This is something that I have wondered for quite some time. Consider the following hypothetical example. You board a space ship, and take it to the very end of the universe (yes, I know that is impossible, esp. now because of speed constraints, etc–but let’s not consider that just now.). What exactly do you find there? A wall? An energy border of some kind? You surely couldn’t just keep finding more space. For that to be true, space would have to be infinitely long, which I assume would be impossible. So what exactly do you find?
Suppose you board a sailing ship, and sail all the way to the edge of the Earth. What do you find there? A wall? A waterfall? You surely couldn’t just keep finding more ocean. For that to be true, the oceans would have to be infinite.
The current understanding is that there is no edge but that space is curved. A reasonable analogy is, as Chronos alluded to, sailing in a straight line on the oceans–you never find and “edge” but eventually get back to where you started. (The analogy has flaws because you’ll never get back to where you started due to the limits of space, time, and space travel but it works as a simple explanation.)
I don’t have a link, but I recall reading an article a few years ago about the Pac-Man effect. You know how Pac-man leaves the edge of the screen, and comes out on the other side of the screen? From Pac-Man’s 2D point of view, his playing feild just goes on infinitely like a giant sheet of paper, but we can see that he is more likely on a limited 2D surface wrapped around a 3D object, like a cylinder.
The theory that the article described was that our 3D universe is sort of wrapped around a 4D object. You could point your space ship in any direction, fly straight, as fast as you can go, and you would eventually end up back where you started, tilted 60˚ to one side and turned 60˚. (I never understood why the tilting occurred, but the article made it clear that it would.)
Remember, Pac-Man thinks he is in a 2D universe, so to him, there is no “down.” It is also not curved in any way.
I found the article, but I don’t have access to it, and the VPN client for my college isn’t working (which might get me into the site). They claim the the universe is a dodecahedron, and you could see yourself in twelve different orientations, if you could see that far away.
Actually, I think that the **Pac-Man **universe is curved is one direction. Since the length of the tunnel from the right side of the screen to the left side is zero the playing surface is effectively a cylinder, curved around from left to right. There is still no “up” or “down” since it’s still just a 2D surface, albeit curved.
The video game universe I loved was Asteroids. Since left and right, as well as up and down, were curved around to meet each other you were playing the game on the surface of a torus! I never got around to designing my video game that was going to be played on the surface of a Klein Bottle.
The latest measurements I’ve seen indicate that space is flat to within our measurement error. And the universe is isotropic - it looks identical in general composition no matter which direction you look in.
These two facts suggest that the universe is either infinite, or so much larger than what we can see, that any curvature it might have happens over such a vast distance that in our own visible universe it approximates being flat.
And in a way, asking what would happen if you hit the ‘boundary’ of the universe is kind of a nonsensical question - like asking what would happen if you traveled a distance between two points farther than the distance between two points.
If there was a boundary where the universe simply stopped existing, I’d expect it to appear as a perfect mirror.
Just like an electrical signal in a cable gets reflected at an infinite impedance mismatch. Only that everything gets reflected. Light, energy, particles, forces. Our universe is a system where everything has to go somewhere. It follows from the laws of conservation that reality must bounce back on itself at the boundary to nowhere.
In science fiction, Larry Niven uses a similar concept of stasis fields in his books. They are bubbles in which time is stopped, appearing perfectly reflective to the outside world.
My understanding is that you would end up where you started. PacMan lives in a 2D world. He would have to travel in a third dimension to exit his universe - but, his pixilated brain doesn’t understand that.
We live in a 3D world. We can travel in any combination of those three dimensions in a “straight” line and end up back where we started. We would have to point our space ship in the 4th dimension to leave our universe. Unfortunatley our meat brains can’t understand that direction. And even if we could, it’s likely we don’t have the means to travel far enough anyway.
So to answer the question “What’s at the end of the universe?”, the answer is that nobody knows or will ever know.
IIRC, the Enterprise encountered the energy border is at the edge of the galaxy, not the edge of the universe. And one of the later books claimed that that was the work of the Q continuum, to keep some especially powerful and unpleasant life-form out.
You’ll keep finding more space because space is expanding at faster than the speed of light so you will never go fast enough to come back around again.
