It’s Raining Particles, Hallelujah!

Day by day, mysterious cosmic rays bombard our planet. They are some of the universe’s most energetic particles. But where do they come from? What creates them? And most importantly, what can they tell us about the universe? These are exactly the kind of questions that the Pierre Auger Cosmic Ray Observatory is trying to answer.

How It Works

Not-so-humbly stretching over 3,000 square kilometers in the vast plains of Pampa Amarilla near Malargüe in Argentina, the Pierre Auger Observatory is currently the world’s biggest cosmic ray detector. Now, you may be wondering how it is possible that these cosmic rays constantly rain down on us and still you have never seen one in your entire life. That’s because the human eye can’t see them. However, when they arrive in Earth’s atmosphere, they create enormous showers of subatomic particles such as electrons, photons and muons. To detect these, the observatory has come up with a clever ‘hybrid’ system consisting of 1660 water tanks and 24 air fluorescence telescopes.

A map of the layout of the UV telescopes and scintillation tanks. Credit: A. Aab et al., Spectral Calibration of the Fluorescence Telescopes of the Pierre Auger Observatory. Astroparticle Physics. 95. 10.1016/j.astropartphys.2017.09.001.

The water tanks rely on the principle of the Cherenkov effect. This effect is observed when light passes through a medium at a speed greater than the speed of light in that medium, it is similar to the sonic boom but for electromagnetic radiation. This means the scientists can observe these cosmic showers due to the soft blue glow left behind by the charged particles while moving through the water tanks. The telescopes, meanwhile, search for very weak ultraviolet flashes produced when the showers penetrate the atmosphere. With the collected data, scientists can track where the particles came from and how much energy they contained.

Schematic of a particle entering the water tank and emitting light due to the Cherenkov effect. Credit: https://cerncourier.com/a/augerprime-looks-to-the-highest-energies/

What We’ve Learnt So Far

One of the most exciting discoveries came in 2007, when the Observatory found that 27 of the highest-energy cosmic rays seem to come from active galactic nuclei. In other words, the answer to where these things come from may be found in black holes at the hearts of distant galaxies. A decade later, the Observatory took another huge step towards solving the puzzle of cosmic rays. It appears that these powerful particles don’t come from within our galaxy. That implies they’re carrying information from the vast space beyond the Milky Way. However, what these other galaxies are remains unsolved for now.

What’s Next: AugerPrime and Beyond.

Pierre Auger Cosmic Ray Observatory is keeping pace with the scientific advancements, and to get a clearer insight into the phenomenon of cosmic rays, it is developing an upgrade called AugerPrime. As the majority of energetic particles are heavy, highly charged nuclei and not just protons, they get deflected due to cosmic magnetic fields. This makes it difficult to trace their origin. However, AugerPrime represents a solution to this problem by enhancing the surface detectors with scintillation detectors and radio antennas. It has been under construction since 2019 and is currently nearing completion.

Sources:
The Pierre Auger Collaboration, “AugerPrime – Pierre Auger Observatory,” Pierre Auger Observatory, [Online]. Available: https://www.auger.org/observatory/augerprime. [Accessed: Apr. 30, 2025].

A. Aab et al., “The Pierre Auger Observatory: Contributions to the 34th International Cosmic Ray Conference (ICRC 2015),” Nucl. Instrum. Methods Phys. Res., Sect. A, vol. 798, pp. 172–213, 2015. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0168900215008086. [Accessed: Apr. 30, 2025].

Department of Astronomy and Astrophysics, “Pierre Auger Observatory,” University of Chicago, [Online]. Available: https://astro.uchicago.edu/research/auger.php. [Accessed: Apr. 30, 2025].