The easiest way to understand curvature is to look at a triangle. If you have three points (not in a line) they form a triangle. If you sum the three angles in any triangle, you always get 180˚ in a flat space. This can be a 2D space, like geometry class on paper, or a 3D space, like the space around the planet.
Consider the surface of the Earth, this surface is curved. If you make a triangle with one side on the equator and the third point at the North Pole, then this triangle has an sum greater than 180.
The thing about the Universe, we see flatness every place we look. That isn’t too convincing, because if you draw a triangle on the parking lot outside it will measure 180˚ and give you the false impression that the surface of the Earth is flat.
Maybe the Universe is flat, maybe the Visible Universe is in a flat spot, maybe we’re just in a flat spot and we need to measure multi-lightyear triangles.
You’re correct, we do live in a 3D world. they’re not talking about dimensions, they’re talking about how geometry works in our universe.
Imagine you’re on the equator of the earth. Walk straight forward to the North Pole, turn 90 degrees, and walk straight down, back to the equator. Then turn 90 degrees again and walk back to exactly where you were at the start. You’ve walked 3 straight lines, and turned 90 degrees three times, and yet you’ve somehow made a triangle. This is called spherical geometry, it’s very different from the “flat” Euclidean geometry were used to on paper, where triangles cannot be made with 3 right turns.
Obviously this is a 2D example, and it’s hard to make a 3D analogue, but that’s what scientists are talking about with flat or spherical space.
Think of the universe like an infinite rubber sheet.
This sheet can have an overall bent in 3 broad ways.
It can be bent downwards like it’s on the outside of a ball. It can be bent upwards like it’s at insides of a ball.
Or it can be perfectly flat.
This isnt my best work but am literally falling asleep so. Might update later
The universe is expanding in all directions, but “flat” in this context is about geometry, not shape. So you’re mixing up the term “flat” as it’s used in everyday life with how cosmologists use it.
When cosmologists talk about the universe being “flat,” they’re talking about its large-scale geometry. Imagine a 2D analogy: If you draw a triangle on a flat piece of paper, the angles add up to 180 degrees, right? But if you draw a triangle on a curved surface, like a sphere, the angles add up to more than 180 degrees. Same deal with the universe. The geometry dictates how parallel lines behave, how shapes form, and whatnot. In a flat universe, general relativity plays out in a way that parallel lines will remain parallel forever, and the angles in a large-scale triangle will add up to 180 degrees. If the universe were “closed” or spherical, parallel lines would eventually converge.
Regarding dimensions, yeah, we live in a 3D world, but when we talk about the shape of the universe, it’s easier to simplify things by using 2D analogies. Our universe is indeed 3D (or even 4D, if you count time), but its “flatness” is a property of its geometry, not its dimensionality.
So, the universe is expanding in all directions, not just “one plane,” but its geometry appears to be flat based on our best measurements. It’s not about how the universe is expanding, but how space-time itself is curved (or not curved, as it appears to be). It’s actually amazing when you think about it.
We live in a three dimensional space.
The question is whether the universe has curvature in a higher dimension.
Think of it like Earth. From our casual perspective, the ground appears “flat” (more or less). But of course, we know we actually live on a sphere and that if you walked long enough in one direction, you’d end up back where you started. So the ground is like a 2D object curving in 3D space.
The question is whether the entire universe has a similar characteristic, a 3D volume curving in a 4D space.
Don’t try to actually imagine what this would look like, our brains aren’t built to process the concept, but mathematically it’s a possibility.
One way we test for this is essentially by measuring the behavior of parallel lines, or the angles between very large hypothetical triangles. On a 2D object a triangle is composed of angles adding up to 180 degrees. But if you draw a 2D triangle on a curved 3D object, those angles can be different. You can build a triangle out of multiple right angles, for example.
As far as we can tell, our universe is flat (this is actually unexpected for mathy reasons). But it’s possible we just aren’t measuring the change over a big enough distance yet (like how you wouldn’t notice a small triangle you draw with chalk on the ground is actually off).
Google “non-euclidean geometry.”
It’s “flat” respective to how things travel through it. What scientists do is shoot incredibly precise lasers into space and watch how they travel. In a “flat” universe they should travel perfectly straight forever. If the universe had positive curvature, the lasers would eventually overlap. If the universe had negative curvature, they would separate from each other more and more.
Contrary to what most cosmologists want us to believe the Big Bang theory though grounded in science still has big unexplained holes in it.
What they will also not speak about is no research that provides answers that don’t uphold the Big Bang is going to ever get funded so long as the creationists lie in wait to attack the Big Bang
The universe is “flat” in a higher dimensional sense. When we think of our existence on Earth, we’re thinking of being 3D beings sitting on the 2D surface of a 3D object.
When physicists talk about the shape of the universe, they’re talking about the experience of sitting on the 3D surface of a 4D object. There is observable curvature in space-time due to black holes and such, but we’ve come to the conclusion that in the absence of gravity or energy, there is no curvature.
Edit: I totally failed at the “explain like I’m 5” part, I’ll admit.
The universe did not expand from a single point. We do not know if the universe is infinite or not. If it is infinite now, then it was infinite in the beginning.
The “observable universe” is a region of the universe that we can see from earth in some form, be it visible light or some other wavelength. We can never see anything beyond the observable universe so we can never directly know if the universe if infinite or not.
The observable universe started as a point-like region. That is, everything we can see was once squashed down into a very tiny region of space and expanded rapidly directly after the big bang.
When we say the universe is flat, we are not talking about the shape of the the universe as a whole, but the geometric properties of space. In a flat universe parrallel lines will never meet (think of parrallel lines on a flat sheet of paper clearly not meeting). In a non flat universe paralled lines could meet (think of lines of lattitude on a globe meeting at the poles).
It’s important to undertand if the universe is flat or not so we can correctly understand the movement of photons over a vast distance. Light always follows a straight line, and in a flat universe a straight line doesn’t follow a curve, where as in a non flat universe it would. So if we observe light following a curve we then know it’s due to a gravitational distortion of space rather than the geometric curve itself in space.
We believe the universe is flat because everywhere we look we see the same average density in space. If space was curved, places foar off in the distance would look more or less dense than or local space.
It’s flat vs spherical in a 3D sense not in a 2D sense. So if it’s spherical, then it’s not a “ball” like the way you would imagine a soccer ball (that’s a 2D ball in a 3D world), but rather a 3D ball in a 4D world.
Basically, if the universe were spherical (in that definition), it means you can go from one point, travel in a straight line and after a long time get back to where you were but from the opposite direction. No you’re not going to run into a “wall” (the edge of the universe) because there’s no such thing. That’s the over-simplification, because of course “straight” line itself is a tricky thing when you have gravity around you (remember gravity bends spacetime itself so your straight line actually is affected by galaxies and blackholes). Also how do you know you get back to the same point, because by that time the Earth will have moved around to a different place.
Anyways that’s the gist, you can travel to one direction and get back from the other direction, that’s what “spherical” means.
If it’s flat, then you can travel to one direction and go forever and keep seeing “new stuff”
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