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In other words, the universe isn't expanding into anything because it is everything. And that's ok."If the universe is expanding, what is it expanding into?"

The universe is expanding---we can see this by observing that galaxies, on average, are moving away from each other. It's natural to ask the next question, "What is the universe expanding into?" It turns out that this question doesn't even make sense. What, after all, is the universe? The meaning we use in cosmology for this word is, "Everything there is." But if this is so, then there is nothing else for the universe to expand into, since it is everything there is.

How can it be expanding if there is nothing for it to expand into? As suggested in another FAQ answer, scientists think that the universe may be infinite. If it is, then the problem is resolved. Consider viewing the expansion backwards in time. The current universe is infinite. At half this size, it was half of infinity, but any mathematician can tell you that half of infinity is equal to infinity. At half that size, it was one quarter the present infinity, and yet still equal to infinity. And so on. The universe has clearly contracted going backwards, and yet at each moment it was infinite.

What if the universe turns out to be finite? Let us make an analogy. A finite universe will curve back on itself, like a sphere or a torus (a doughnut shape). Imagine a balloon in the shape of a sphere, with a grid of dots drawn on it. As the balloon is blown up, it will expand into the third dimension, and all these dots will move away from each other. Now remove the third dimension. Believe it or not, it is mathematically possible to describe the expansion of the sphere's surface (otherwise known as a 2-sphere) without using a third dimension to do so. Similarly, the universe would be curved into a 3-sphere. A 3-sphere is an object of finite volume (just as the 2-sphere is an object of finite surface area), and finite circumference. If you could travel in a 3-sphere in a straight line for long enough, you would return to where you started. The 3-sphere expands, but remains an object solely contained by its original three dimensions, like the 2-sphere expanded without using a third dimension.

To read more about how a 2-sphere can exist without the third dimension, check out this FAQ answer!

But I don't understand how that's reconciled with inflationary cosmology, which if I haven't misunderstood anything predicts that our visible universe is most certainly

*not*"everything" and is in fact only a tiny piece of a tiny bubble/pocket (of which there are unimaginable numbers) in a much larger (probably infinite)

**U**niverse. So the expansion of the

*visible*universe is a local characteristic of our particular bubble--in fact, there's even an article at space.com today in which Mario Livio discusses the possibility that the value of dark energy varies from pocket universe to pocket universe (and I assume he's talking about the type of pockets that arise in inflation).

Is that last bit correct? That the expansion we observe regards only our little piece of the

**U**niverse and not the entire

**U**niverse itself?

Anyway, my main question: if the expansion of spacetime is just the expansion of the tiny amount inside our little bubble of

**U**niverse then isn't the generic "the universe isn't expanding into anything because it

*is*everything" flat out incorrect? Isn't the expansion of the universe then similar to a bubble in my can of Sprite expanding (in that there clearly is an outside of the bubble to expand into)? Or am I missing something?

I guess I just don't know how the accelerating expansion of "the universe" meshes with the superuniverse that inflation seems to say we're in.