The universe is full of cosmic radiation that can be measured here on Earth.
This cosmic radiation consists of –- produced by black holes, gigantic supernovae, or rotating neutron stars – a type of dead star. These particles contain energy.
Sometimes, however, cosmic rays have a much higher energy than usual. We have known about this since 1962, but we still don't know why.
We also don't know where this ultra-high-energy cosmic radiation comes from. Or do we?
Supermassive black holes may be the cause
Foteini Oikonomou is working on the case. She is an associate professor at the Norwegian University of Science and Technology's (NTNU) Department of Physics. In a newly published article, she and her colleagues present a completely new and plausible explanation.
We suspect that this high-energy radiation is created by winds from supermassive black holes.
The lead author is PhD research fellow Domenik Ehlert from the same department. The team also includes postdoctoral fellow Enrico Peretti from the Université Paris Cité. Their work focuses on astroparticle physics, which studies the relationship between the smallest particles in the universe and the universe's largest phenomena.
"We suspect that this high-energy radiation is created by winds from supermassive black holes," said Oikonomou.
But what on earth does that mean?
Active black holes create winds
The Milky Way is the neighbourhood in the universe where you and I live. Our Sun and solar system are part of this galaxy, along with at least 100 billion other stars.
"There is a black hole called Sagittarius-A* located right in the centre of the Milky Way. This black hole is currently in a quiet phase where it isn't consuming any stars, as there is not enough matter in the vicinity," explained Peretti.
This contrasts with growing, supermassive, active black holes that consume up to several times the mass of our own Sun each year.
"A tiny portion of the matter can be pushed away by the force of the black hole before it is pulled in. As a result, around half of these supermassive black holes create winds that move through the universe at up to half the speed of light," said Peretti.
We have known about these gigantic winds for approximately ten years, and by blowing away gases, they can affect the surrounding galaxies and prevent new stars from forming. This is dramatic enough in itself, but Oikonomou and her colleagues looked at something else, much smaller, that these winds could be the cause of.
"It is possible that these powerful winds accelerate the particles that create the ultra-high-energy radiation," said Ehlert.
To understand this, we also need to explain a little bit about atoms.
Atoms and enormous amounts of energy
Atoms consist of a nucleus, which is made up of protons and neutrons. These particles are made up of quarks, but we don't need to go into that right now.
One or more electrons can be found around this nucleus in the so-called cloud.
"The ultra-high-energy radiation consists of protons or atomic nuclei with energy up to 1020 electron volts," explained Oikonomou.
A particle like this, which is smaller than an atom, contains about as much energy as a tennis ball when Serena Williams serves it at 200 kilometres per hour.
If that number doesn't mean anything to you, you should know that in this context, it is an absolutely enormous amount of energy.
"A particle like this, which is smaller than an atom, contains about as much energy as a tennis ball when Serena Williams serves it at 200 kilometres per hour," said Oikonomou.
It corresponds to approximately a billion times more energy than the particles created by researchers in the Large Hadron Collider in Switzerland and France.
Wow!
Fortunately, these cosmic rays are destroyed by the Earth's atmosphere. When they reach ground level, they are as harmless as all the other cosmic radiation we receive.
"But for astronauts, cosmic radiation is a very serious problem," Oikonomou said.
Airline crews don't need to worry about this because they don't fly that high.
"The main concern for astronauts is cosmic low-energy radiation produced by our own Sun, because it is much more common. The rays we study are infrequent enough that it is extremely unlikely they would pass through an astronaut," she explained.
Other suspects
Previously, researchers have looked into whether these high-energy particles come from gamma-ray bursts, from galaxies that are creating new stars at an extremely high rate, or from plasma outflows from supermassive black holes.
However, Oikonomou and her colleagues have another hypothesis.
"All the other hypotheses are very good guesses – they are all sources that contain a lot of energy. But no one has provided evidence that any of them are the origin. That is why we decided to investigate the winds from the supermassive black holes," said Ehlert.
Guilty? Maybe
So what do we actually know? Is it the winds that create the high-energy particles in the cosmic radiation?
When researchers ask questions like this, they often feel a sense of excitement and think "YES, that might just be the case!"
"Our answer is more of a cautious 'maybe', said Oikonomou.
That doesn't sound particularly dramatic. However, when researchers ask questions like this, they often feel a sense of excitement and think "YES, that might just be the case!", but that does not mean it is the case in this instance.
"We find that the conditions related to these winds align particularly well with particle acceleration. But we are still unable to prove that it is specifically these winds that accelerate the particles behind the high-energy cosmic radiation," she concluded.
However, the model they are using can explain one specific aspect of these particles that we still don't understand. Within a certain energy range, the particles have a chemical composition that other models cannot explain in any meaningful way.
"We can also test the model using neutrino experiments," said Oikonomou.
That, however, is something for a completely different article.
"In the years to come, we hope to collaborate with neutrino astronomers to test our hypothesis," added Oikonomou.
Perhaps they will then find more evidence, one way or the other.
Reference: Domenik Ehlert, Foteini Oikonomou, Enrico Peretti, Ultra-high-energy cosmic rays from ultra-fast outflows of active galactic nuclei , Monthly Notices of the Royal Astronomical Society, Volume 539, Issue 3, May 2025, Pages 2435–2462, https://doi.org/10.1093/mnras/staf457
