…it would be a ‘border’ between nothing and nothing?
In: Planetary Science
You sort of got why “edge of the universe” doesn’t make sense if you define the universe as “everything there is.” As far as we know, the universe is flat, so it just goes on to infinity with no edge. It’s also possible for the universe to be slightly curved so that it’s closed, but that doesn’t give it an edge either since traveling in a straight line just eventually leads back to where you started.
Since all interactions are limited by the speed of light, we can talk about the _observable_ universe (i.e. the region of space around us with which we theoretically could have interacted with) but even that doesn’t have a real edge, and it’s constantly expanding anyway as older and older light catches up to us.
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I like the explanation that our dimension (3D) is like the surface of a balloon and the ballon itself is so to say in the 4th dimension. If the balloon inflates our universe expands. But there is no edge or border, though it might be interesting if one could hypothetically travel into one direction and come back from another direction one day.
Space is not empty, and in the known dimensions of the universe, has no edge. Space is constantly expanding into itself, creating more space. As light is the “speed limit” of the universe, the observable universe is ~13.8 billion years.
But, since that is how we measure “the beginning” everything since then has been expanding, and the farther we look, the faster the universe seems to be moving away. In the idea of cosmic inflation, *space actually moved faster than light.*
The lamba cold dark model presents this idea that “dark energy” (basically a place holder for something not yet understood) is the force behind this expansion, and most of the matter in the universe being “dark matter” that doesn’t interact with light or electromagnetic fields (another place holder for something not understood.)
So, since the universe has always been expanding with the expansion of space, this light has now traveled ~46.5 billion light years. With an euclidean “width” of the universe of around 93 billion light years “edge to edge.”
The universe isn’t expanding into anything. It is everything. Space is appearing between matter and pushing it further apart. The only border is the observable universe beyond which any light will never reach us in time for us to observe it.
We don’t know what is at the “edge”, but space as we think of it, isn’t exactly nothing. Laws of physics like gravity and time have an effect on it. Lawrence Krauss talks about the topic and defines nothing as, no space, no time, no physics. Here are 3 videos of 3 lengths, if it holds your interest.
It’s not going to make sense sadly. When you get to either extreme of size, very big like the universe or q black hole, or very small like quantum particles, our everyday experiences and expectations just no longer work. The universe is infinite. Why? Because it is. Just like there was no time before the big bang, all objects with mass warp space time, all objects without mass travel at the speed of causality, black holes are singularities with infinite density, etc. Things just get weird sometimes.
So there’s quite a lot to unpack with your question.
First when someone says edge, it’s usually or should be followed by “observable universe”. This is the edge at which light has been able to travel to reach us. So from our point of view, the furthest it’s possible to “look” and see something.
Space being a vacuum is another loaded piece of your question. A vacuum is measured in how little stuff there is to cause pressure. This doesn’t mean there’s no stuff. But just that it’s very spread out. On earth at sea level we will have 1 atmosphere. In space it’s something like 1×10^-20 atmospheres depending where you are. There will still be stuff and in star systems there’s more stuff than the space between galaxies.
Space is not the same thing as the universe. Space is just… the space between the stuff that’s in the universe and there is a lot of space in the universe.
Finally there’s the “border” part. If you were inside a donut (or torus) you could keep travelling around and around without coming to an edge. Now you might say that the donut has an outside. If you take the concept of this donut or edgeless shape into 4 dimensions, then there’s no edge as defined by the 3 coordinates of x,y and z. Outside our universe will be a different dimension that we couldn’t see and so the universe (not to be confused with the observable universe) is infinite in all directions.
Space isn’t nothing. There’s gravity, radiation, and molecules/atoms and the base fundamental. In some parts there are heavy concentrations (planets, stars, and solar systems). In other parts there are an extreme lack of those things which is called the interstellar medium (interstellar space or the empty space between solar systems). There’s about 3 to 10 atoms per cubic meter in this ’empty space.’ The edge of the universe is a point where there is no longer the effect of gravity, radiation of any kind, and a true 0 atoms per cubic meter. There is true nothingness not even the fundamental laws of physics apply.
The “edge” of the universe is the edge of what we can see. This is an oversimplification, but because the Big Bang happened about 14 billion years ago, we can only see things that are closer than 14 billion light years away. Because for anything further out, the light hasn’t had time to reach us yet.
The edge is just the border of what’s observable. What’s beyond the edge? There’s no way to know. But it’s almost certainly just more stars and galaxies and stuff. Maybe it goes on infinitely in every direction; maybe it doesn’t. We cannot make claims about unobservable phenomena.
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