Axionic Shortcuts for High Energy Photons

We study the photon axion mixing in the presence of large extra dimensions. The eigenvalues and eigenstates of the mixing matrix are analyzed and we establish the resonance condition for the total conversion of a high energy photon into a Kaluza-Klein (KK) axion state. This resonant transition, a photon transformed into a KK axion travelling freely through the bulk and converting back into a photon, may provide a plausible explanation for the transparency of the universe to energetic photons. If the brane we live in is curved, then there are shortcuts through the bulk, which the axion can take. Within our model, the photons having the appropriate resonance energy are using the axionic shortcut and arrive earlier compared to the photons which follow the geodesic on the brane. We suggest that such axionic shortcuts are at the root of the dispersion of time arrival of photons observed by the MAGIC telescope. We indicate also the cosmological significance of the existence of axionic shortcuts for the photon.