  A model is presented where there exists another U(1) gauge group which is
extremely weakly coupled to that of QED except inside the core of domain walls.
It is possible to choose parameters such that standard model photons crossing
such a wall are mixed maximally with the 'para-photons' of the other U(1). We
use this model to explain the apparent low luminosity of high redshift
supernovae. A possible mechanism is outlined where the domain walls correspond
to changes in the relative orientation of branes in a compact space. The model
is rather contrived but has the advantage that for a wide range of values it
can solve the cosmic coincidence problem - an observer at any redshift would
come to the conclusion that their universe had recently started to accelerate.
