Introduction

Cosmic rays… what are they? Where do they come from? These are the big mysteries of these high energetic particles that bombard the Earth. Because that’s what cosmic rays are: particles, like electrons, protons, or heavier atomic nuclei. They travel through space at enormous speeds and collide with the atmosphere of the Earth. This is why we can detect them. And this is how we can learn about what they are, and where they come from.




The origin of cosmic rays

We know that cosmic rays with relatively low speeds (or energies) come from our own Sun, or stars like the Sun. More energetic particles come from, for example, supernovae. A supernova is a star that explodes at the end of its life. However, we observe cosmic rays at ultra high energies. Stars and supernovae cannot be the source of these cosmic rays. The acceleration of the particles is a result of the interaction with magnetic fields near the sources. We think of this acceleration as a cosmic tennis: the magnetic fields hit the particles, thereby giving them more energy. Just like a tennis ball gains energy every time it is hit by a racket. Stars and supernovae do not have magnetic fields strong enough or large enough to accelerate particles up to the highest energies. We think that these particles come from nearby galaxies with a super massive black hole at its center. But how do we know this?

“Looking” at the universe

We observe our universe with telescopes and satellites. They operate at different wavelengths: we see, for example, the visible light that is emitted by stars. But we also study astronomical objects at wavelengths that are invisible to our eyes, like with radio waves or infrared light. However, we know that stars and other objects emit more than light: they create and accelerate all sorts of particles. These messengers carry new information about the regions of space where they come from. Detecting them gives us a “sixth” sense for studying the universe. This is what scientists do at the Pierre Auger Observatory for ultra-high-energy-cosmic-rays in Argentina.

Detecting cosmic rays

Cosmic-ray particles collide with molecules (like oxygen and nitrogen) in our atmosphere. In these collisions, exotic particles like pions are created. They, in turn, collide with atmospheric molecules, or decay into other particles, like muons or electrons. In this way, the primary cosmic-ray particle creates a cascade, or shower, of millions of secondary particles. These particles reach the surface of the Earth and leave a “footprint” that can be detected with particle detectors. The more energetic the cosmic ray, the bigger the footprint.

The Pierre Auger Observatory

However, the most energetic cosmic rays are also the most rare ones. We see only one particle of the highest energy per square kilometer per century. And exactly these rare particles are the ones we want to study, because they can tell us where they come from. Cosmic rays with lower energies are deflected by the weak magnetic fields that are present in intergalactic space and in our own Milky Way. But cosmic rays with the highest energies are hardly affected by these magnetic fields. Their trajectories are therefore almost unaltered. If we can detect them, we can see where they came from. Because they are so rare, we need a big surface of detectors to detect a good amount of them each year. In Argentina we have 1600 particle detectors spread out over an area of 50 by 60 kilometers: the Pierre Auger Observatory. Here, we have seen, for the first time, that the highest-energy-cosmic-rays do not come from all regions of space. They come mostly from parts of the universe where we know there are galaxies containing a super massive black hole. Whether these black holes are the actual sources of cosmic rays, is still under investigation.

How can we demonstrate that cosmic rays exist?

Our scientific discovery led to a publication in Science, which is well read by scientists. But we want to tell a much broader audience about our discovery, and about the existence of cosmic rays. Therefore, we came up with a way to explain this to lots of (young) people: Cosmic Sensation! We will host an event with dance music and light effects triggered by the messengers from space. Thus, we make cosmic rays visible and audible. You can experience that cosmic rays are everywhere, all the time, flying through everything here on Earth: flying through you! With Cosmic Sensation we won the Academic Year Award 2009, a prize of 100.000 euro from the Dutch Organisation for Science (NWO). The Radboud University Nijmegen decided to help the organizers, a team of scientists and students from the University, by contributing an equal amount of money. The Cosmic Sensation event will take place somewhere in the near future, so stay tuned!

Discovery of cosmic rays

On top of the recent results, it is now 100 years ago that Theodor Wulf discovered that there is radiation in the atmosphere. Victor Hess measured this “mysterious” radiation higher up in the atmosphere, with balloon flights. He discovered an increase in radiation with height, concluding that it comes from outer space onto the Earth: this is what we now call cosmic rays. Many years later, Pierre Auger discovered that these cosmic rays from outer space create a shower of particles in the atmosphere that reach Earth. The detected radiation is that of the particles in the shower. He also found a way to relate the information of the detected particles in the shower to the primary cosmic ray entering the atmosphere. This paved the way to a quantitative analysis of cosmic rays. Therefore, the cosmic-ray observatory in Argentina is named after Pierre Auger.