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According to inflation theory, the inflation itself took place at some short yet finite time after Big Bang, so there should be some gravitational interactions present before inflation, some objects started accelerating towards other objects etc. However after inflation many of those objects were scattered outside of their neighbours' light cones and as gravity propagates with the speed of light the interaction between such objects should be interrupted. Yet what about the initial acceleration that these objects gained before the inflation occurred? Shouldn't these objects still be moving in their original pre-inflationary direction? And how would such forces shape the present Universe?

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  • $\begingroup$ Would you mind clarifying what you mean by "objects", bearing in mind the conditions immediately after the big bang. Thanks $\endgroup$ – user81619 Jun 23 '15 at 0:56
  • $\begingroup$ By objects I mean any form of matter which could exist at that time, so probably nothing more than the most elementary particles. $\endgroup$ – Ardath Jun 23 '15 at 20:22
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Objects moving in an expanding universe experience an apparent drag force called the Hubble drag and given by:

$$ D = 2\frac{\dot{a}}{a}\mathbf{u} $$

where $\mathbf{u}$ is the comoving velocity and $a$ is the scale factor. This isn't a real force, it's the result of the universe expanding away from the moving object, but the end result is that in an expanding universe freely moving objects appear to slow down.

The key parameter is that factor of $\dot{a}/a$. The numerator, $\dot{a}$ is the rate of change of the scale factor, and during inflation this was absolutely enormous and produced a huge Hubble drag. The end result is that any peculiar velocities that particles had before inflation were reduced to essentially zero by the time inflation ended.

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  • $\begingroup$ A footnote: I couldn't find a good article on the Hubble drag. Wikipedia doesn't have an article on it and the only articles I found were rather technical. If anyone knows of a good article on the Hubble drag please feel free to edit my answer to include the link. $\endgroup$ – John Rennie Jun 23 '15 at 11:32
  • $\begingroup$ I can't find anything describing HD too, so I hope you won't mind if I ask you more about that. Let's say that there were some elementary particles being part of hypothetical primordial plasma that were affected by huge and non-uniformly spread mass. In such scenario those particles would gain speeds very close to C. Then space expands and distances become suddenly huge, so the same particles would now have to move for longer time to reach others, but why would their velocity in respect to space/rest of the Universe be affected? Why wouldn't they move close to C after inflation any longer? $\endgroup$ – Ardath Jun 23 '15 at 20:20

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