Astrophysics with UHECRs at the Energy Frontier

Ultra-high energy cosmic rays (UHECRs) are the most energetic particles ever detected, reaching energies up to ~10^20 eV. As such, UHECRs offer unparalleled opportunities to explore the most extreme processes and environments in the Universe as well as probe fundamental physics at energy scales that are far out of reach of terrestrial accelerators. While invaluable clues about the nature of UHECRs have slowly emerged from almost a century of observations and measurements, key pieces of their phenomenological puzzle remain missing, particularly the identities of their sources and how they attain their energies. While contemporary UHECR and multimessenger experiments have offered significant glimpses into the UHECR phenomenon, further revelations will require the enhanced capabilities of a new generation of UHECR experiments. Accordingly, the new measurements will require new theoretical investigations to decipher their phenomenological underpinnings, particularly by leveraging insights from multiple areas of astrophysics and the emerging multimessenger landscape. In this talk, we will discuss the key open questions and challenges in building a comprehensive interpretive framework of UHECRs.