Where do cosmic rays come from?

The Earth is constantly bombarded by cosmic raysrains of high-energy particles destroying our planet at almost the speed of light from all directions.

While this may sound like the precursor to a sci-fi alien invasion, it is a real phenomenon that scientists have been aware of for more than a century. Despite their dramatic description, cosmic rays are actually quite common: They pass through our planet so regularly that a person gets about a million cosmic rays through their body during an average night’s sleep, according to the University of Birmingham in the United Kingdom

Despite their ubiquity, cosmic rays still present scientific mysteries. While cosmic rays hit with slow energy Soil They are known to originate from the Sunothers with higher energies flow the solar system from deep room. The origin of these extrasolar cosmic rays is less known, including suspect sources black holes and the supernova explosions that mark the death of massive stars.

Related: Space mysteries: why are there no gas moons?

Cosmic rays “have been observed here on Earth for more than 100 years. Yet their origin remains largely unknown,” says Julia Tjus, professor of physics and physics. astronomy at Ruhr University in Germany, Space.com told us via email. “These tiny particles reach energies far beyond what we can achieve here on Earth. We are trying to solve a mystery that is now more than 100 years old and slowly but steadily put the pieces together.”

What are cosmic rays?

Cosmic rays are streams of high-energy particles that impact each other the Earth’s atmosphere at almost the speed of light. They were discovered in the 1900s and the The term “cosmic rays” was coined in 1925 by physicist Robert Millikan.

Since then, scientists have determined that trillions of cosmic rays hit Earth every day, but the vast majority are blocked by the planet’s radiation. magnetosphere and atmosphere.

More than 90% of cosmic rays are hydrogen nuclei (single protons), 9% are the nuclei of helium and 1% are the nuclei of heavy elements up to iron, according to the University of Chicago. These are called “hadronic particles” because they are composed of hadrons, such as protons And neutronswhich are made up of fundamental particles, called quarks.

“There are also electrons and positrons [the antiparticles of electrons] come to us in cosmic rays, but in smaller numbers than the hadronic particles. These are often called cosmic ray electrons,” Tjus said. ‘Sometimes people also include the neutral high-energy particles – photons and neutrinos – in the term cosmic rays, but in most definitions these are kept separate.”

How do cosmic rays become so energetic?

Cosmic rays are constantly raining down on Earth, but no one knows exactly where they come from.

Cosmic rays are constantly raining down on Earth, but no one knows exactly where they come from.

The core of the cosmic ray mystery is how these particles can reach such incredible energies that they accelerate to near speed of light.

‘We know the energies of cosmic rays quite well – the universe somehow accelerates particles up to 10²⁰ (1 followed by 20 zeros) electron volts (eV),” said Tjus. “For comparison, earth-bound accelerators, such as the Large Hadron Collider (LHC) at CERN can only accelerate particles to 10¹³ eV, many orders of magnitude lower than what the universe can achieve. The mechanism by which particles can be accelerated to these extreme energies is not understood.”

One suggestion is that particles can be accelerated to such energies by a shock front that forms when a material moving at an incredibly high speed hits a slower-moving medium, causing a sudden change in the latter. This would generate a turbulent magnetic field that could act as a natural and powerful cosmic particle accelerator.

One possible way to create such conditions would be a supernova, the explosion that occurs when a massive star dies. This explosion would cause the star’s outer layers to hurtle at incredible speeds until this stellar material finally hit the star interstellar medium – slow-moving gas clouds in between stars – creating a glowing supernova remnant.

‘Supernova remnants are reasonable candidates for cosmic rays coming from within Milky Way. There is evidence that supernova remnants can accelerate particles to approximately GeV energies [around 10⁹ to 10¹² eV]’, Tjus said. ‘At the highest energies, around 10²⁰ eV, we know that these particles must come from other galaxies.’

She said a reasonable source of these higher-energy cosmic rays is active galactic nuclei (AGN), the centers of active galaxies powered by feeding galaxies. supermassive black holes with masses of millions or billions of times as much mass of the sun.

AGN supermassive black holes are surrounded by matter that they gradually feed on, which they also spin with their enormous gravitational pull, causing them to glow brighter than the combined light of every star in the area universe. Matter in these regions that is not fed into the central supermassive black hole can be channeled to the black hole’s poles, where it is blown out as jets of matter at nearly the speed of light. When these jets hit surrounding interstellar material, the collision can also generate cosmic rays.

Another possible source of cosmic rays could be so-called starburst galaxies – that is, galaxies undergoing exceptionally high rates of star formation – that host gamma-ray burstsTjus said.

But if cosmic rays come from some of the universe’s most violent and striking events, why do astronomers struggle to trace these charged particles back to these sources?

A cosmic pinball machine

One reason why it is difficult to find the sources of cosmic rays is because they consist of charged particles that are affected by magnetic fields. When these particles encounter magnetic fields during their long journeys through space to reach us, they are deflected.

So by the time extragalactic cosmic rays reach Earth after traveling millions or billions light yearsThey have been deflected and redirected countless times, ricocheting through the cosmos like a ball in a celestial pinball machine. This makes reconstruction of their original path virtually impossible, but there may be an indirect way to do this.

“When cosmic rays interact with gas, these interactions lead to the production of photons and neutrinos. These are neutral particles that travel straight and can therefore indirectly reveal the origin of cosmic rays,” Tjus explains.


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Currently, cosmic rays are studied over a wide range of energies, from 10⁹ eV to 10²⁰ eV, with scientists looking at everything from the composition of the cosmic rays to their favorite directions in the sky. Tjus thinks that with a combination of 3D modeling and precision measurements of neutrinos and photons associated with cosmic rays, progress can be made in understanding where cosmic rays come from and how they are launched at such incredible energies.

“The riddle of cosmic rays can only be solved by putting together different pieces of information from the different observed messengers,” Tjus said. ‘Today there is a wide variety of observatories providing different types of information.’

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