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If some galaxies expand away from each other faster than the speed of light then do they collide faster than the speed of light?

Update 1:

  • I guess my question is what's the max speed/velocity at which galaxies can move towards each other?
  • How long does it take for galaxies to collide?

If you could use this example that would be awesome. Galaxy A: diameter of 3,000 light years Galaxy B: diameter of 300,000 light years

If Galaxy A collide with Galaxy B , what would be the velocity at the collision? I have no idea about what the time and distance would be.

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  • $\begingroup$ I don't think anything moves faster than c. In the case of expansion of the universe, things may cover a distance in less time than it'd take for light to traverse said distance, but that's because the space behind the thing that's moving is expanding. $\endgroup$
    – rodrigo-silveira
    Dec 27, 2020 at 6:31
  • $\begingroup$ Your question is confusing. Things that are expanding away from each other don't collide. $\endgroup$
    – PM 2Ring
    Dec 27, 2020 at 6:45
  • $\begingroup$ @PM2Ring Thanks. I updated my question. $\endgroup$
    – Larry Battle
    Dec 27, 2020 at 8:22
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    $\begingroup$ You have completely changed the question. If you have new questions it is better to ask them as new questions rather than completely change an old one. THe new question also doesn't make sense. If I asked "A lorry is 10m long and a bike is 1 m long... how fast do they collide" Well... it depends how fast they are going. The size of the lorry and bike don't have anything to do with their speed. You might ask about the typical proper velocity of galaxies during a collision rather than the "maxium" as there is no meaningful max $\endgroup$
    – James K
    Dec 27, 2020 at 9:25

1 Answer 1

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If some galaxies expand away from each other faster than the speed of light then do they collide faster than the speed of light?

The short answer is "no".

The expansion of the distant universe at apparent speeds faster than light is due to the expansion of space. But the velocity of that expansion depends on how far apart the observer and object are. If there are two objects A and B which we see apparently moving away from us at faster than the speed of light due to the expansion of spacetime then that is not how the objects will see each other. They will be close to each other so from their reference frames the expansion of spacetime is negligible (just as it is for us and our "local" galaxies).

At some future time we think the universe may expand even faster at "small" ranges but in that case the two objects could not collide. Once they are able to collide, they are close enough so that the effects of the expansion of the universe are not going to make them move apart faster than light. Once objects move apart faster than light they cannot collide (unless the entire universe starts to contract).

If they are expanding away from each other faster than light (due to the expansion of space-time) then there is no way for them to collide. No attraction could overcome the expansion of spacetime if it has already reached the point where they are moving apart faster than light. It would require the entire universe to start contracting to do that.

So if it's going to collide with you, an object cannot be moving faster than light relative to you.

How long does it take for galaxies to collide?

This depends on how fast they are approaching each other. In theory they cannot approach faster than light, so the upper limit is (simplistically) the distance divided by the speed of light.

A more realistic example would be Andromeda and the Milky Way which are approaching each other at a relative speed of 300 km/s. Andromeda is about 2.5 million light years away and current estimates are for a collision in about 4.5 billion years, so that's moving much, much slower than the speed of light.

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  • $\begingroup$ It's not correct to say two objects can never meet if they are receding from each other at gretaer than or equal to c. In the currently favoured LCDM model the cosmic event horizon has a slightly larger radius than the Hubble sphere in the current epoch. $\endgroup$
    – John Davis
    Dec 27, 2020 at 19:19

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