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jiggiwatt t1_j2narkx wrote

Reply to comment by mfb- in How do galaxies move? by modsarebrainstems

I've heard the analogy used that you can think of 3D space as flattened onto the surface of a balloon which is expanding. No matter which direction you take, you always end up back where you started (after a relativity breaking amount of time.

Edit: as the balloon expands, the actual material expands similar to how spacetime is expanding. Interestingly, at some point eons into the future, this expansion will make the milky way an island where everything else is so far away, any future civilization will never receive its light and will think our galaxy is all that exists in the universe.

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BlinkOnceForYes t1_j2p1qkv wrote

Expansion-wise, sure. But we haven't concretely proven the 'shape' of the universe. Would we end up at the other side? Or would we keep going infinitely far?

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Wroisu t1_j2p747l wrote

The 3D universe can be thought of as the surface of an expanding hypersphere. If the universe weren’t expanding, you could go all the way around and come back to where you started.

But since it’s expanding, you’ll never be able to move fast enough to come all the way back around again.

A “finite but unbounded universe”

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TheMace808 t1_j2puavw wrote

It’s not proven to have positive curvature like a balloon, at the largest scales we could measure the universe has no base curvature. It expands in every direction all at once like dots on a balloon but isn’t shaped like it as far as we know

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Wroisu t1_j2pureb wrote

the argument I’d give in return is that it only appears locally flat (local as in the entire observable universe) because the total thing is much larger than 93 billion light years across. Like if your entire observable universe was Kansas, but you didn’t know Kansas was part of a globe.

The margin of error for positive curvature is 0.4% so… within the limits of things that are known and possible.

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TheMace808 t1_j2puvor wrote

Yeah that’s why I said as far as we know we can think up and theorize many a thing but the evidence we have suggests it’s flat

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koebelin t1_j2py05v wrote

There’s probably an infinite number of areas of space like what we call “the universe” for trillions of light years in every direction, some expanding, some colliding, some contracting. This is one idea some people have.

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