Cosmological redshift is primarily caused by the metric expansion of space, which stretches photons as they travel through the universe, leading to a loss of energy and a shift to longer wavelengths. Compton scattering is not a viable explanation for this phenomenon, as it would not result in the systematic redshift observed in emission lines from distant galaxies. The redshift is measured by analyzing specific emission lines, which remain consistent despite the stretching effect of expansion. Energy conservation does not apply in the same way on cosmological scales due to the dynamic nature of spacetime in General Relativity. Ultimately, the observed redshift reflects the increasing distance between the source of the light and the observer, rather than local scattering effects.