
#19
Mar1612, 09:27 PM

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#20
Mar1612, 09:33 PM

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Since we can't tell by looking at redshift, why do we say "space is expanding" instead of "galaxies are moving away from us THROUGH space". I'm guessing that this is a much bigger can of worms than I thought it was. 



#21
Mar1612, 09:49 PM

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#22
Mar1612, 10:07 PM

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#23
Mar1712, 01:50 AM

Astronomy
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It can be thought of either way but becomes very complicated if thought of as doppler.
The basic underlying thing is the assumption that we live in a solution to the Einstein GR equation. GR has been tested a lot and is considered pretty reliable. Plus the model derived from GR fits the data well. So there is an expansion history a(t) and the redshift satisfies 1+z = a(now)/a(then). Now this CAN be treated as the result of a concatenation of an infinite number of infinitesimal doppler effects, occurring in little almost flat Minkowski patches along the way. You can hop from one coordinate patch to the next all along the way the light traveled, and perform a cumulative doppler shift. Because the cumulative redshift depends NOT ON A SINGLE SPEED or rate of recession, say at the time of emission, but on a whole history of recession rates that prevailed at different times during the light's travel. But you can analyze the cosmo redshift as the cumulative result of many many small dopplers. And therefore you can say that it IS A DOPPLER. what you say depends on how you treat it mathematically. Poincaré said that mathematics is not right it is *convenient*. It is a sophisticated attitude, he was a smart man. Don't ask what nature IS, ask what is the most convenient way to describe it. Call it a succession of many tiny doppler shifts if you like. Or call it stretching out the lightwave, if you like. They give the same answer. 



#24
Mar1712, 01:55 AM

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You're saying that because of the way GR works simply thinking of the redshift as doppler only works if you make little short hops and stay "local" the whole time? Alright, I can understand that. Thanks Marcus.




#25
Mar1712, 11:54 AM

P: 832

It seems to me there are two different ways to create redshift. The first is if the light source is moving away from us in a static spacetime. The second is if spacetime is expanding and the source is being carried away with it. In general there would be some combination. How can we observe the difference? Is the difference real or just in the coordinate system used?




#26
Mar1712, 12:04 PM

P: 44

Well, I know it might be too late to point out, but there's this really awesome link for anyone who hasn't understood.
http://www.exploratorium.edu/hubble/tools/center.html The tiny widget they have created, makes things really clean. 



#27
Mar1712, 12:54 PM

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#28
Mar1712, 12:54 PM

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#29
Mar1712, 09:44 PM

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#30
Mar1712, 11:54 PM

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#32
Mar1812, 06:34 AM

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But in the mean time, in General Relativity the only time vector subtraction is welldefined is at a single point. When you try to subtract one vector at one point from a vector at another point in spacetime, ambiguities arise as to how to do that. To take a simple example, one method in General Relativity that allows you to subtract two vectors at different locations is through parallel transport. Parallel transport moves one vector across some path towards the other vector, keeping this vector parallel to itself along the entire path. The problem is that in curved spacetimes, parallel transport can lead to different answers depending upon which path you choose. And there is no a priori way of saying that one path is better than any other path. 



#33
Mar1812, 03:58 PM

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I see. Alright, I'll see if I can find some more information on it. Thanks guys.




#34
Mar1812, 04:12 PM

P: 297

I would like to point out that the quality of discussion on this thread has been phenomenal.
They have given a huge boost to my conceptsThanks :) 



#35
Mar1812, 06:20 PM

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Drakkith, another way of saying it is that is is a difference between "moving in space" and "space expanding". Far distant galaxies ARE "moving in space" relative to us (towards, away, sideways, whatever), but by utterly trivial amounts compared to the effect of the expansion of space. Nearby objects, however, are NOT moving relative to us due to the expansion of space, they really are moving IN space relative to us.
Hope I haven't just made it less clear instead of more clear. EDIT: Hm ... I think I skipped a full page of replies and didn't say anything that wasn't already said. Guess I just like the sound of my own keystrokes. 


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