So general relativity gives us some great ideas about the shape of the universe, But truly, No one can know for certain what shape the universe is until we are able to see it from the outside, But knowing the shape of the universe Can give us many insights into the nature of the cosmos itself. It all really comes down to two basic models, Does the universe have a positive curvature or a negative curvature? I mean, There is the option it has no curvature and is flat, We just don't know, But A positive curvature fits nicely in a spherical bubble like universe model, while A negative curvature more or less tells us, The universe didn't have enough mass to form a positive curvature and therefore may expand infinitely, While the closed positive curvature does fit nicely with multiverse theory, But one needs to mention in a positive curvature universe it may not expand infinitely and will eventually collapse back in on it'self. So Pros and cons to both I suppose :/ As I understand it, Knowing the shape of our universe could tell us things like, if this universe were infinite then there would be no multiverse, Because this one universe was infinite, So, negative Curvature equals an infinite universe with no possibility of a multiverse, While positive curvature equates to, A Finite universe with the possibility of others because a universe is a finite thing. I propose this theory, It's just a thought, What if the Cosmic Microwave background isn't an end, But a horizon the hides a curvature.

how is this even a question? the only shape that makes sense in a 3d world is the postive curvature one, it doesn't matters if its infinite or finite, the universe expanded in a spherical shape

I can't disagree with you there, I believe fully the universe is spherical, However, It's a science kind of glitch, We can't say factually it has a positive curvature because we can't prove it, It's one of those ideas, People really like, But we really can't prove that. Things like proof are very important when it comes to debating science factual discussion bro.

It's a huge question in astrophysics. And if the universe has positive curvature, then it has to be finite. The Baryon Oscillation Spectroscopic Survey Collaboration, aka (BOSS), measured the curvature of the universe and found that it's flat to within observational error, which points to an infinite universe: https://phys.org/news/2014-01-baryon-oscillation-spectroscopic-survey-universe.html We only see a spherical cosmic horizon because looking into deep space means looking into the distant past. At a specific distance this means seeing the "surface of last scattering," which is the moment when the universe became transparent to light after its initial opaque plasma state. So let's say that you could jump a distance of billions of light-years in any given direction. The universe would look spherical from that position as well, and areas of the universe that we can't see from our position would become visible, and other areas in the opposite direction wouldn't be visible any longer.

i don't think this should even be a question, the inverse curvature and flat model don't make sense in a 3d world

It's not a 3D universe; it's a 4D universe. An infinite, flat 4D universe, according to our best measurements. Here's how an infinite universe can exist within a Big Bang model: Cosmology FAQ: How can the Universe be infinite if it was all concentrated into a point at the Big Bang?

Well, flat is a shape: an infinite shape, as opposed to a closed, finite shape. But just to be clear - it's flat in all directions equally, not flat like a pancake. We may not ever know if the universe has an overall shape, however; we can only go with the precision of our observational measurements. That level of precision will always increase, but there's always the possibility of a curvature that's so small that we'll never be able to detect it - a universe that's almost infinite, but not quite. We can only really go with what we can measure. So as far as we know, the cosmos is infinite in scope. Which is totally mind-boggling. I always assumed that the universe was finite, so if you could travel far enough, you'd end up back where you started. That doesn't seem to be the case though, as far as we can observe anyway.

yeah but calling it a flat shape is weird, it implies bi-dimensionality, it would be better to call it the no shape model

Then don't think of it as Flat, Because it's not really, Think of it as a plane of expansion, Does it expand on a curve or does it expand in a flat way, it's not really suggesting dimensions of the universe itself, Just the expansion process. I mean how can we suggest shapes of the universe when the question is, What's the shape of the universe? So we study its expansion.

There are some thoughts on the curvature of the universe. as Relativity suggests, The shape of space can be curved by mass, I mean, Spacetime may play a small part in proving positive curvature, Who knows.

No, the term "flat" is more illustrative than "no shape" - it means that in any given slice of spacetime, the overall topology is Euclidean. Which implies that the net mass of the universe is zero. The term "flat" is chosen to define a specific (zero net mass-energy) condition within the context of the general theory of relativity, not because of anything to do with the expansion. I can't really tell what you're saying here, but the whole point is that when you add up all of the components of the universe - whatever they may be ranging from baryonic matter to photons to dark matter and dark energy and gravitational fields and neutrinos etc., the sum of all of that stuff adds up to zero. That's why the overall geometry of the cosmos is Euclidean, aka "flat." If the sum of everything added up to a positive mass-energy, the curvature would be positive, aka "closed." If the sum added up to a negative magnitude, the universe would be "saddle-shaped" - negative curvature. The fact that it's neither of those to within observational error is very important and very interesting, because it not only indicates an infinite universe, but it also indicates that the entire universe could've arisen from nothing, because it adds up to zero - which suggests that the universe could've been born from a quantum event that obeyed the conservation of energy.

Gravity, Curves spacetime yes? So then we can assume mass curves space, Hinting at a positive curvature imo, I know we can't perceive net mass-energy, But it doesn't negate that we just haven't identified it yet, in the end, we base this on observable data the observable universe is only what we can perceive and we don't perceive curvature, That doesn't suggest it's not present though. The possibility exists, we can't empirically claim the universe to be flat brother.

I mean, The earth seems flat in some places, until we hit that horizon, we know the earth to be round. But as we look into the cosmos, What is our assurance that the universe is flat and we simply don't have a large enough FOV?

I like to be optimistic, I like to think those fantastic things are possible, But I'm analytical, I have to support the numbers. The Zero net mass-energy thing is pretty damning, It's basically proof. In fairness, Thomas has earned the point with this, So far, In this Debate. The universe is decidedly Flat. Massâ€“energy equivalence - Wikipedia The truth can be rough, But We have to take the truth and go with it. As it stands right now, That the universe is flat is almost empirical. So then, I'm off to postulate an infinite cosmos

It's possible that we just happen to be within a flat observable section of a vast unobservable universe which has an overall curvature on larger scales, or that there's a very small curvature that we can't yet detect. The first possibility would require an explanation for why our part of the universe is different than the rest though. It's not almost empirical, it's empirical: astrophysicists have actually measured the geometry of the universe and found that it is flat to within observational acuity - so that is by definition an empirical determination. We now know that gravity curves spacetime in both directions: positive (around planets and stars and galaxies), and negative (between the galaxy clusters). That's why test bodies fall toward positive masses, and why the galaxy clusters are accelerating apart. It's very interesting that both of these curvatures, summed together, yield no detectable curvature overall, on the largest observable cosmic scales. That seems to indicate, to me anyway, that the two are intimately connected at a fundamental level. Because if local positive gravitational effects, and the long-range "dark energy" effect, weren't deeply connected, then we'd have to explain why they happen to equal the exact same magnitude on cosmic scales. It's almost as if the universe is like the surface of a liquid, and for every body of matter that floats on the surface, forcing it down...an equal volume is forced upward above the "zero line," so that overall, the whole system adds up to zero curvature: neither positive or negative.

or even better, call it the regular model, because its has regular expansion and not a strange non-euclidean one