Connecting the Lab to the Cosmos: The Legacy of the Relativistic Heavy Ion Collider

The Relativistic Heavy Ion Collider (RHIC) recently celebrated 25 years of operation. Over this period, it has produced transformative discoveries, including unambiguous evidence for the formation of the quark–gluon plasma—even in the smallest collision systems, new insights into the role of nuclear structure at high energies, and a systematic mapping of the phase diagram of dense Quantum Chromodynamics (QCD) matter. These results have had far-reaching impacts across multiple disciplines. Heavy-ion collisions pushed relativistic viscous fluid dynamics to its limits, driving theoretical advances that have influenced astrophysics, gravitational physics, and even condensed-matter systems. The detection of subtle signatures of nuclear deformation has brought together traditionally distinct low-energy and high-energy nuclear physics communities in unexpected ways. Furthermore, the RHIC Beam Energy Scan has opened an experimental window into high-baryon-density matter, offering constraints on the QCD phase diagram and potential hints of a critical point, with important implications for neutron-star interiors and binary neutron-star mergers.