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Comment by thaumasiotes | Hacker News Reader

OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly.

After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before?

And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?

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>And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?

The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.

But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?

The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.

The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?

>The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?

In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter.

Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.

> More space appeared for it to exist in.

This is a very different idea from "it's the same amount of space, but bigger", which is what I understand by the claim that a scale factor is increasing.

Well, space isn't a tangible (and finite) thing. It's the manifold in which the universe exists. Increasing the scale factor doesn't "stretch" space, it just makes... more of it.

> But the scale of quarks increases just as much as every other scale

No, there is no scaling of matter or light. It just has a bigger space to play in.

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