The universe is expanding so that light received by an observer is red shifted. This means it has less energy

Question

Where did it go?

Answer 1

Energy conservation is a result of a time translational invariance assumption of the laws of physics. Due to universal expansion, this premise breaks down on that scale. Energy as we presently know it is not conserved in General Relativity.

Nonetheless, epidavros is correct in the context of a classical description. You see, the gas that eventually formed earth and the molecule that emitted that red shifted photon were receding from it other from the very start. That means it was red shifted from the day it was emitted *in our reference frame*. Note too that the photon in the reference frame of that now distant atom that emitted it long ago still has the same high energy as it did the day it was emitted.

Answer 2

I don't know the details of the answer, but I have heard and (once) followed the explaination. The "color" of the light is equal to the energy of the photons, which carry also momentum. The momentum depends on the reference frame, even though the speed of light itself does not. Since we are moving away from the light, there is less momentum in our reference frame, so the energy is less, thus the color is shifted in the red-from-blue direction.

Answer 3

It didn't go anywhere.

The light was always red in our frame of reference. And its still blue in the frame of reference of the star that emitted it.

Energy is relative, not absolute.

Answer 4

i think that no one really knows all that stuff. i think that the energy is still there, maybe in some unknown format. maybe it is in other light somewhere else. scientists who study outerspace is iffy buiseness. i wouldnt believe everything.

Answer 5

Nowhere...it's more like "where did we go"..seeing that we're practically running from it ourselves.