Time delay of photons of different energies in multi-dimensional   cosmological models
  We consider the general expressions for the time delay of photons of different energies in the framework of multi-dimensional cosmological models. In models with compactified extra-dimensions (Kaluza-Klein type models), the main source of the photon time delay is the time variation of the electromagnetic coupling, due to dimensional reduction, which induces an energy-dependence of the speed of light. A similar relation between the fine structure constant and the multi-dimensional gauge couplings also appears in models with large (non-compactified) extra-dimensions. For photons of energies around 1 TeV, propagating on cosmological distances in an expanding Universe, the time delay could range from a few seconds in the case of Kaluza-Klein models to a few days for models with large extra-dimensions. As a consequence of the multi-dimensional effects, the intrinsic time profiles at the emitter rest frame differ from the detected time profiles. The formalism developed in the present paper allows the transformation of the predicted light curves of various energy ranges of the emitter into the frame of the observer, for comparison with observations. Therefore the study of energy and redshift dependence of the time delay of photons, emitted by astrophysical sources at cosmological distances, could discriminate between the different multi-dimensional models and/or quantum gravity effects. 
T. Harko, K. S. Cheng