Photon intensity interferometry for expanding sources

Using Quantum Field Theory we derive a general formula for the double inclusive spectra of photons radiated by a system in local equilibrium. The derived expression differs significantly from the one mostly used up to now in photon intensity interferometry of heavy-ion collisions. We present a covariant expression for double inclusive spectra adapted for usage in numerical simulations. Application to a schematic model with a Bjørken-type expansion gives strong evidence for the need of reinvestigating photonphoton correlations for expanding sources.