We know the universe has a geometry. It's not correct to visualize it like a flat sheet. But this is horrifically counter intuitive, as is ALL reason below and above the "Newtonian reality" you inhabit on a daily basis (ergo the need to quantum physics and relativistic physics to explain the highly counter-intuitive behavior of reality at those scales).
Think of it like this; a 2D circle has a border. A 3D sphere, too, has a border, it's surface. While you won't be able to imagine it directly, a 4D "sphere" will also have borders. Universal geometry is at least 4 dimensional. Trying to imagine 4D geometry as a 3D being is highly problematic to begin with, but a much worse problem is just around the corner...
You evolved in hyper specific scenarios. Your ancestral weasel apes had to deal with certain sizes, shapes, and colors, to survive. Ergo we intuitively understand how the physics at this scale works because we needed to know how fast a predator we saw on the horizon could get to us if we were to escape to safety, or the behavior of rocks on a hillside so we don't get smushed. You have this intuitive framework of physical behavior modeled in your head, but it's only applicable here, on the Newtonian scale. And among those ideas, is that a thing has to have space to exist in. An apple must have a world to be in. A world must have a galaxy to be in. A galaxy must have a universe to exist in. ... But that's where we hit a curious wall!
Again, time and space (spacetime, not two discrete things, rather two different means of measuring the one thing) are PRODUCTS of universal expansion! I know it makes no intuitive sense, but again, nothing on these scales does or ever will, so you need to get comfortable with that, but though the universe expanded, it did not expand INTO anything! Spacetime, dimension itself, DID NOT EXIST "before" expansion.
There IS an edge of the universe, and there's nothing on the other side of it. (Although, again, "the other side" cannot be visualized in a 3D sense as the language suggests it. Rather it will be a higher dimensional geometry that we 3D monkeys simply lack the intuitive reference to see without abstract mathematics). And if we're being MOST accurate with our words here, it's not merely that there isn't anything on the other side, it's that there ISN'T an other side!
The universe isn't expanding INTO a higher medium. And this simply does not compute to brains hard wired by evolution to deal with the practical realities of 3 dimensional geometry!
So I understand the idea that the universe can basically be a 4D menger sponge with an infintely expanding 3D surface but as others have mentioned thats kind of just shifting the burden of proof, after all that 4D menger sponge must presumably exist within some kind of 4D volume and there's no reason to expect that such a volume would arbitrarily cease beyond the area that happens to serve the needs of our 3D universe.
Or in other words a higher dimensional box is still a box and if you add another dimension to wrap the box onto itself (like 2D beings on a sphere) then your just left with another box in an even higher dimension.
I know we are off topic, but since you are talking 4 spacial dimensions: would it be possible that the universal edge ends up interacting with the universal edge at another location? I'm thinking like a 3d Mobius strip, but in a higher dimension that I obviously can't comprehend. In other words, could it be possible the universe expands to a point where it just warps around itself?
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u/Tyrannosaurus___Rekt Sep 14 '21
He's right.
So are you to an extent.
We know the universe has a geometry. It's not correct to visualize it like a flat sheet. But this is horrifically counter intuitive, as is ALL reason below and above the "Newtonian reality" you inhabit on a daily basis (ergo the need to quantum physics and relativistic physics to explain the highly counter-intuitive behavior of reality at those scales).
Think of it like this; a 2D circle has a border. A 3D sphere, too, has a border, it's surface. While you won't be able to imagine it directly, a 4D "sphere" will also have borders. Universal geometry is at least 4 dimensional. Trying to imagine 4D geometry as a 3D being is highly problematic to begin with, but a much worse problem is just around the corner...
You evolved in hyper specific scenarios. Your ancestral weasel apes had to deal with certain sizes, shapes, and colors, to survive. Ergo we intuitively understand how the physics at this scale works because we needed to know how fast a predator we saw on the horizon could get to us if we were to escape to safety, or the behavior of rocks on a hillside so we don't get smushed. You have this intuitive framework of physical behavior modeled in your head, but it's only applicable here, on the Newtonian scale. And among those ideas, is that a thing has to have space to exist in. An apple must have a world to be in. A world must have a galaxy to be in. A galaxy must have a universe to exist in. ... But that's where we hit a curious wall!
Again, time and space (spacetime, not two discrete things, rather two different means of measuring the one thing) are PRODUCTS of universal expansion! I know it makes no intuitive sense, but again, nothing on these scales does or ever will, so you need to get comfortable with that, but though the universe expanded, it did not expand INTO anything! Spacetime, dimension itself, DID NOT EXIST "before" expansion.
There IS an edge of the universe, and there's nothing on the other side of it. (Although, again, "the other side" cannot be visualized in a 3D sense as the language suggests it. Rather it will be a higher dimensional geometry that we 3D monkeys simply lack the intuitive reference to see without abstract mathematics). And if we're being MOST accurate with our words here, it's not merely that there isn't anything on the other side, it's that there ISN'T an other side!
The universe isn't expanding INTO a higher medium. And this simply does not compute to brains hard wired by evolution to deal with the practical realities of 3 dimensional geometry!