Faster than light galaxies?

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brian livesey
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Faster than light galaxies?

Post by brian livesey »

In a radio interview some time ago, astronomer Martin Rees said that galaxies beyond the visible universe are travelling faster than light because of the expansion of the universe - the farther away and the greater the acceleration.
According to Einstein ( where would we be without him? ), nothing with mass can exceed the speed of light. Does Martin Rees know something that Einstein didn't?
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Re: Faster than light galaxies?

Post by joe »

From our point of view they appear to be travelling faster than light (if we could observe them) but they are not physically (locally) travelling that fast. They are, like all galaxy clusters, receding at the current cosmic expansion rate which is waaay below c. I believe it's around 74 kilometers/second/megaparsec.

The common analogy is to imagine a rubber band that has regularly placed dots along its length being pulled at both ends with a constant force/speed. If we imagine that we are at the centre then the dots will be receding from us at increasing speed the further away they are from the centre.

But not having heard the radio program he may have been saying something else altogether. I suspect not.
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brian livesey
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Re: Faster than light galaxies?

Post by brian livesey »

Thanks Joe. It's easy to forget that it's space itself that's expanding, taking the matter embedded in it along with it ( I think ).Rees didn't have time to "expand" on what he was saying.
There's still a puzzle here: if the universal expansion is, as you say, way below the speed of light, why should galaxies appear, from our point of view, to be travelling faster than light? Why not much less?
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Re: Faster than light galaxies?

Post by joe »

As you've pointed out Brian, space itself is expanding. This results in all galaxies receding from all other galaxies in all directions*. If you can get your head around that then it inevitably leads to a situation where galaxies further away will be receding faster than those nearby.

To simplify the description, imagine a simple line of galaxies. The space inbetween each galaxy is expanding at 74km/s/Mpsc. You will see the nearest galaxy, your neighbour, recede at 74km/s. The galaxy beyond that one will be receding from your neighbour at 74km/s. From your point of view that further galaxy will appear to receding at 148km/s. The next galaxy further on will appear to be receding at 222km/s. The further away the galaxy is situated, the faster it will appear to be receding. It's therefore inevitable that there will be galaxies so far away that they will appear to be receding at a rate faster than 300,000km/s.

But any distant galaxy receding from us at a rate faster than light will only be receding from its neighbouring galaxy at 74km/s.

*Actually large clusters of galaxies.
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stella
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Re: Faster than light galaxies?

Post by stella »

"The space in between each galaxy is expanding at 74km/s/psc."

That should be 'per megaparsec'.
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Re: Faster than light galaxies?

Post by joe »

Oops thanks stella,
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brian livesey
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Re: Faster than light galaxies?

Post by brian livesey »

And thanks Joe. The way you've explained the phenomenon makes it easy to comprehend.
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Re: Faster than light galaxies?

Post by brian livesey »

Hey! hold on Joe. You've said that all galaxies are receding from each other. How do you explain that the Andromeda spiral M31 is heading our way, on course for a collision?
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Re: Faster than light galaxies?

Post by joe »

I just said galaxies for simplicity. In my reply above I did actually state that it is in fact large clusters of galaxies that are the objects affected. Individual galaxies within clusters are not really affected by universal expansion because gravity overcomes the expansion. The distances between clusters is so vast that expansion is the dominating "force".
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Re: Faster than light galaxies?

Post by Gubbins »

These are interesting arguments but something most of you keep overlooking is the distant galaxies speed relative to ours. You say they are not over light speed relative to ones near them but what about to us? You keep leaving that out of your replies. So does Einstein mean that light speed is the limit relative to anything else every where, as he seems to imply or does he only mean objects close by - in which case how far away does his limit fall apart and allow greater than light speed? I have never heard of any such definition myself but I'm happy to have it explained to me. I understand the idea of expansion theory (be careful what you consider as fact or theory) and that space itself is expanding but regardless of the energy applied Einstein's theory should hold, shouldn't it?
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Re: Faster than light galaxies?

Post by joe »

Gubbins wrote:You say they are not over light speed relative to ones near them but what about to us? You keep leaving that out of your replies.
Not sure what you mean when you say "what about to us?"
But anyway, to explain again - light speed (c) is the limit; it cannot be exceeded. Galaxies are moving relatively slowly through space. Einstein's Special Relativity Theory refers to matter moving through, or in, space. Distant galaxies only appear to be moving faster than c because of the expansion of the space that the galaxies are moving in. The galaxies themselves are not actually moving through space at speeds >c. The theory holds.
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