Galaxies

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Cyclostrophic

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Why is it that the further a galaxy is from us the faster it's travelling away ?

My initital thought was the inflation of the universe. But that would mean all galaxies in the universe would never get any closer or just fly away from one another, but that doesn't happen as we've seen galaxies merge and our very own Milky Way is set to be swept up by Andromeda. Is localised gravity in play here ?

Also, i'm not sure that inflation sits well with increased acceleration for galaxies that are more distant ? Although it's really, really giving me a head ache this one. I started assuming that we are at the very centre. Then I moved us to one side of the inflation looking over to the other side to prove that we would be moving apart, only for me to realise that there would need to be a stationary middle point in the whole of the universe for this to happen - and i'm not sure that there is ?

I think i'll stop before I get in to expansion within the inflation as I can feel a migraine coming on.

Any input is much appreciated.

Thanks
 
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MeteorWayne

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The Universe is expanding, so galaxies that are far away, are moving away from us.

For galaxies in our local group (like Andromeda) the acceleration toward each other from the mutual gravity causes them to approach us much faster than the expansion of the Universe at this "small" distance.
 
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neilsox

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Galaxies in the local group are typically in elliptical orbits with respect to each other, so some are approaching and some are moving away. It is expected that Andromeda will be very close in a few billion years, but the galactic hubs may not collide, in which case the two will likely continue in their orbits after the disks pass though each other. Most of the stars will however be perturbed at least slightly so the shapes of both will change. Neil
 
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Mee_n_Mac

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Cyclostrophic":1xfz06uy said:
Why is it that the further a galaxy is from us the faster it's travelling away ?

My initital thought was the inflation of the universe. But that would mean all galaxies in the universe would never get any closer or just fly away from one another, but that doesn't happen as we've seen galaxies merge and our very own Milky Way is set to be swept up by Andromeda. Is localised gravity in play here ?

Also, i'm not sure that inflation sits well with increased acceleration for galaxies that are more distant ? Although it's really, really giving me a head ache this one. I started assuming that we are at the very centre. Then I moved us to one side of the inflation looking over to the other side to prove that we would be moving apart, only for me to realise that there would need to be a stationary middle point in the whole of the universe for this to happen - and i'm not sure that there is ?
I think i'll stop before I get in to expansion within the inflation as I can feel a migraine coming on.
Any input is much appreciated.

Thanks
Well it is due to expansion (inflation has a different meaning in cosmology) of the universe. The reason we seem to be in "the middle" of this expansion is easier to understand with a visual. The one most often used is a 1 dimension case of the rubber band. Let's pretend I have a rubber band and it extends out of sight in both the left and right directions. On it are a series of dots placed 1 inch apart. They go on as far as I can see, 1 inch apart, in both directions with 1 placed immediately in front of me. Now let me have the rubber band stretch so that in 1 minute the dots are now 1.5 inches apart, dot to dot. The dot in front of me hasn't moved at all but the ones just to the right and left have moved from 1 in to 1.5 in away. The dots just next to them also moved, apart, from 1 to 1.5 inches. But how far have those "second" dots moved from my "center" dot ? Well they were 2 inches away to start and now they're 1.5 ("center" to 1'st dot) + 1.5 (1'st dot to 2'nd dot) = 3 inches away. So at 2x the distance from my "center" dot the dots moved 2x the amount (of the closest ones) in the same time ... they're moving at 2x the rate. Go 3 dots down and it's 3x the displacement and so 3x the rate. It appears I'm at the center of the expansion and things move away from me faster the further away they are. This is analogous to what we see in 3 dimensions.

So I go 100 miles to the right and sit down with a dot right in front of me. What do I see ? The rubber band still stretches out of sight to the left and right. The dot in front of me isn't moving (relative to me, this is the important part to understand) and the dots to my right and left are still moving away. And just as they were 100 miles to my left, they move away faster the further away they are. Again it appears I'm at the center of the expansion. But as you can now understand that's not really the case. Or, if it is, it's by chance and there's no way for you to determine this to be true by observing the dots. Is there a center to the rubber band ? Maybe, maybe not but I can't see it from where I am.

Look here ... (about 2 mins in) ...

[youtube]http://www.youtube.com/watch?v=IWbc54u8k8c[/youtube]

or here ... (sidewalk is like the rubber band above)

[youtube]http://www.youtube.com/watch?v=i1UC6HpxY28[/youtube]

Now I left out parts that pertain to how this expansion seems to be accelerating but let's keep to the basics first.
 
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