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In reading various sources about universal inflation, it got me wondering if said inflation, creates paradoxes.

Let's say I have a galaxy moving away from me at the speed of light. I lay an imaginary string that is 1/2 a light year across so it is equal distances away from the Milky Way and the other galaxy I want to measure. As I understand it, if I am at the end of the string furthest away from the Milky Way, then the second galaxy will appear to be moving away at 1/4 the speed of light, and the Milky Way will appear to be moving away at 3/4 the speed of light. This is due to the space in between expanding. What happens if I measure my string now. If the space between the galaxies is expanding, then my string should have expanded as well as it occupies that space.

If it is the same size, then how did the space it occupies expand? Isn't this a paradox?

If the string is expanding too, would the string even be there anymore as the space between the atoms has expanded?

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  • $\begingroup$ I think that there is an interesting GR question hiding in here. However, some readers will be distracted by your incorrect assumption that the expansion of space is relevant on scales as small as a light year. Even between here and the other big galaxies in our Local Group, space expansion is much smaller than the "peculiar motions" of the galaxies. Does your question still make sense if your string is ten million light-years long? I'm not sure. $\endgroup$
    – rob
    Jul 19 '20 at 0:57
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    $\begingroup$ Your string is way too short. 1/2 light year is only a fraction of the distance to the nearest star (other than the sun). It would have to be billions of light years long to be out in the Hubble flow. Slightly different, but interesting, questions have already been asked about long strings tethering galaxies together. For example this one: physics.stackexchange.com/q/3062 $\endgroup$
    – D. Halsey
    Jul 19 '20 at 1:31
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    $\begingroup$ Make that millions of light years, not billions. $\endgroup$
    – D. Halsey
    Jul 19 '20 at 4:19
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A couple of things up front: first, you seem to be talking about the present-era expansion of the universe, not inflation. Inflation, if it happened at all, ended about 14 billion years ago, long before the formation of large structures like galaxies and very long ropes.

Second, a galaxy with a recession velocity of c in the present era is going to be some tens of billions of light years away, not 1 light year. For the rest of this answer I won't mention any specific distances.

The short answer is that your string is a physical object and you haven't specified its properties well enough to determine what will happen.

One possibility is that the string is extremely stretchy (very small spring constant), and of low mass, and each part of it is initially comoving with the local Hubble flow (the averaged speed of nearby galaxies). In that case, the string will expand along with everything else.

Another possibility is that the string is essentially unstretchable (high spring constant). If it still starts out with each part moving with the local Hubble flow, it will almost immediately snap into many pieces. If it starts instead in some large-scale approximation of inertial motion, so that in the short term it doesn't snap, then it will continue to move that way for a while. In that case only a small part of the string can be locally at rest relative to the Hubble flow; the rest of it will be moving (relatively) toward that part of the string, with at least one of the ends moving (relatively) quite rapidly. But that doesn't mean it will break. In the short term, it's just like a very long thin galaxy with its own peculiar velocity.

In the longer term, the string could stretch and break under the influence of the cosmological constant, or it could collapse on itself, depending on its construction, its initial motion, and the local density of galaxies.

Regardless, the important thing to understand is that there isn't an expansion force making things move with the Hubble flow. The expansion of the universe is just the motion of the matter in it, subject to inertia and gravity. The reason that the galaxies are moving away from each other is that they condensed out of primordial matter that was moving apart. But your string didn't simply condense out of primordial matter; you constructed it somehow and moved it into place for the purpose of your thought-experiment. Its subsequent behavior depends on the details of how you did that, and not on what any of the nearby naturally occurring galaxies may be doing.

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