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Why space expansion affects matter?

Imagine two tiny spacecrafts that are moving with the Hubble flow and so are moving away from each other. Let's assume that they've been that way since the very early universe, never firing their engines, just drifting along their Hubble flow geodesics in a homogenous isotropic universe.

They then momentarily fire their engines so that they "cancel" the Hubble flow and have a fixed proper separation. Will they now start drifting apart again (presumably due to expansion)? Or will they stay at fixed proper separation, and maybe very slowly move towards each other due to their mutual gravitational attraction?

I guess this is another way of asking whether expansion is kinematic and hence can be forgotten (so that "Brooklyn isn't expanding" because gravitational collapse and structure formation have erased memory of the expansion). Or maybe someone will help me refine this question and make me realize I just haven't thought things through completely?

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dbrane
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  • I'm afraid I can't quite make heads or tails of this question... – Noldorin Jan 16 '11 at 00:55
  • The Hubble expansion affects the space between gravitationally bound objects such as galaxies, but does not affect the sizes of the bound objects themselves. In any case, this is a tricky question for which even cosmologists can fail to provide a clear resolution so +1. –  Jan 16 '11 at 00:55
  • Brooklyn is held together by a huge amount of concrete which is not true of the space-time itself :-) Is that sufficient as an answer? – Marek Jan 16 '11 at 00:56
  • @space_cadet: what's tricky about it? Hubble expansion depends on the relative distance of the two objects that are supposed to be getting away from each other and thus requires macroscopic scales to be relevant. For Brooklyn one can compute that its effect is completely negligible by many orders of magnitude. – Marek Jan 16 '11 at 00:58
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    @Marek the Brooklyn example is only illustrative I'm sure. I don't think the OP is particularly concerned about the imminent sundering of the east coast due to Hubble expansion. The question is simple and has been covered before in different forms on this site - "why does hubble expansion seemingly have no effect on bound systems such as galaxies, solar systems and even protons?" There was a time when the universe was much smaller in scale that it is today. So why has matter never been stretched out like silly putty due to this expansion - especially if it affects ALL of spacetime? –  Jan 16 '11 at 01:02
  • I used to be convinced by @Marek's response until I read this article by John Peacok, which genuinely confused me http://www.roe.ac.uk/~jap/book/expandspace.pdf – dbrane Jan 16 '11 at 01:04
  • @space_cadet: surely you don't intend to compare current small scales with small scales shortly after BB ;-) Back then it was hotter, there were quantum effects going on and there was inflation, making the expansion relevant also at the small scales. Nothing like that is around today. – Marek Jan 16 '11 at 01:09
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    Because the Dodgers left? – SplashHit Jan 21 '11 at 03:24
  • I don't think this is a duplicate. The answer on this particular question depends on the distance between the spaceships and the impulse they make towards each other. If the distance too big and the impulse too small, the spacecrafts still will move away of each other, if the distance smaller or impulse bigger, then they will first approach each other and then still continue to move away. And finally if the impulse large enough or the distance is too small, they will approach each other until they meet. Note also that if the distance is very large, there is no impulse that can stop separation. – Anixx Feb 07 '11 at 16:53
  • This was one of the first questions I asked. I postponed reading the other question (http://physics.stackexchange.com/questions/1801/why-space-expansion-affects-matter) because I was having exams back then. I've read it now and it is clear to me that this is far from being an exact duplicate. I am asking something very specific - whether the expansion can be considered as due to kinematic initial conditions (memory of which can be erased to stop expansion in a region) as is claimed by Peacock in the paper I referred to above. The other question doesn't emphasize this and... – dbrane Feb 12 '11 at 20:57
  • ...none of the answers address this either. So I'm voting to reopen. – dbrane Feb 12 '11 at 20:57
  • @dbrane: okay, but what does that question have to do with the reason Brooklyn doesn't expand? – David Z Feb 13 '11 at 00:55
  • According to my reading of Peacock's paper, the reason Brooklyn doesn't expand is the same why anything that is part of a region that has undergone gravitational collapse doesn't expand - because the gravitational collapse has erased memory of the inertial push that the Big Bang gave the constituents of the region. – dbrane Feb 13 '11 at 03:04

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This is a good question, but one that has a mundane explanation. Hubble expansion only holds over long distances at scales near the seperation of galaxies for systems that are not gravitionally bound. So for us, our fate is likely one where we will eventually lose sight of other galaxies, but our galaxy will continue on alone for some time. There are some interesting questions about what happens when the temperature of the cmb drops below the temperatures of black holes in our galaxy (e.g. evaporation of black holes via hawking radiation). Eventually, if black holes evaporate substantially, gravitational forces will relax and the kinetic energy of bodies may be sufficient to allow the galaxy to gradually expand. At that point, some of the cold bodies of matter may have sufficient velocity to escape the galaxy's pull and become further flung. Those may eventually be removed from view due to cosmological expansion. In any case, the universe will eventually be composed of very far flung, cold objects.