Starts With A Bang

What Are The Most Energetic Particles In The Universe? (Synopsis)

The production of a cosmic ray shower, produced by an incredibly energetic particle from far outside our Solar System. Image credit: Pierre Auger Observatory, via http://apcauger.in2p3.fr/Public/Presentation/.

“Energy is liberated matter, matter is energy waiting to happen.” –Bill Bryson

When it comes to the Universe, you might think that energy really is only limited by rarity: get enough particles accelerated by enough supermassive, super-energetic sources, and it’s only a matter of time (and flux) before you get one that reaches any arbitrary energy threshold. After all, we’ve got no shortage of, say, supermassive black holes at the hearts of active galaxies.

Galaxy NGC 1275, as imaged by Hubble. Image credit: NASA, ESA, Hubble Heritage (STScI/AURA).

And yes, we do find cosmic rays hundreds, thousands or even millions of times the energy that the LHC can achieve. But when we think about the Universe in detail, these cosmic rays aren’t unlimited in their energy, but are rather stopped in their tracks by the most unlikely of sources: the ultra-low-energy cosmic microwave background, left over some 13.8 billion years after the Big Bang.

The spectrum of cosmic rays. Image credit: Hillas 2006, preprint arXiv:astro-ph/0607109 v2, via University of Hamburg.

Come get the full story on the most energetic particles in the Universe, and learn why we have those limits at all!