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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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  • $\begingroup$ I'm afraid I can't quite make heads or tails of this question... $\endgroup$
    – Noldorin
    Commented Jan 16, 2011 at 0:55
  • $\begingroup$ 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. $\endgroup$
    – user346
    Commented Jan 16, 2011 at 0:55
  • $\begingroup$ 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? $\endgroup$
    – Marek
    Commented Jan 16, 2011 at 0:56
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    $\begingroup$ @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? $\endgroup$
    – user346
    Commented Jan 16, 2011 at 1:02
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    $\begingroup$ Because the Dodgers left? $\endgroup$
    – SplashHit
    Commented Jan 21, 2011 at 3:24

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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.

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