Every second, the Earth is bombarded by thousands of tiny atomic nuclei called cosmic rays. Most of them scatter in the upper atmosphere miles above the Earth’s surface before ever reaching us, although a stray one occasionally gets through, hits a computer and scrambles a few bits.
These dime-a-dozen cosmic rays originate either from the sun or from somewhere within the Milky Way; sorting out the probable origins of different kinds of cosmic ray particles based on their energies has been the bread and butter of many an astrophysicist (which is why sometimes it feels like we keep discovering their origins in the headlines).
But then, every field has its white whale, and for cosmic rays this is a category known as “Ultra High Energy Cosmic Rays,” or UHECR. The first one ever detected, in 1991, was named the Oh-My-God particle, and for good reason: that one little atomic nucleus had as much energy as an entire baseball traveling at 58 mph (94 kmh). This is sixty times more energy than the Large Hadron Collider (LHC), the world’s most powerful particle collider, can create. You’d need an accelerator more than 70 million miles in diameter—about the size of Mercury’s orbit—to produce one UHECR. What’s more, such a tiny particle packing so much energy challenges our understanding of physics. Theoretically, UHECR should basically not exist.
But they clearly do, and we don’t know of a process in our galaxy or its immediate surrounds that could give an UHECR its immense energy (although the jets of particles that spew from supermassive black hole jets are one suspect). On top of that, the cosmic rays that hit our planet have to come from somewhere relatively local: after 160 million light years of travel, every cosmic ray in the universe will almost certainly collide with a photon from the Cosmic Microwave Background, the radiation still left from the dawn of the universe. To make things even more mysterious, no one knows exactly what kind of atomic nuclei UHECR are. All of this makes them intriguing, not just for cosmic ray physicists, but for astrophysicists and particle physicists as well because they give us our only chance to study particles we can’t create on Earth.
Unfortunately, UHECR are extremely rare. On average, a square kilometer of the upper atmosphere gets hit by one just once every century. You could never collect enough observations from one spot to work out where they’re from and what they’re made of, so physicists had be clever about it. The answer: an UHECR that hits the upper atmosphere triggers a cascade of particles, known as a particle shower, which can be detected by a ground-based water tank just a few meters across. Put one of these water tanks every square kilometer, to cover 1,160 square miles, and you should get enough UHECR detections after a few years to say where they come from.
And that’s exactly how the Pierre Auger Observatory, located in the remote, flat plains of the Mendoza province of Argentina, was designed. It consists of 1,600 particle detectors covering an area larger than Rhode Island (or Luxembourg, if you use the metric system). The project involves 400 scientists from 18 countries around the globe, and slowly, over time, they’ve been gathering UHECR detections.
And now, according to a couple of new papers in Science, there is strong evidence from the Pierre Auger Observatory that UHECR clearly appear to come from outside our galaxy. This is not an easy conclusion to make: because UHECR are charged particles, their trajectories are altered by galactic and intergalactic magnetic fields, which deflects their paths by up to a square degree on the sky before they reach Earth. (For comparison, the full moon covers a half square degree in area.) After modeling to account for this, the data show UHECR come from a part of sky away from our galactic plane, and where the distribution of galaxies is fairly high. This is very exciting, because it means UHECR are the first cosmic rays that we know originate from outside our galaxy.
There are still a lot of mysteries to sort out. What are UHECR actually made of? What process creates them? Can we narrow down their point of origin even better?
But every scientific journey begins with a single step, and that is what we are seeing today.