I think this have been discussed a lot, however, here it comes again right in front of me. I'm talking about this article (and also in my class notes): http://en.wikipedia.org/wiki/Sachs–Wolfe_effect#Integrated_Sachs.E2.80.93Wolfe_effect and in particular at this paragraph: So my problem is, how can you talk about an accelerated universe when you look at a supercluster object? The accelerated universe makes sense in the FRW metric. However a supercluster (a group of galaxies+clusters that I believe are gravitationally bound -and so the gravitational redshifting of ISW) cannot be described by FRW metric... So how does the well gets "stretched" during the photon's stay in it?