Photon-photon dispersion of TeV gamma rays and its role for photon-ALP   conversion
  The propagation of TeV gamma rays can be strongly modified by B-field induced conversion to axionlike particles (ALPs). We show that, at such high energies, photon dispersion is dominated by background photons - the only example where photon-photon dispersion is of practical relevance. We determine the refractive index for all energies and find that, for fixed energy density, background photons below the pair-production threshold dominate. The cosmic microwave background alone provides an "effective photon mass" of (m_gamma)^2 = -(1.01 neV * E/TeV)^2 for E < 1000 TeV. The extragalactic background light is subdominant, but local radiation fields in the galaxy or the source regions provide significant contributions. Photon-photon dispersion is small enough to leave typical scenarios of photon-ALP oscillations unscathed, but big enough to worry about it case by case. 
Alexandra Dobrynina, Alexander Kartavtsev, Georg Raffelt
astro-ph.HE hep-ph