Can we ever know if there is anything on the "other side" of the universe?
If the universe is expanding, it must be expanding into something, right? If so, then if the universe is expanding at the speed of light, is it even possible to ever find out what's on the "other side"? Not looking for religious answers. I really want to know if this is even physically possible?
Answers ( 2 )
To talk about the "other side" of the universe implies that you consider us to be located on "this side". Arguably we are in the centre of the universe, although anyone anywhere else can make the same claim. It works like this:
At the Big Bang, everything was in the same place. After that it all headed "away" from that centre. The "edges" of the universe would thus be defined by how far away the bits moving as fast as possible (i.e. the speed of light) have gotten since then. That makes a sphere about 14 billion light years in radius. (I'm ignoring the period of Cosmic Inflation because that makes it weirder still, and this bogus explanation is already weird enough.)
From our point of view, we aren't moving; the other stuff is just shooting off in all directions from where it began, namely right here. That puts us at the centre of the "light sphere", therefore we are at the geometrical centra of the universe. But so is everyone else. (Run the same argument for them and see!)
Whether you buy this premise or not, we are never going to have any interaction with anything moving away from us at the speed of light, unless it hits a mirror and bounces back; and then we'll still have to wait at least 14 billion years to see the reflection. And I doubt that there are any mirrors out there.
No, it's not "expanding into something", it's just expanding. The distance between every pair of separate galaxy clusters is growing. You could perhaps interpret that as more new space being constantly created, locally, at every point in deep space. Or as space being stretched. Or gravity constantly destroying space within galaxy clusters so that it's us who are getting smaller.
It doesn't matter if there is "other side" or not (e.g. a higher-dimensional space the universe is embedded in, or some sort of unreachable Edge of the Universe), the Universe is not expanding into it. The phenomenon of expansion, as observed and measured, is here and now (and at the observable distances and in the observable past).