Hi PF members, I have a question about electromagnetic radiation propagating in an expanding universe. We know that because of space expansion, light from a distant source gets redshifted, its wavelength becoming longer and its amplitude flatter. If we move far enough from the source, we should reach a point where the space expansion rate equals precisely the speed of light propagation so the light wave becomes totally flat. There is zero amplitude and the wavelength is equal to the space expansion rate. If we move still farther, the light wave can not catch up anymore with the space expansion. That space region has become causally disconnected from the light source. This presents some question to me: At the point where the space expansion equals the speed of light, the light wave has effectively disappeared. It's amplitude is zero and its wavelength equals the rate of space expansion so there is actually no physical EM wave anymore. Where has the energy of the EM wave gone to? In the regions where expansion has exceeded the speed of light, causality with the light source is gone. The light wave has died "before the appearance of that space". Is this a correct interpretation? Thanks !