Cosmic rays are incredibly powerful invisible particles, and we can’t be sure where they come from. Not much in the way of a comforting thought, but it makes for a cosmic mystery that astronomers have been trying to solve for decades. And now they have come one step closer.
Here’s what we do know. Cosmic rays are energetic atomic nuclei travelling at near the speed of light. They hit our atmosphere and rapidly interact with the molecules there to break into billions of smaller, less energetic particles that shower down on the life on Earth, without giving us much notice at all. In fact, we didn’t know they existed until 1912, when Victor Hess ascended in a balloon with an electroscope, finding it discharged more rapidly the higher he went. And this trend continues right outside of the solar system. The Voyager I spacecraft proved its departure from the solar bubble with a marked increase in the number of cosmic rays it detected.
Since Hess’ experiment, for which he won a nobel prize in 1936, cosmic rays have been studied extensively, their energies measured and characterized, yet they still don’t have an origin story. It’s been theorized that they originate in the breath of a dying massive star, a supernova, as these events are energetic enough to accelerate cosmic rays to their high energies. Even if they do originate in Supernovae, we don’t know whether they came from within the galaxy or beyond….until now.
The Auger collaboration, a group of over 400 scientists from 18 countries, has been using the single largest cosmic ray detector ever built, the Pierre Auger Observatory in Argentina. The observatory measures cosmic rays by looking at the Cherenkov light that is produced in over 1600 individual detectors, each containing 12 tons of water, spread over a distance of 3000 square kilometres. Cherenkov light is a bright blue glow that occurs when a particle is moving through a medium faster than the speed of light in that medium (slower than the vacuum lightspeed, so no issue there).
By spreading the detectors out, the arrival times of particles at the different detectors allow researchers to pinpoint the direction of origin of the cosmic ray to within one degree, to see if there is any favourable direction.
And it turns out that there is a favourable direction for cosmic rays that is statistically significant. It is from a direction that has a high density of galaxies, suggesting these cosmic rays come from outside of the Milky Way.
Of course, there is more work to be done, and researchers are focussing on the highest energy cosmic rays. This is because the highest energy cosmic rays are the least deflected by interactions in the atmosphere, and so their origin can be pinpointed with greater accuracy.
In 2018 the Auger Observatory is getting an upgrade that will help researchers zero in on the precise locations of higher enery cosmic rays, hopefully giving us their origin for the first time since Hess flew into the storm in 1913.