  We study the collective radiative decay of a system of two two-level emitters
coupled to a one-dimensional waveguide in a regime where their separation is
comparable to the coherence length of a spontaneously emitted photon. The
electromagnetic field propagating in the cavity-like geometry formed by the
emitters exerts a retarded backaction on the system leading to strongly
non-Markovian dynamics. The collective spontaneous emission rate of the
emitters exhibits an enhancement or inhibition beyond the usual Dicke super-
and sub-radiance due to a self-consistent coherent time-delayed feedback.
