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Comment by btilly | Hacker News Reader

This is a well-known phenomena.

The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other.

If you understand manifolds and general relativity, this analogy is surprisingly exact.

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I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.

Cupcakes have a center.

That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.

This is physics, so it is best to assume an infinite cupcake.

I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.

Good point, I guess they're good for illustrating different things :)

What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?

Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds.

Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.

And since the even the whole groups of galaxies are gravitationally bound, it seems that the galaxies, including our own galaxy, are not "stretching" but remain the same size. And, based on the same principle, everything inside of them also, including us.

So back to the cupcake analogy, the raisins are the whole galaxy groups.

Also:

https://medium.com/starts-with-a-bang/this-is-why-we-arent-e...

"As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”"

https://www.newscientist.com/article/dn8082-cosmic-expansion...

https://arxiv.org/abs/gr-qc/0508052

Here "bonded to other matter" means "electrostatically attracted together".

It's not strong enough to overcome the forces keeping us together.

But that might not be the case for forever: https://en.wikipedia.org/wiki/Big_Rip

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