What are the Universe’s Most Powerful Particle Accelerators?
Every second, every square meter of Earth’s atmosphere is pelted by thousands of high-energy particles traveling at nearly the speed of light. These zippy little assailants are called cosmic rays, and they’ve been puzzling scientists since they were first discovered in the early 1900s. One of the Fermi Gamma-ray Space Telescope’s top priority missions has been to figure out where they come from.
“Cosmic ray” is a bit of a misnomer. Makes you think they’re light, right? But they aren’t light at all! They’re particles that mostly come from outside our solar system — which means they're some of the only interstellar matter we can study — although the Sun also produces some. Cosmic rays hit our atmosphere and break down into secondary cosmic rays, most of which disperse quickly in the atmosphere, although a few do make it to Earth’s surface.
Cosmic rays aren't dangerous to those of us who spend our lives within Earth's atmosphere. But if you spend a lot of time in orbit or are thinking about traveling to Mars, you need to plan how to protect yourself from the radiation caused by cosmic rays.
Cosmic rays are subatomic particles — smaller particles that make up atoms. Most of them (99%) are nuclei of atoms like hydrogen and helium stripped of their electrons. The other 1% are lone electrons. When cosmic rays run into molecules in our atmosphere, they produce secondary cosmic rays, which include even lighter subatomic particles.
Most cosmic rays reach the same amount of energy a small particle accelerator could produce. But some zoom through the cosmos at energies 40 million times higher than particles created by the world’s most powerful man-made accelerator, the Large Hadron Collider. (Lightning is also a pretty good particle accelerator).
So where do cosmic rays come from? We should just be able to track them back to their source, right? Not exactly. Any time they run into a strong magnetic field on their way to Earth, they get deflected and bounce around like a game of cosmic pinball. So there’s no straight line to follow back to the source. Most of the cosmic rays from a single source don’t even make it to Earth for us to measure. They shoot off in a different direction while they’re pin balling.
Photo courtesy of Argonne National Laboratory
In 1949 Enrico Fermi — an Italian-American physicist, pioneer of high-energy physics and Fermi satellite namesake — suggested that cosmic rays might accelerate to their incredible speeds by ricocheting around inside the magnetic fields of interstellar gas clouds. And in 2013, the Fermi satellite showed that the expanding clouds of dust and gas produced by supernovas are a source of cosmic rays.
When a star explodes in a supernova, it produces a shock wave and rapidly expanding debris. Particles trapped by the supernova remnant magnetic field bounce around wildly.
Every now and then, they cross the shock wave and their energy ratchets up another notch. Eventually they become energetic enough to break free of the magnetic field and zip across space at nearly the speed of light — some of the fastest-traveling matter in the universe.
How can we track them back to supernovas when they don’t travel in a straight line, you ask? Very good question! We use something that does travel in a straight line — gamma rays (actual rays of light this time, on the more energetic end of the electromagnetic spectrum).
When the particles get across the shock wave, they interact with non-cosmic-ray particles in clouds of interstellar gas. Cosmic ray electrons produce gamma rays when they pass close to an atomic nucleus. Cosmic ray protons, on the other hand, produce gamma rays when they run into normal protons and produce another particle called a pion (Just hold on! We’re almost there!) which breaks down into two gamma rays.
The proton- and electron-produced gamma rays are slightly different. Fermi data taken over four years showed that most of the gamma rays coming from some supernova remnants have the energy signatures of cosmic ray protons knocking into normal protons. That means supernova remnants really are powerful particle accelerators, creating a lot of the cosmic rays that we see!
There are still other cosmic ray sources on the table — like active galactic nuclei — and Fermi continues to look for them. Learn more about what Fermi’s discovered over the last 10 years and how we’re celebrating its accomplishments.
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