When a light source moves in a particular direction, the light gets blueshifted along the direction of motion and redshifted against the direction of motion. This Doppler redshift is superimposed atop, and independent of, any cosmological redshift that’s due to the expanding Universe. If the Universe were contracting, there would be a cosmological blueshift instead. (WIKIMEDIA COMMONS USER BREWS OHARE)

Is Energy Conserved When Photons Redshift In Our Expanding Universe?

When the Universe expands, photons redshift to longer wavelengths and lower energies. So where does that energy go?

Imagine the ultimate version of a toy Universe: it’s expanding, it’s full of material, and through it all, there’s one photon ⁠ — or quantum of light ⁠ — that we keep track of and forbid from interacting with any other particle. The photon, at any given time, will have all the properties you expect a quantum of electromagnetic radiation to have, including a direction of propagation, a polarization for its electric and magnetic fields, and a wavelength that dictates how much energy is inherent to that photon.

Well, as photons travel throughout the expanding Universe, they experience the effects of that expansion, which stretch it to longer wavelengths. Longer wavelengths imply a decreased energy, and a decrease in energy implies that either energy is not conserved, or that energy must go somewhere. Either way, it’s a massive cosmic puzzle.

In a light-emitting process like combustion, energy is still conserved. Light and heat is emitted as a by-product of the combustion reaction, but if we include all the stored chemical energy in the molecular bonds of the wood and the atmosphere’s oxygen, we find that energy is conserved between the initial and final states. (WIKIMEDIA COMMONS USER DARIO CRESPI)

After all, if there’s one thing we’ve learned about energy, it’s that it can neither…