Do redshifted cosmological photons really violate the principle of   energy conservation?
  Although the Universe is far from understood, we are fairly confident about some key features: Special Relativity (SR) describes the kinematics of inertial frames; General Relativity (GR) explains gravitation; the Universe had a beginning in time and has been expanding since. Nevertheless it is quite difficult to see the 'big picture', although the idea of applying GR to the entire Universe has been very successful with a model emerging that is consistent with observation. One unpleasant feature of the model is that cosmological photons appear not to conserve energy, and the only explanation forthcoming is the claim that GR is exempt from the principle of energy conservation. It is demonstrated here that cosmological observations may legitimately be projected onto flat spacetime and can then be treated Special Relativistically, whereupon energy conservation is restored. This is not to say that the concordance General Relativistic cosmological model is incorrect, just that in observational terms there is no energy conservation anomaly. 
Alasdair Macleod