Black holes might hold the key to a 60-year cosmic mystery

Interestingly, "cosmic rays" are not actually rays at all. The name comes from early scientific history, but these are in fact tiny particles -- mostly atomic nuclei -- accelerated to immense energies somewhere in the universe. Their true origins remain uncertain, but they are likely linked to some of the most extreme environments known, including black holes, exploding stars (supernovae), or spinning neutron stars (a type of dead star).

Every so often, scientists detect cosmic rays with energy levels far higher than usual. They have been aware of this since 1962 but still have not uncovered the reason. The sources of this ultra-high-energy cosmic radiation remain one of the biggest mysteries in physics.

Researchers at the Norwegian University of Science and Technology (NTNU) may now be closer to solving it.

Supermassive black holes may be responsible

Foteini Oikonomou, an associate professor in NTNU's Department of Physics, and her team have proposed a new and credible explanation for these mysterious high-energy particles. The study was led by PhD research fellow Domenik Ehlert, with contributions from postdoctoral researcher Enrico Peretti of the Université Paris Cité. Their work focuses on astroparticle physics, which explores how the smallest particles in the universe connect with its largest and most powerful phenomena.

"We suspect that this high-energy radiation is created by winds from supermassive black holes," said Oikonomou.

But what exactly does that mean?

How active black holes create winds

The Milky Way, our galactic home, contains a central black hole called Sagittarius-A*. "There is a black hole called Sagittarius-A* located right in the center 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," Peretti explained.

Other galaxies host much more active, supermassive black holes that swallow matter equivalent to several Suns each year. "A tiny portion of the material 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," Peretti said.

Astronomers have known about these powerful outflows for about a decade. Such winds can shape entire galaxies by pushing away gas and halting star formation. Yet Oikonomou and her colleagues focused on a much smaller consequence -- these winds might accelerate particles to extreme energies.

"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.

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 kilometers 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.

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 that reaches us at the Earth's surface.

"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 high enough.

"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 said.

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 source. 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?

"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 doesn't 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," Oikonomou said.

However, the model the researchers 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," Oikonomou said. Perhaps they will then find more evidence, one way or the other.