Black holes may be the secret force behind cosmic rays

Black holes may hold the key to one of space’s most puzzling mysteries: ultra-high-energy cosmic rays. These particles – and various forms of radiation – constantly reach Earth. Some, like sunlight and radio waves, are familiar. Others are far more mysterious.

Among the lesser understood are ultra-high-energy cosmic rays – tiny particles, mostly atomic nuclei, traveling near the speed of light with staggering amounts of energy.

Most cosmic rays come from known events like exploding stars. But sometimes, we detect particles with much more energy than expected. These supercharged cosmic rays were first described in the 1960s, but their origin remains unknown.

Now, researchers at the Norwegian University of Science and Technology (NTNU) are offering a possible explanation that adds a new twist to the story. Their study suggests that winds from supermassive black holes might be the missing link in understanding intense cosmic rays.

Are black holes creating cosmic rays?

At the heart of the Milky Way lies Sagittarius-A*, a black hole that’s unusually calm right now. It’s not drawing in much matter compared to its more active counterparts across the universe.

“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,” said Enrico Peretti from Université Paris Cité, a collaborator on the project.

Active black holes behave differently. They devour large amounts of matter – sometimes even more than the mass of our Sun each year – but not all of that matter gets pulled in.

“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.

These winds have been known for about a decade. They can push gases out of galaxies, stopping new stars from forming. But now, scientists are wondering if these winds are also responsible for something much smaller – and stranger.

“It is possible that these powerful winds accelerate the particles that create the ultra-high-energy radiation,” said PhD researcher Domenik Ehlert.

Power in a tiny particle

To understand the magnitude of this radiation, you have to think small – very small. We’re talking about atomic particles.

A typical atom has a nucleus made of protons and neutrons, surrounded by a cloud of electrons. The particles being studied here are even smaller – just protons or atomic nuclei.

“The ultra-high-energy radiation consists of protons or atomic nuclei with energy up to 10²⁰ electron volts,” explained Professor Foteini Oikonomou from NTNU’s Department of Physics. That amount of energy, packed into a single particle, is staggering.

“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.

To put it another way, it’s a billion times more energetic than the particles smashed together in the Large Hadron Collider in Europe.

Cosmic radiation and astronaut safety

Luckily, these particles don’t pose a risk to us here on Earth. The atmosphere absorbs their energy long before they reach the ground. “But for astronauts, cosmic radiation is a very serious problem,” Oikonomou said.

Airline pilots don’t need to worry – they fly too low. But astronauts face more frequent exposure to solar particles. Even so, the ultra-high-energy rays this study focuses on are so rare that they’re unlikely to ever hit a human in space.

“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,” noted Oikonomou.

Extreme winds from black holes

This isn’t the first attempt to explain where these powerful particles come from. Scientists have looked into gamma-ray bursts, rapidly star-forming galaxies, and plasma jets from black holes. Each theory has its strengths but lacks direct evidence.

“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.

The team’s idea is based on something simple: we already know black holes can produce huge winds. Could those winds also be particle accelerators?

Black hole winds and cosmic rays

So, are these winds the cause of ultra-high-energy cosmic rays? “Our answer is more of a cautious ‘maybe,’” said Oikonomou.

Still, their model aligns well with the particle acceleration needed to reach these energy levels.

“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.

The most intriguing part? Their model can explain a chemical pattern in these particles that other theories can’t.

“We can also test the model using neutrino experiments,” said Oikonomou. “In the years to come, we hope to collaborate with neutrino astronomers to test our hypothesis.”

Until then, the winds of supermassive black holes remain a compelling suspect in one of physics’ longest-running mysteries.

The full study was published in the journal Monthly Notices of the Royal Astronomical Society.

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