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Comment by wutwutwat | Hacker News Reader

But what is it expanding into? If the universe is expanding like a balloon then there's some sort of "edge". What is that edge expanding into? Imagine we had a craft that could take us to the edge and you could stand on it, the way you place your finger on the outside of a balloon. If you could stand on the outside/edge of the expanding universe, what would you see, that we're expanding into?

It can't be nothing. Something can't expand into nothing.

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It doesn’t expand into anything. It has no edge. It expands within itself, like it’s distorting itself. Alternatively, you can picture it as groups of gravitationally bound objects (= galaxy clusters) moving away from each other everywhere in the universe. It’s a bit like raisins in a cake being carried away from each other by the dough expanding in the oven. Where the cake already fills all space infinitely in all directions, from the very start. Space is the cake dough. There is no “outer” space that space expands into. The geometry of space itself expands.

Still alternatively, imagine an infinitely extended graph paper whose grid size slowly and continuously increases, carrying with it the dots already drawn on it. Since the grid has infinite size from the start, it doesn’t expand into anything. It just expands.

> It doesn’t expand into anything. It has no edge. It expands within itself, like it’s distorting itself

That's not an explanation, that's incoherent ramblings that make zero logical sense

To paraphrase Feynman, I can't explain to you how the universe expands in terms that you are familiar with because I don't understand it in terms that you are familiar with.

https://www.youtube.com/watch?v=MO0r930Sn_8

It takes effort to understand those fundamental terms, so if you want to understand this, you'll have to meet people halfway instead of rejecting how somebody is trying to explain something to you.

The math from general relativity is crystal clear. It’s just difficult to translate into natural language, because it doesn’t match our intuitions, which are primed by local experiences of non-curved space. You have to question your intuitions, and if you’re serious about it, look into the actual math. It all follows from a few basic assumptions, and is consistent with actual observations to an unmatched degree.

Thinking of this kind of "expansion" using an analogy like a balloon does a disservice. When you get to large enough or small enough distances then our intuition often falls down completely.

In this case it's entirely possible for the topology of space to change such that it takes longer to travel from one point to another over time. If you consider a trip from point A to point B in topology A, and then topology A expands during the trip from point A to point B such that new distance is created, then the "distance" you have to travel to get to B is changing because at each moment along the trip you're at a different point.

If you think about it this way, the universe can be both expanding because it takes progressively longer to get from point A to point B, and it can also not be expanding into some "outer" area because there's nothing other than the universe.

It may make more sense to stop thinking in terms of distance and start thinking in terms of time. In that case, when you think of "expansion" what you're really describing is that at a constant speed, the closer you get to B, the more time it takes you to advance toward B. And if you change direction mid-trip to go to C, the "expansion" means that the amount of time required to reach C also increases as time elapses.

It's hard to say what's causing that expansion, but we can measure it by its effects on light traveling the same distance.

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...

> 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.

Like a lot of language we use when discussing things at the universe scale, the word “expanding” operates as a placeholder with analogs in day to day life but isn’t a perfect representation of what we mean.

The “universe” itself is space and time. When we say “expanding” we simple mean that galaxies are observed to be moving farther away from each other. That does not at all imply some kind of “expansion into another space” - space itself is exhibiting this property and we are observing it. That’s all.

The “balloon” analogy and the usage of the word “expand” in this context are both imperfect metaphors for physical phenomena we are observing.

It is a bit like trying to discuss what happened “before” the Big Bang - there is no “before” - time was created.

There are many phrases you can construct which may seem like they “make sense” but are actually combining a set of word concepts in ways that are self-contradictory.

“What is space expanding into?”

“What happened before time was created?”

Etc.

I don't agree. You're assuming that the universe is a distinct 'something' expanding into a distinct 'nothing'. I disagree, I think that the 'universe' is an arbitrary border we created.

If we were somehow able to accelerate a particle out past the edge of the known universe, now the bubble includes that particle too and essentially we've 'expanded' the universe. In other words, the arbitrary bubble containing things that we know about and consider significant is larger than it was before - because there's something out there we can observe.

> I think that the 'universe' is an arbitrary border

I don't think that the scientific definition of the universe matches this. It's a physical thing, not a construct we made up. It's not an imaginary line between two countries.

The universe is full of things which have mass. Those things are rapidly expanding outward. What are those masses expanding into as they "blow up the ballon" and push the boundaries of the universe outward? It can't be nothing, it's physically impossible.

EDIT: might as well quote people smarter than I

> The universe is all of space and time[a] and their contents.[10] It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of energy and matter, and the structures they form, from sub-atomic particles to entire galaxies. Space and time, according to the prevailing cosmological theory of the Big Bang, emerged together 13.787±0.020 billion years ago,[11] and the universe has been expanding ever since. Today the universe has expanded into an age and size that is physically only in parts observable as the observable universe, which is approximately 93 billion light-years in diameter at the present day, while the spatial size, if any, of the entire universe is unknown.

https://en.wikipedia.org/wiki/Universe

The edge of the Mandelbrot set expands out to infinity and yet the area within the edge is finite. Who is to say that the universe isn't a hologram on a higher-dimensional Mandelbrot set-like object?

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