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I was going through John Rennie's answer and I had a issue in understanding it.

So if my understanding were to be right. The universe were to be a grid and scaled at different points in time, that would mean that 7 billion years after the big bang (about halfway back), the distance between atoms were half the length they are today.

So if I were to just change the unit length of the grid (by hypothetically moving to a different time), I would be larger/smaller than my present self which would offset the apparent moving away of the galaxies.

In other words, changing the spacing is like zooming into a group photograph (or zooming out). If I were to zoom into myself in the photograph, I become bigger on the screen that also results in my sister who was standing next to me in the photograph move away from me (in terms of pixels on my screen). But while this is all happening, I were to go inside the conscience of my 'image self', I have become bigger than I was before that means that my perception of length has also changed (my eyes also become bigger so hence the standard meter that my image self knows about has increased) which then means that I haven't noticed the change at all.

Doesn't the same go for galaxies? I zoom into 1 galaxy but since the universe is scaling (not growing), our definition of length itself changes, the speed of light increases, all in a way that we don't even notice that the universe is growing.

Qmechanic
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Dev Not Taken
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3 Answers3

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Only the distances grow, not the size of objects themselves, such as rulers or atoms or galaxies. This might seem contradictory, but here is why it is not:

Any bounded object, something that is kept together by forces, will remain at the same size, the space expanding inside it does not push the subparts appart (at least not at the current rates of expansion, because the amount of space created inside is very small and the forces keep the size constant. A super fast expansion on the other hand, could even rip atoms apart).

It is only when you look at unbounded objects, such as galaxies that are far enough apart so that they are not gravitationally bound, is when you can see the effect of the expansion of space (such as objects that seem to move faster from each other the larger the distance between them).

The big bang balloon analogy is not one in which matter dots are painted in the ballon, and as the balloon expands, every dot moves apart from each other and also grows in size. It is more like a balloon in which the little rigid dots are attached to the ballon, and as the balloon is inflated, they move apart from each other, but the rigid dots themselves remain of constant size due to the internal forces, so at the point in which the balloon is attached to the dot, the rubber expands but somehow "flows" outwards from below the dots, as the dots cannot expand.

Pato Galmarini
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The normal interpretation is that fundamental sizes (most importantly particle sizes) are fixed and that the empty space in between changes. This interpretation is mainstream, there is nothing wrong with it (as far as I know) and you should use it if you want to be understood.

But a mathematically equivalent interpretation (the "Self-Similar model") is that the empty space in between stays fixed, and all particle diameters shrink over time. The advantage of this interpretation is that it gets rid of the question often asked 'where is the universe expanding into?', but it introduces the question 'why are particles shrinking?'.

I am not advocating for this alternative interpretation, because it has not led to new insights or better understanding. Play with it at your own risk.

user132647
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The universe expands, but not everything within it does. That's because the forces of attraction within some small objects (relative to the size of the universe) is stronger than the force of repulsion due to dark energy. To quote an answer of mine on the Astronomy SE:

One way to think about this is that dark energy begins to have an impact when its energy density becomes comparable to the energy density of matter (or radiation). The energy density of dark energy is about $7 \times 10^{-30} g/cm^3$, which is much smaller than the density of interstellar space (~one hydrogen atom per cubic centimeter). So galaxies don't expand.

The dark energy energy density is larger than the density of intergalactic space (which is about $1 \times 10^{-30} g/cm^3$), and hence this is where the expansion happens.

Allure
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