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Comment by 16bytes | Hacker News Reader

The balloon analogy is a 2 dimensional example. If you are on the surface of the balloon, there is no "edge". You can go forever in any direction. If the balloon is inflated, it doesn't expand into something (remember we're in a 2D space, it's not a 3D balloon), it "expands into itself".

If you drew two dots on the surface of that balloon, those dots would get further apart as it inflated. What did those dots expand into? Well, nothing right? They just got further apart from each other on the balloon.

So, if you translate this into 3D space, that's what's going on. It's hard to visualize. It's not that the universe is expanding into something, it's that the space between every "point" in the universe is getting further apart.

https://bigthink.com/starts-with-a-bang/what-universe-expand...

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> The balloon analogy is a 2 dimensional example. If you are on the surface of the balloon, there is no "edge". You can go forever in any direction. If the balloon is inflated, it doesn't expand into something (remember we're in a 2D space, it's not a 3D balloon), it "expands into itself".

I'm talking about 3d that we live in. We're currently standing on a ball of dirt, if the earth started to swell up, the surface we're standing on would expand into space. The thing we're standing on that's pushing outward is what I mean by "edge".

What is the universe expanding into? If it's pushing outward in all directions, it's pushing into something, the same way a swelling earth would expand into space

The universe is everything. There's no outside the universe, since whatever that "outside" was would also be part of our universe. It's not empty space, since that too would be part of the universe. There is no edge of the universe since that would imply that there is something beyond that edge. In scientific terms, the universe is homogeneous and isotropic.

We've proved the universe itself is getting bigger, which means that yesterday it was smaller and the realization of that fact was how the big bang theory came to be.

There are lots of resources out there if you want to build an intuition around how a universe that's everything can get larger without needing something to expand into.

You can imagine the balloon’s curved surface geometry without assuming that it is embedded into a 3D space. This is what Riemann discovered in the 19th century. Such a non-embedded space is called a manifold. In the case of the balloon, it’s simply a non-Euclidian two-dimensional space. A third dimension doesn’t enter into the picture in that formulation.

Four-dimensional spacetime is considered such a manifold. Its three-dimensional space “slices” are predicted to expand with time by general relativity (first predicted by Alexander Friedmann in 1922), and actual observations have confirmed those predictions.

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