early, much earlier than the moment when protons and neutrons, the particles that everything is made of, were formed,” Alonso-Monsalve said.
“In our everyday world, we don’t see protons and neutrons breaking apart and they behave as elementary particles. But we know they’re not elementary particles because they’re made up of even smaller particles called quarks, which are held together by other particles called gluons,” she added.
“The Universe is too cold today for quarks and gluons to exist freely on their own,” Alonso-Monsalves added, “but early in the Big Bang, when the universe was very hot, quarks and gluons could exist freely on their own. So primordial black holes formed by absorbing free quarks and gluons.”
This formation makes them fundamentally different from the astrophysical black holes that scientists typically observe in the universe, which are the result of collapsing stars. Primordial black holes are also much smaller – on average the mass of an asteroid condensed into the volume of a single atom. But if enough of them survived the Big Bang and survived to this day, they could potentially explain all or most of dark matter.
According to the study, during the formation of the primordial black hole, another type of black hole never seen before must have formed as a by-product. These are even smaller and have a mass of RhinoIt condenses into a volume smaller than the volume of a single proton.
Because of their tiny size, these miniscule black holes would have been able to acquire a strange and unusual property from the soup of quarks and gluons they formed from, called “color charge,” a state of charge unique to quarks and gluons that is never seen in ordinary matter, Kaiser said.
This color charge would make these black holes unique among black holes, which normally do not have any charge. “It is inevitable that these smaller black holes also formed as a by-product” of the formation of the primordial black hole, Alonso-Monsalves said. “But they would no longer exist today because they have already evaporated.”
But if it was still around when protons and neutrons were formed just a few ten-millionths of a second after the Big Bang, it could have left an observable signature by changing the balance between the two types of particles.
“The balance between how many protons and neutrons were produced is very delicate and depends on what other matter was in the universe at that time. If these color-charged black holes still existed, they could have shifted the balance of protons and neutrons (in favor of one or the other), and we could measure that in a few years,” she added.
Kaiser said the measurements could come from telescopes on Earth or sensitive instruments on orbiting satellites, but added that there may be other ways to confirm the existence of such exotic black holes.
“Creating a population of black holes is an incredibly violent process that sends giant ripples through the surrounding space-time. They decay over the history of the universe but never go to zero,” Kaiser said. “The next generation of gravity detectors may be able to glimpse low-mass black holes – exotic states of matter that are unexpected by-products of more mundane black holes that could explain today’s dark matter.”
What does this mean for ongoing experiments trying to detect dark matter? LZ Dark Matter Experiment In South Dakota?
“The idea that there are new exotic particles remains an intriguing hypothesis,” Kaiser says. “There are several other large experiments, some of which are still under construction, looking for clever ways to detect gravitational waves, and they may indeed pick up some of the stray signals that arise from the very violent formation process of primordial black holes.”
Alonso-Monsalves added that primordial black holes could also just be a small part of dark matter. “It doesn’t have to be all the same,” he said. “There’s five times as much dark matter as regular matter, and regular matter is made up of a lot of different particles, so why should dark matter be a single type of thing?”
Primordial black holes have received renewed attention due to the discovery of gravitational waves, but little is still known about their formation, said Nico Caperutti, an assistant professor of physics at the University of Miami who was not involved in the study.
“This work is an intriguing and viable option for explaining the elusive dark matter,” Cappelluti said.
The work is provocative, suggesting a new mechanism for the formation of first-generation black holes, said Priyamvada Natarajan, the Joseph S. and Sophia S. Fulton Professor of Astronomy and Physics at Yale, who was also not involved in the study.
“All the hydrogen and helium currently in the universe was created in the first three minutes. If enough primordial black holes had been around by that time, they could have influenced that process in a way that we might be able to detect,” Natarajan said.
“The fact that this is an observationally testable hypothesis is what really gets me excited, apart from the fact that it suggests that nature has probably been creating black holes through multiple pathways since the dawn of time.”
