Given that the universe is expanding over time, in the sense that the (spatial) metric is changing over time, corresponding to the physical distance between objects increasing, naïve intuition leads me to the conclusion that the wavelength of a photon travelling from a distant galaxy (receding us) will be stretched, and consequently its frequency will decrease, leading to the energy of the photon decreasing (with this energy simply being lost, since time translation symmetry is broken due to the big-bang). The problem with this is, relative to another observer, wouldn't the wavelength be stretched by a different amount and hence the redshift the photon will be different, corresponding to a different amount of energy being lost by the photon.
This all leaves me feeling confused on the subject. Does the photon actually get redshifted and lose energy due to cosmic expansion, or is it simply an observer effect (akin to the standard Doppler effect). It does seem a little counter intuitive that a photon would lose energy simply due to its propagation through space?!
Is the whole point of this that it is an observer dependent phenomenon and the energy of an object is an observer dependent quantity (energy is not conserved one moves between two different frames of reference)?!