The title is a bit misleading. The particle doesn't break any "speed limits", in the sense that there is nothing fundamental preventing particles from traveling

The title is a bit misleading. The particle doesn't break any "speed limits", in the sense that there is nothing fundamental preventing particles from traveling faster (or more precisely, having more energy). The GZK cutoff simply implies that particles are likely to interact with relic photons above this speed. So, the correct interpretation is that either 1) the observed particle started out with much more energy, and lost some via the GZK mechanism, 2)it didn't travel very far to get to us, or 3) we were just "lucky" to see it (which is already built into the rarity of such high energy particles). See also this figure [1], showing expectation energy vs. propagation distance for various initial proton energies. You can see that above 10^20 eV, particles will on average loose energy pretty quickly until they are at or below this energy, on about a 250 million lightyear distance scale. [1] http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_proton.jpg http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_prot...