An infrared background--TeV gamma-ray crisis?
  We consider the implications of the recent determination of the universal infrared background for the propagation of photons up to 20 TeV from the active galaxy Markarian 501 as observed by HEGRA. At 20 TeV the mean free path for photon-photon collisions on the infrared background would be much shorter than the distance to Markarian 501, implying absorption factors of the order of exp(-10), or greater, and consequently an excessive power output for this active galaxy. Possible solutions of this problem are discussed. 
R.J. Protheroe and H. Meyer