[kijin]

No, it's observed as 93 billion light years. That's why we call it the "observable" universe.

There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion.

But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.

[Udik]

Where I supposed that "observed as <distance>" just means how much red-shifted its light is- we don't really have any other way to measure such distances. This in turn only measures how much the space has expanded between us and the original starting point, much closer than 13 billion light years away. So we can say that we're seeing a star that is "now" 46 billion ly from us, but must have been only a few billion ly away when it emitted the light we're receiving. Correct?

[kijin]

You're right. The star could not have been more than a few billion light years away when it emitted the photons that we're seeing. Otherwise, not only would the photons have not reached us yet, they will probably never reach us because the universe is expanding faster than the speed of light.

[pkofod]

I feel so stupid when talking and hearing about physics... but we are not in any meaningful way "at the center" of the universe are we? Or is every point in some sense at the center (a point of reference thing)? I'm asking because why would it be 46.5 "each way"?

[irishsultan]

We are at the center of the observable universe. And not that's not special, any star/galaxy is at the center of it's observable universe.

[spc476]

Imagine we exist in a 2D universe, but one that happens to be the surface of a sphere. Any point you pick on that surface is "at the center of the universe."

[eru]

Hmm, but it looks like the universe is flat, doesn't it?

(Or is it flat like a torus, so doesn't need any curvature to go loop on itself?)

[kijin]

Cosmologists talk about "horizons" a lot, and the analogy of standing on a sphere actually works quite well. Remember that horizons only make sense on curved surfaces. You can't see beyond the point where certain features of spacetime (black holes, expansion, or sheer distance) prevent signals from reaching you, just as you can't see beyond a mountain range or the curvature of Earth itself. Of course you'll need to extrapolate the analogy to three, four, or more dimensions, but the basic idea is the same.

[eru]

I'm not talking about our own light cone. I'm talking about https://en.wikipedia.org/wiki/Shape_of_the_universe

Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe?

Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition?

You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature.

Or the universe might be weirdly shaped, and we just happen to live in the flat part.

So I guess the question comes down to:

* assuming no edges * assume Copernicus at least for space (we might have a special position in time) * What's the simplest theory about the shape of the global universe that satisfies our observations?

I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.

[simiones]

We are by definition almost at the center of the universe that we can observe - we can see as far away in any direction.