  Cosmological perturbations in an expanding universe back-react on the
space-time in which they propagate. Calculations to lowest non-vanishing order
in perturbation theory indicate that super-Hubble-scale fluctuations act as a
negative and time-dependent cosmological constant and may thus lead to a
dynamical relaxation mechanism for the cosmological constant. Here we present a
simple model of how to understand this effect from the perspective of
homogeneous and isotropic cosmology. Our analysis, however, also shows that an
effective spatial curvature is induced, indicating potential problems in
realizing the dynamical relaxation of the cosmological constant by means of
back-reaction.
