# Star trace in the sky

If the space of universe is expanding at fast rates..(even faster than light)

Why we do not see the trace of the stars?

A possible answer could be that the stars are only moving away but mantaining in geodesic respect earth and each other, that is strange, in a 2D model of an inflating balloon with dots as analogy, one can suspect that, but I can't imagine how that structure should be in 3D.

Any other explanation or possible idea or analogy to explain why we don't see those traces are welcome (or perhaps the information that they are already seen)

Thanks

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Expansion of the universe goes out radially from any observer. So we never see any side-to-side motion from expansion, only the Hubble redshift. We do see Hubble redshifting though, which is effectively the "trace" you are looking for. But there's no transverse motion because the expansion of the universe is isotropic.

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said: "there's no transverse motion because the expansion of the universe is isotropic." (I don't even believe in Big Bang), but perhaps that's the current science interpretation, then this kind of answer is what I was looking for. – HDE Mar 10 '11 at 12:58
@HDE You should consider this experimental evidence in favor of the isotropic expansion model. – spencer nelson Mar 10 '11 at 18:09

If by "trace of stars", you mean the sort of light trail you see when a very fast light source whizzes by you, here is the answer:

Such a light trail occurs because of persistence of vision, the phenomenon by which an image on the retina persists for about 1/25 of a second. So a trail will be present only if the image on your retina is moving sufficiently fast. This is determined not just by how fast the object is actually moving, but also by how far away it is. If two objects are moving at the same speed, the one that is farther away will appear to move slower because it covers a smaller angular displacement $\theta=l/r$ in the same time. This is of course why an aircraft appears to be moving slowly in the sky even though it is travelling at several hundred kmph.

Stars are so ridiculously far away that it doesn't matter how fast they are moving - their image will still move so slowly on your retina that you won't see any trail.

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hello, I am not referring to that kind of trail on eye image persistence, imagine a tenis ball thrower machine, moving left-right in front of you, then if the left-right speed is faster than the ball speed, an horizontal line of balls could reach you, that's not caused by eye perception – HDE Mar 9 '11 at 14:47
@HDE Ah, ok, then the persistence of vision part doesn't apply - but you should realize that if the tennis ball machine were very far away, the back and forth motion would not matter at all, since the angular displacement rate is practically zero. – dbrane Mar 9 '11 at 14:53