Strong Gravity Crossroads: fundamental fields in the High-Energy Universe

Historically, each new observational window on the Universe has revealed unexpected phenomena, reshaping our understanding of fundamental laws. Gravitational wave (GW) observations are driving such a revolution. While LIGO, Virgo and KAGRA have illuminated the most violent astrophysical events, next-generation detectors promise to fully unlock the discovery potential of GW astronomy for probing fundamental physics. They will extend our reach to the early Universe, enabling precision tests of General Relativity (GR) and the detection of new particles and fundamental fields.Realizing the full scientific potential of future observations—and analyzing the expected wealth of high-quality data—requires an intensive theoretical effort. This effort must model the diversity of sources and the wide range of potentially detectable effects, exploiting the exquisite sensitivity of both ground- and space-based GW observatories. Equally essential are data-analysis developments capable of interpreting and disentangling the richness of the emitted signals.This quest demands a highly diverse set of expertise that can only be assembled through a collaborative network of scientists who have made pioneering contributions to GW astronomy and strong-gravity research, and who are committed to exchanging ideas and developing new synergies.The STRONG project aims to bring together such a network, with the goal of addressing questions at the interface of relativistic astrophysics, high-energy physics, and cosmology: Is GR a complete description of gravity in the strong-field regime and across all mass scales? Are all dark, compact objects observed in the Universe truly Kerr black holes, as predicted by GR? What are the properties of matter and fields in binary environments, and how do they interact under strong gravity? Could astrophysical environments reveal new fields or dark particles that influence black hole dynamics?