Why would it only expand in one plane not every direction like you’d expect?
Also how is a flat universe even possible? Surely since we live in 3 dimensions the universe needs to be a 3 dimensional shape.
Im probably misunderstanding what physicists are trying to say but that’s why im here.
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The thing is that the “spherical” you are thinking is a volume, a flat volume. Let me give you an analogy in 2d: imagine expanding from a point in the plane (like in a table). The point would expand as a circle, a flat circle. Geometry inside the circle is perceived as how would be the fastest way of getting from one point to another point in that circle. Since that circle is on the table, the fastest way is a straight line, and we call that a planar (or euclidean) geometry.
Now, imagine that instead of expanding the point on a plane, it expands on the surface of a sphere (but still imagine we still live on the surface). The closest path between 2 points on that surface is not a straight line, but a “geodesic” (that is the term that mathematician give to “straight lines” on non-planar geometries). This geodesic is actually illustrated in how planes fly long distances (because they want to find the shortest path between two points) on the surface of the earth. The geometry on the surface of the earth is in fact a spherical geometry.
Now, for the fabric of space-time itself (i’m not a physician, so i hope i dont get most things wrong here), you are talking about a 4 dimensional “surface” that could “live” in a higher dimensional “table” (if planar geometry satisffies) or “sphere” (if spherical geometry satisffies). There’s also a third option which is hyperbolical geometry but i dont want to get into many details.
This is hard to visualize even for physicists (i cannot visualize myself) but most scientists use analogies. There are however methods to measure “straight lines” in order to verify the geometry of the universe, and it seems that these measures have shown until today that the universe geometry is flat.
I hope the explanation is not so obscure and makes things clear.
We have the globe, a spherical depiction of Earth. We also use Mercator projection maps, which is a flat depiction of our spherical Earth. As a consequence of this model of Earth we made (the Mercator projection map), some of the distances and sizes get warped.
You see, Earth is 3-dimensional but the Mercator projection map is 2-dimensional. The universe is a lot more dimensions (we think), but since we’re stuck in this 3-dimensional scope, we can only render maps of it with less dimensionality. According to our models, this depiction is flat. Reality isn’t **really** flat, though, it just appears so from our models.
This Mercator projection of reality is also why we get weird results from our equations. Like length and time contraction due to Special Relativity. Spacetime isn’t **really** contracting and expanding, they’re just a consequence of our Mercator projection in the same way distances and sizes get warped on our Mercator Earth map.
TL:DR – reality isn’t “flat”, our models of reality are “flat” – which is all we get.
It is three dimensionally flat, which just means that it isn’t curved in the fourth dimension.
Gravity curves space in the fourth dimension which is why we fall towards the center of gravity wells. Overall, the whole of space, that we can see, is flat, not curved.
Any deeper and this is way past a ln eli5.
Others have gone more into the specific answer to this question, but I just want to point out that “flat” is a terrible term for the layman and should not be taken at face value. This is not uncommon in physics; terms stick around by inertia and nobody wants to change them. It’s important in physics to actually look into the explanation of a thing, and leave at the door any assumptions about it based on a name. Other examples:
“Color Charge” – quarks have no color. This isn’t even really an analogy, it’s more of a mnemonic. The charges could have been named after the three musketeers with equal validity. By extension, “Quantum Chromodynamics” has nothing to do with color or chromaticity.
“Big Bang” – it was not big, nor a bang. The term originated as a sarcastic joke.
“Virtual Particles” – are very much real. In a similar vein, imaginary numbers are not imaginary.
“Black Holes” are not holes, they are compact objects at their center (white holes are not an accepted theory at this time).
“Theory of Everything” – can be taken to imply that finding it means the end of physics research. Nothing could be further from the truth.
“Recombination” – there was nothing “Re-” about it, this is the first time *ever* that protons/electrons combined into atoms
“Spin” – this one is weird because particles are not actually spinning, but they do have angular momentum. So the term isn’t terrible but somewhat misleading.
Scientists say a lot of stuff, for good reason, but they have no idea what “shape” the universe is. Spherical or flat are just two possibilities.
Flat like a blanket, which is a 3D shape with top, bottom, and sides. Like a blanket stretched taut, heavy objects like planets make it sag in spots, that’s gravity. Incredibly heavy objects like black holes tear a hole right through all 3 dimensions of it. String theory (M theory) talks about whether the blanket is pulled taut or fluttering like a flag in the breeze, and how that might work
It is assumed to be flat in the fourth dimension but it is round in the third dimension as you would imagine from a point expanding equally in all directions.
Flat in 4d means it will expand endlessly in all directions until everything is very far apart, this is because there is slightly more pushy out force than sucky in force. If the universe were round in 4d there would be more sucky in force so it would eventually shrink again
Take a piece of paper and draw a circle on it. Label that circle “the universe”.
Now, is the universe flat or circular? Well that depends on who you ask. If you ask yourself, it looks pretty flat, you know, like a piece of paper. But imagine someone that lives inside that circle, and they’ll say it looks like a disk.
You said “we live in 3 dimensions”. Well, are you sure about that? Are you sure it’s not 4 dimensions but since you live inside it you only see 3 of them? Because in that case, you would say that the universe looks like a sphere, even if it was really flat.
And the universe is indeed 4 dimensional. For whatever reason, we can’t see that. Confusion abounds as a result.
I wonder how many people used ChatGTP to generate their answer, since three of the top answers are nearly identical.
What they mean is if you would walk in 1 direction around the Earth you you would eventually end up in the same spot, the Earth is curved.
If you were were to walk, or float along space forever you would never end up in the same place if space is flat. That raises the question of how could space be curved? Well that’s a more complicated question that gets into very complicated physics and ideas
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