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

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... f 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.