The Highest Energy Particles in Nature

Observations of new cosmic messengers have challenged our understanding of the most energetic processes in the universe. The origin of the highest energy cosmic particles are still a great mystery. Giant extensive air-showers observatories, such as the Pierre Auger Observatory and the Telescope Array (TA), have shown that the sources of ultrahigh energy cosmic rays, particles with energy above EeV (10¹⁸ eV ), are extragalactic, but which extragalactic sources produce these particles is yet to be determined. The promise of using neutrinos to solve this mystery has started to be realized. IceCube has detected the first very high-energy cosmic neutrinos with energies that reach above PeVs  (10¹⁵ eV). The sources of these neutrinos are also not easily identified. Finally, the recent announcement of the first gravitational waves to be detected opens yet another dimension into understating the most energetic events in the Universe. The next decade will see these fields come together to solve these long-standing mysteries.