Researchers from the Norwegian University of Science and Technology (NTNU) may have found the answer to one of the big, unanswered questions in physics.

The universe is full of different types of radiation and particles that can be observed here on Earth. This includes photons across the entire range of the electromagnetic spectrum, from the lowest radio frequencies all the way to the highest-energy gamma rays. It also includes other particles such as neutrinos and cosmic rays, which race through the universe at close to the speed of light.

Curiously, "cosmic rays" are not actually rays – this name has historical reasons – but small particles, mostly atomic nuclei, which are accelerated to enormous energies somewhere in the universe. Although their sources are not yet fully understood, they are most likely associated with some of the most extreme environments in the universe, such as black holes, supernovae, or rotating neutron stars (a type of dead star).

But occasionally cosmic rays have much higher energy than usual. We've known about this since 1962, but we still have no idea why.

We also don’t know where this ultra-high-energy cosmic radiation comes from. Or do we?

Foteini Oikonomou, an associate professor at NTNU's Department of Physics is working on the case. In a recent article, she and her colleagues present a completely new and plausible explanation.

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?

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 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,” emphasized Oikonomou.

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.

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.

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.

Maybe then they will find more evidence, one way or the other.