# Interpretation of red shift

We see light as having travelled in a straight line from stars or galaxies light years away from us. However it's path is more likely of multiple curves as a result of gravity along the journey (gravitational lensing) . How can we be sure that the accelerations thus experienced have not produced braking radiation (bremsstrahlung) thereby reducing the energy and therefore frequency of the light spectrum leading to an apparent red shift?

Another way to see that a photon cannot simply emit another photon is by conservation of energy-momentum. If the process $\gamma \rightarrow \gamma +\gamma$ were possible, one should be able to go to the center of momentum frame of the two final particles. However, conservation of momentum tells us that then, the momentum of the initial particle (and hence the energy, applying $E=pc$) must vanish, an absurd situation!