Ultra-high-energy cosmic rays (UHECRs) — extraterrestrial charged particles with the highest energies detected — have been observed for more than fifty years, yet their origin is unknown. They are purportedly made in powerful cosmic accelerators, though none has been identified. Thus, discovering the sources of UHECRs is fundamental to understanding the high-energy Universe.
Neutrino physics is booming with 2 recent Nobel prizes and the first detection of neutrinos above 1012 eV with the IceCube experiment. On the other hand, ultra-high-energy neutrinos remain unchartered territory. Their existence is guaranteed as they are bound to be produced by the interactions of UHECR with the cosmic backgrounds, on their way from their sources to the Earth. Neutrinos should also be produced directly at the sources. Because they are produced with 5% of their parent cosmic-ray energy and travel undeflected by cosmic magnetic fields, neutrinos with E > 1017 eV are unique messengers to identify the sources of UHECRs.
The recent observation of neutron-star merger GW170814 has brilliantly shown that the challenges of high-energy astronomy will be solved by combining data from a large number of multi-messenger experiments. Ultra-high-energy neutrino astronomy will be central to the quest of understanding the violent Universe in this new multi-messenger era.