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Recent findings by Imaging Atmospheric Cherenkov Telescopes indicate a large transparency of the Universe to gamma rays, which can be hardly explained within the current models of extragalactic background light.
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This value is in agreement with the WMAP upper bound n e < 2.7 ⋅ 10 7 cm − 3 n_{e}<2.7\cdot 10^{7}\,{\rm cm}^{-3}
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The alternative possibility—concerning a modification of the source emission spectrum—turns out to be hardly compatible with current AGN models [ 1 ] . This is due to the fact that even a small change in the photon mean-free-path λ γ {\lambda}_{\gamma} entails a big change in the observed spectrum, because of the exponential dependence on λ γ {\lambda}_{\gamma}
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Parity conservation requires X X to be pseudo-scalar
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