system
Given the following post, respond as if you're commenting on Reddit:
human
If the earth was placed at the border of our universe (some planet must have ended up there?). Will half of the sky be completely black?

Or what will we see or be able to measure from the other side?
gpt
The observable universe — which is often what people mean when they say "the universe," and that ambiguity of terminology causes a lot of confusion — is simply a sphere centered wherever the observer in question happens to be. The radius of the sphere is a function of the age of the universe. Every point that lies within the sphere is sufficiently close to the observer that light has had time to reach the observer from that point in the time that the universe has existed.

So if you could magically teleport yourself (and your telescope, of course) to a point at the very edge of the observable universe as seen from Earth, you would find yourself at the center of a sphere of stars and galaxies that looks entirely similar to what we see all around us. You'd be seeing stars and galaxies that no human has ever seen before, because light hasn't had time to reach us from those stars and galaxies since the beginning of the Big Bang, but other than that, there'd be nothing particularly interesting about your view.

This raises the obvious question: If light cannot yet have reached us from those places, how can we know what you'd find there? Well, the honest answer is we don't. Not for sure. But it follows logically from the assumption that the laws of physics are the same everywhere. The technical term for this is *translational invariance.* Wherever you go, the same basic laws of physics apply.

The reason we have stars and galaxies *here* is because the laws of physics caused them to form out of the soup of matter that emerged from the early stages of the Big Bang. Those same laws of physics apply *there,* so it follows naturally and inevitably that there will also be stars and galaxies wherever you go.

Of course, it's technically possible that the laws of physics are *not* the same everywhere. But in many centuries of people observing the universe on scales both large and small, there's never yet been any evidence at all that the laws of physics aren't translationally invariant. So while it's an assumption, it seems to be a pretty darned safe assumption to make.