Let me try to establish two things up front. First, is why I am making this post. And that is fairly straightforward: we have a pool of knowledge here that may provide clarification/refutation on some of the things covered below. Second, why do I even have a notion that the expansion is one of the types in the thread title. This, also, is fairly straightforward. A ballon, while expanding, gets thinner. To put it another way, the ballon is made of no more material when blown-up then when deflated. The material is stretched thinner. An empire which expands, on the otherhand, will do so by acquiring more territory. It has grown. Expansion is a general word. It can be applied to two quite different processes and yet say nothing of how a process is occuring - other than via context. And in the context of the expansion of space it would seem to say very little. I am, or should more accurately state, was open to either form - or a third if anyone can think of a suitable analogy. I am, in a sense, still open to either, yet also feel that with only a little additional reasoning one can be laid to rest, even if the other remains with a question mark over it. Where did I get these ideas from? The ether. I'm sure you'll be pleased to know I'm not talking about that ether. No, it's really just random thoughts colliding. I have reached the point of making this post primarily because of a series of Susskind lectures on cosmology, available on youtube. After an initial post on these forums using a thought experiment, which, while terribly erroneous, still managed to reveal alot, I was still left unsatisied about the mainstream view on how space expands. Susskind's lectures add considerable weight to the view of a growing space. He seemed quite clearly to be telling his students that new space forms and this is why the cosmological constant stays constant, not diluting as the universe expands. Multiple times he offered an analogy of a rubberband being stretched, yet never breaking as "an invisible reservoir of atoms" filled in the forming gaps between the bands existing atoms. So how do you view this? I can find nothing wrong on the surface. Space must be expanding in some fashion and the idea of 'growth' does not preclude any of the observed phenomena, such as redshift. Redshift, however, is much trickier to explain in a stretching space. Well, the redshifting itself isn't. But as the light enters a 'less-stretched' region of space, you would expect it to begin blueshifting. But by how much? Enough to entirely undo the redshifting (which of course would be contrary to observation)? Does redshifted light in a growing space also experience some amount of gravitational blueshift? This would make objects further away still than they are already calculated to be. As you can see, my inability to provide answers to these questions, is partly what leaves the issue so wide open. From here we move into speculation, some mine: some not. Following Susskind's lead, rather than turn every sentence into a mouthful of "if, buts, maybes" and "it is suggested/expected" I will just say it plain and you can take it with as big a pinch of salt as you care to, as your leanings on the question of inflation go. During inflation the vacuum energy was considerably higher than it is today. The bottom dropped out after a number of e-foldings had occured. In Susskind's words "the universe went over an edge". As well it did. Most of the VE was turned into thermal energy and other particles. The Vacuum was then left with very little "in the tank", so to speak. So, what's this got to do with space? If new space is formed during expansion, does it come into existance containing the post-reheat energy or the inflation period energy? If the former, why? If the latter, it might suggest the energy spreads out around the region leading to the notion that the expansion has steadily been filling the tank back up, a drop at a time. The stretching space scenario seems to have a superficially simpler explanation for the constancy of the vacuum energy, via concepts such as tension increasing the potential energy. This would seem to move toward string theory. There are questions about how either idea plays out in a matter dominated region, with stretching space coming out ahead, compressing then in the presence of gravity in accord with the curvature of General Relativity. Growing space, however, has a lurking can of worms labelled with a big question mark of ultra-speculation waiting for us beyond the more straight-forward acceptance that while it may grow, once grown, space simply acts according to GR. Part of this is related to a very simply question: does space stretch, then fill in with new space, or does new space form, pushing out the 'old' space? This question provides a neat cut-off between a direct, potentially meaningful querying of current views, versus baseless speculation. I hope I have stayed on the rightside of this line throughout and apologise if any of it goes too far. Having said that, one last area does remain. And I feel it shouldn't be over-stepping the mark to make some logical deductions about it. The Big Bang. If space has been expanding through stretching, then it implies there is no more space today than there has ever been. However, the growth of space would seem to lead inevitably to the idea that t=0 also means space=0, such that the undesired singularity is reduced to non-existance. I make no assertion that this explains the existance of the universe, and only reveals another in the series of Russian nesting dolls. Indeed, I'm fairly certain the general idea isn't even original. I came here to learn. Not to teach.