# (Physics Newbie) Reaching the edge of the expanding universe

Say you have a space ship so fast that it was able to instantly reach the edge of the expanding universe.

and you decided to try and drive straight into that edge, would you :

Ram into something similar to a wall OR Be Ripped apart ?

Note: I did read some other similar'ish posts and one specifically said "there is no edge of the universe", how can that be if the universe is supposed to be expanding ?

Say you have a space ship so fast that it was able to instantly reach...

I don't think there could be any such ship, but hey ho, I'll go with the flow.

...the edge of the expanding universe.

We don't actually know that there is any such edge. Nor do we know that there is no such edge. Some people will tell you the universe is flat and is therefore infinite, but that's a non-sequitur. Unfortunately it's on some respectable-looking websites such as this one from NASA: "We now know (as of 2013) that the universe is flat with only a 0.4% margin of error. This suggests that the Universe is infinite in extent". Oh no it doesn't. Other people will tell you the universe is "closed" such that if you keep going thataway you end up coming back thisaway. We have no evidence for that either. Google on toroidal universe asteroids. Asteroids is an early video game.

and you decided to try and drive straight into that edge, would you : ram into something similar to a wall OR Be Ripped apart?

I don't know. But I do know there's another option. Remember the wave nature of matter, and think of what would happen to a wave inside a water droplet. It propagates through the droplet and encounters the surface, and then it undergoes total internal reflection. For all I know you drive straight into the edge, and all of a sudden you find yourself driving back the way you came.

Note: I did read some other similar'ish posts and one specifically said "there is no edge of the universe", how can that be if the universe is supposed to be expanding ?

I don't know. I just don't know how a universe can expand if it doesn't have an edge. You find people being emphatic when they say there is no edge, but I'm old enough to remember articles like this which talks about the universe being the size of a grapefruit: "The linear dimensions of the early universe increases during this period of a tiny fraction of a second by a factor of at least 1026 to around 10 centimetres (about the size of a grapefruit)". Nowadays you hear people saying the observable universe was the size of a grapefruit. When you ask about the whole universe, they say it's infinite, which IMHO just doesn't square with big bang cosmology. When you press for an explanation, they tend to come up with the cosmological principle, and claim that some observer 43 billion light years away sees the same sort of thing as us. But we just don't know this. For all we know he sees half the night sky as black, or the mirror image of the other, or something else.

• Really insightful, great answer ! – Bernard Walters Jan 13 '17 at 13:50
• @chimchillas : many thanks. I try. There's a lot of I don't knows in the above, but shucks, the universe is usually a bit tricky. – John Duffield Jan 13 '17 at 14:11
• Can you please clarify what you meant by size of grapefruit. What would happen if someone reached the edge of the "grapefruit"-sized universe? – Arvin Singh May 20 '18 at 0:06

There is indeed no known edge to the universe, although we experience the limit that we can only see a certain distance into the universe, this is called "the visible universe" and is due to the fact that light has only had a limited amount of time to travel as the universe is a finite age.

We do observe everything we can see in the universe getting further away from us, which is how it is known to be expanding. There is a particular effect called doppler red shift that makes this measurement easier.

• This answer doesn't answer any of the questions. – Parrotmaster Jan 13 '17 at 11:45
• so there's no wall, being at the "edge", wherever you mean by that is assumed to be the same as being right here. (Of course no-one has actually been there, but there is no reason to suggest anything else.) – JMLCarter Jan 13 '17 at 11:47