Thermal Radiation Mapping the Space–Time Evolution

Over the past decade heavy ion experiments at RHIC and now also at LHC uncovered many properties of quark matter produced in collisions of nuclei. Generally, the data is consistent with the formation of a strongly coupled plasma, which is in or close to equilibrium within a fraction of a fm/c; evolves like low viscosity/entropy liquid as described by hydrodynamic models; and is largely opaque to colored probes. Many models have been developed to describe different aspects of the observations. However, the detailed data that is now available present many challenges to theoretical models, which hints towards how incomplete our understanding currently is. In particular, PHENIX data on electromagnetic radiation is seemingly inconsistent with the standard hydrodynamic space-time evolution of strongly interacting matter and requires new sources of radiation early in the collision.