The Science and Philosophy Forum

Is there a conservation of energy problem there? The "lost" energy of the photon (it can't slow down, so it has to shift red) is actually contained in the increasing volume of space?

TheVat wrote:Is there a conservation of energy problem there? The "lost" energy of the photon (it can't slow down, so it has to shift red) is actually contained in the increasing volume of space?

That's a good question. Things get confusing with dark energy, which acts like negative gravity. Here's how I think of it.

When you throw a ball up from earth it slows, losing kinetic energy (KE), while gaining potential energy (PE) of altitude. At some point it stops and the process reverses as it returns. I think of light's "radiant energy" (E=hf) analogous to KE of massive objects.

Similarly, light looses KE (gets redshifted) climbing out of a gravity well. But if you reflect it back with a mirror, it will regain KE (blueshift) on the return trip. So, light effectively gains PE as it looses KE climbing out of a gravity well.

Light leaving a distant galactic cluster, because of expansion, finds itself at a higher altitude from its emitter, than if there was no expansion. If we absorb it redshifted, it should still be the case that our system now has the energy of that low KE photon at a higher altitude (PE) above its source galaxy than it would have been without expansion.

Still, I think it may be too soon to rigorously include dark energy in our conservation laws.