How can we still receive photons from last scattering, i.e. the CMB? Does our receiving the photons (not other evidence from CMB) require a constraint on the curvature of the universe or the speed of expansion? I can see how a curvature that described a closed universe would have CMB around forever (kinda like going off the edge of the screen in snake (old nokia phone game) and appearing at the opposite side). Alternatively if parts of the universe were expanding very close to (or greater than) the speed of light relative to us I can also see that we could still receive their photons. If neither a restriction on the curvature or rate of expansion is required, which I don't find mentioned in my books, then I can't see how we are still receiving CMB radiation. It would be like our universe was an expanding balloon, all points shined a torch, very quickly all photons would be outside the balloon! Another related problem I am having is The Horizon Problem. Does it require inhomogeneities from quantum processes? My understanding is that the Universe at last scattering was about 100 (or 300 or 900, one of them) million light years across. The CMB shows that all 4/3 pi (100 million light years)^3 of it was all very much the same temperature. And this thermal equilibrium is very hard to achieve when parts of the universe have never been within each others particle horizon. My issue are, wouldn't they have been within each others particle horizon when the universe was a lot smaller? Is it that the universe is expanding so close to (or faster than) c that some bits still won't be in each others particle horizon? Or is it that quantum fluxuations will have been expanded to greater fluxuations universe before last scattering and the CMB is even more isotropic than that? Or is it something else? Finally, if the universe was a lot smaller and the same mass, why on earth didn't it collapse into a black hole?