Expanding photons.

Just a quick point - all of this occurs in 4d space-time. We think in 3d, which means whatever picture/explanation we put forward for the red-shift is actually some projection of the "true" 4d story, into some shadow "3d" picture. There are many different possible projections and thus many different pictures, which is why you can hear of many very different explanations - just as the question "how big is your shadow" has many different answers...From the photon's own POV, there is no expanding space. It doesn't even move. The distance between the point of emission and point of absorption is zero

I'm rather busy so please forgive the short replies... Aspects of both. Time and distance and are same thing from the 4d perspective (just like distance on a piece of graoh paper is not usually differentiated into x-distance and y-distance) Two points (events) in space-time that are connected by a light-ray (null-ray as we call it) have zero time/distance between them. All the events along the light-ray are collapsed into one point. This seems strange but we don't often think of single space-time pomits/events as being the end markers of a stretch of time or distance. We tend to think of Newtonian time between slices of Newtonian space, and Newtonain distances at one particular moment of Newtonian time.So not really wormhole-like behaviour, more like the "folding space" concept introduced in Lynch's verson of Dune. It is more of a change in the way we perceive the photon. No, it doesn't change anything. But that is not to say that the subject is not intriguing! GR is bizarre and wonderful enough without trying to muddy the waters further

The curvature of space-time depends upon the matter distribution. Far away from matter (in the immense voids of the large-scale structure of the universe, far away from the clusters and super-clusters which are confined to the filaments), the global distribution of matter in the universe dominates and gives rise to expanding space (which is just how we see a particular form of space-time curvature from our 3d POV).In proximity to matter (inside the clusters, galaxies, solar systems, planets, atoms), it those distributions which dominate and dictate the local space-time curvature. The Earth itself dominates our curvature here, and local space-time takes on the Schwarzschild geometry.Consider the global curavture as a hill, and local Schwarzschild curvature as a hole. From inside the hole, the hole looks like a hole, with no clue to the fact that it is situated on the side of a hill. From afar, you simply see the slopes of the hill, and no hole.

'Fraid so... Go read some Brian Greene and you'll soon be on the right track

Exactly! There are no dynamics in GR. Each space-time solution of GR is a fixed, static, immuatable object. So there can be no expansion of the 4d universe. Just as there is no expansion of the Earth. But you could think of the lines of latitude as expanding if you were to journey South from the North Pole... It is our 3d perspective and our passage along the time axis that gives rise to the impression of expansion. Yes. Different projections will represent a different observers with differnt velocities wrt the local rest (comoving) frame. Some projections will result in "unphysical" observers.Edited by cavediver, : added a bit