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If you placed a giant ruler between the sun and our nearest star for example, wouldn't the divisions on the ruler expand at the same rate as the universe - therefore maintaining a constant distance? If you draw 5 divisions between 2 points on a balloon, there will always be only 5 no matter how much you inflate the balloon.

How can this be reconciled with observed red-shift?

Qmechanic
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Mark
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2 Answers2

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By expansion, we don't mean only the surface of the sphere is expanding; it is a volume expansion rather than surface expansion. We believe that in a short period of time (compared to the age of Universe,) the rate of expansion is fixed. However, this doesn't mean that the relative velocity between any two objects is the same. Depending on the "distance" between two objects, the relative velocity would be different. This is why the "distance" will never be fixed in such a volume expansion. And, if you do some research, you will be surprized that we have different definitions of distance in large scales and that's why I used quotes when referred to it.

Thanks,

Benjamin
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  • Thanks for your answer Benjamin. Do you regard the expanding volume of space as a medium which moves the galaxy clusters within it as it expands, being somehow attached to the matter within it, grabbing onto it and dragging it along with it? Why doesn't space just expand around the galaxies instead of causing them to move with it? – Mark Mar 12 '16 at 06:50
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The raisin bread analogy of the expansion of the universe is useful to get an intuition on what is the process.

raisin bread

Why are the raisins not expanding at the rate of expansion of the bread? Because the dough is a soft elastic medium while the raisins are solid.

In exactly the same way, the stability of clusters of galaxies , galaxies and planetary systems is retained because the attractive force of gravity is much stronger than the effective dispersive force exerted by the expansion of the universe . This is more so for systems bound by electromagnetic and strong forces, as your imaginary ruler.

anna v
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  • In the raisin bread model, the raisins are fixed in the bread so that as the dough expands the distances between all the raisins increase. The dough does work on the raisins. If two raisins were attached by a thin filament it would be broken by the forces applied by the dough. Do you think that this would also occur between galaxies, or could two distant galaxies be fixed in place relative to each other by a thin filament stretching between them maintaining a fixed value for their separation without applying any force to them? – Mark Mar 12 '16 at 05:49
  • Not between galaxies, effective gravitational attraction is too strong still, but between clusters of galaxies. The raisins are clusters of galaxies – anna v Mar 12 '16 at 06:41
  • Ok so between galaxy clusters - would such a piece of string be enough to stop the expansion of space between the clusters? – Mark Mar 12 '16 at 06:43
  • Well, one would have to do the equations; a material string would not be expanding because of the electromagnetic cohesion, the question would be if it would break because the two clusters were drifting away from each other or it would keep the clusters as one cluster. – anna v Mar 12 '16 at 06:45