(February 24, 2018 at 1:39 am)Anomalocaris Wrote:(February 24, 2018 at 1:33 am)vulcanlogician Wrote: think I get what you mean. The universe expands, but not into something. But I'm curious, according to the big bang model (and assuming the universe is finite), is there a place "on the edge" of the universe where one could stand and see the universe in front of him, but if he turns 180 degrees he will see nothing but an abyss? Or does that question even have an answer?
I meant a particular spot in space in the known universe at a particular time.
I know. But for that spot to be “on the edge”, presumably spatially as you mention the edge be be characterized by rotational asymmetry in the sense rotate one way and there is universe in front of you, rotate 180 degrees and there is not, then there must be something on the other side of the edge that is not part of the universe. Thus the issue comes back to how do you define universe in such a way that there can be, for the lack of a better term, regions, that are not part of the universe.
A commonly used definition of universe is all of space spatially and all of time temporally. In such a case spatial edge of universe appear to have no meaning.
You may say is there somewhere that, if you rotate one way, then there are familiar stars, galaxies, and galaxy clusters in front of you, and rotate another and there will by no stars, galaxies and galaxies no matter how far you look. In this case, you are not really at the edge of the universe. You are at the edge of a particular region of a edgeless but non-homogenous universe. We can not really exclude the possibility that the universe may be nonhomogenous in this way. However, we are pretty sure if it is nonhomogenous this way, then the region of the universe that resembles our familiar region is likely to be much larger than the observable part of the universe.
Exactly. Almost all models from the basic BB viewpoint assume spatial homogeneity except for some perturbations. Small variances from complete homogeneity certainly happen and those variances do, in fact, show themselves on the background radiation.
But, as far as we have been able to observe, there is no 'edge' in the sense of all matter behind me and nothing in front. And, if there is such an edge, it is several times the current size of the observable universe (otherwise the gravitational effects during the early universe would have left signals).
