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Comment by tux3 | Hacker News Reader

It is not expanding inside of something.

The picture I've been given is living on the surface of a balloon. Draw a few dots on the surface, blow in the balloon. Every points expands away from every other point.

Of course with a balloon there is always an outside, but the point is that it isn't expanding like if it were pushing against an external wall.

It's expanding everywhere. It's expanding in the space between your fingers. If you look there, it's not pushing against any outside. There is just more space per space, the longer you let it age.

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> It's expanding in the space between your fingers

That's not the case (at least not yet [0].) Anything that's bound together by local forces such as gravity or electromagnetism isn't affected by the universe's expansion. You have to get to the scale of the space between galaxies (and even then, galaxies which aren't orbiting/colliding with one another) before the expansion has any effect.

[0] One of the hypotheses (of many: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...) of the future of the expanding universe predicts that the expansion rate will increase indefinitely, do the point where the expansion energy (dark energy) actually does become enough to expand the space between stars, planets, and even matter (and eventually the atoms in your body), but we don't really have any proof of this. This theory is known as the "big rip": https://en.wikipedia.org/wiki/Big_Rip

My understanding is that it does expand on small scales (such as between your fingers), but at that scale the expansion is so weak/slow that it is immediately compensated by the local attractive forces.

Sure. You could model it as being there being an extremely weak expansion force happening between your fingers, but the force not being enough to overcome any other local forces (gravity, molecular bonds, etc)… said force is not strong enough to overcome planetary orbits either, or orbits of stars around the galactic center, or the orbits of nearby galaxies around each other… but once you start getting into intergalactic scales where the force attracting galaxies together is very weak, the expansion force starts to actually make a difference.

But the point is the space between your fingers isn’t literally expanding. It’s not like the universe is just expanding everywhere uniformly, which is what tux3 seemed to imply. The distance between your fingers is staying the same, and so is everything else nearby.

My point though is that yes space is literally expanding everywhere uniformly, it’s just expanding “through your fingers” (i.e. your fingers don’t expand along with it) because the local forces keep your fingers (and the planet and the galaxy) together.

This is roughly similar to how planets don’t collapse due to gravity, because the stronger electromagnetic repulsion makes them stable bodies, but this doesn’t mean that the gravitational field still isn’t there throughout the planet.

If you inflate a balloon, you're not stretching out the dust on its surface.

> It’s not like the universe is just expanding everywhere uniformly, ...

I thought this is exactly what most modern physicists believe.

The wavelength of photons travelling in space is supposed to scale perfectly with the expansion of the universe, like an ink dot on a balloon getting wider as the balloon is being inflated.

This is why we have the Cosmic Microwave Background consisting of photons that would have been maybe in the UV range when emitted by excited atoms back when the universe became "transparent".

This is hard to communicate so I’m sure I’ve been misleading.

“Space” is expanding uniformly, yes. But not the matter that exists in space, nor is the distance between local matter increasing.

It’s not like the entire universe is undergoing a magnification of scale where everything is enlarging including each individual atom.

So it’s misleading to say “the space between your fingers is expanding”, because it might give the impression that the distance between your fingers will actually be larger as time passes. This is not the case.

If “space is expanding”, but the matter inside this space is unaffected by said expansion, then you’re left with a very subtle phenomena which is easy to misunderstand.

> This is hard to communicate ...

True. I think that the best way to explain this to someone who does not know about the phenomenon is to start with the phrase "existing space continuously creates new space" and everything should follow from this fact.

I actually find your "it is and it isn't" to be much more confusing than "it is but the other forces recompact matter faster than the new space emerges".

So then all our parts are "falling" together constantly through this expanding space? I say falling, but I understand it is other local forces besides gravity.

I struggle to wrap my mind around what that means for concepts I learned long ago in college physics... what is an inertial frame of reference then? What about the speed of light and relativistic effects?

I suppose the answer is back to the original statement that the effect is really only noticable at cosmological scales. Locally, it's well below any margin of error?

I've seen this tied up with one idea where space-time itself is ripped apart, leaving nothing that can expand and instead "everything" once against occupying the same space less than an atom (if it can even be quantified), reduxing a Big Bang.

Otherwise we increasingly approximate a transition to "nothing" into infinity.

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