This doesn't make sense to me. The red shift occurs at the moment the light left the object - what is stretching it out after that time?
As the light is traveling through space, space itself is expanding, thereby stretching the lightwave. This phenomenon is independent of whether the object itself is physically moving towards us or not. Physical motion is reflected in standard Doppler shift. Further, if the light source has a strong gravitational field associated with it, there would be yet another (a 3rd) shifting of the light frequency.
As the piece I posted above notes, there are 3 (known) types of shifting: shifting due to actual motion of a light source, shifting due to a strong gravitational field, and shifting due to universal/cosmic expansion. Cosmic expansion and gravitational shifting is always redshifted, whereas standard Doppler involves an object physically moving EITHER towards OR away from us.
For example, light from a remote high mass/high gravity star which, say, happens to be physically moving towards us, would be shifted in all 3 ways.
There would be blueshifting due to the fact that it (the star) is physically (actually) moving towards us, And separate redshifting due to its strong gravitational field AND cosmic expansion. There actually are ways to sort out which effect is causing each shift.
In my example above, instead of star, I should have said galaxy, as it would be impossible, or nearly impossible, to detect such subtle shifting in individual stars at distances where cosmic expansion comes into play.
Gravitational shifting can be towards the blue, as when a photon approaches a black hole. Or the re-tuning of the atomic clocks on GPS satellites.
Thank you for the reply. I can see I am way out of date. I was okay with gravitional curving of space, but space expanding in and of itself, not just the things in the space moving apart, is going to take some effort to get my head around. Could you recommend a good book to read?