  A theoretical framework is developed for treating the quantization of the
photons in a spacetime with a longitudinal expansion. This can be used to study
the production of the photons through the non-equilibrium relaxation of a
disoriented chiral condensate presumably formed in the expanding hot central
region in ultra-relativistic heavy-ion collisions. These photons can be a
signature of the formation of disoriented chiral condensates in the direct
photon measurements of heavy-ion collisions.
