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Old Aug 28th 2017, 04:13 PM   #1
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expanding space

Why would the expansion of space entail the simultaneous expansion of matter? Doesnt that require space to have a significant amount of friction if somehow it can drag entire galaxies with it? If space were expanding given any normal definition of space why wouldnt it expand without disturbing matter? For example if my swimming pool were to suddenly expand some of the mass in it would be dragged along by virtue of the friction of the water but if there were no water in it merely expanding its space would have no effect on any mass within it.

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Old Aug 28th 2017, 04:27 PM   #2
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Originally Posted by jlr01 View Post
Why would the expansion of space entail the simultaneous expansion of matter?
It wouldn't. Many people make the mistake of thinking that because the expansion of space is evidenced by the separation of galaxies also means that galaxies are getting large. That's not true.

Originally Posted by jlr01 View Post
Doesnt that require space to have a significant amount of friction if somehow it can drag entire galaxies with it?
Are you trying to be humorous again?
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Old Aug 28th 2017, 05:53 PM   #3
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Are you suggesting that the expansion of space is not happening on all scales?
Is space only expanding between the galaxies?
Or is it only on cosmic scales that the expansion is noticeable?
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Old Aug 28th 2017, 10:28 PM   #4
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Originally Posted by Pmb View Post

Are you trying to be humorous again?
not at all . I should explain myself better. If the galaxies are receding away from us I understood that it was because space was expanding. Doesnt this require something like friction ?It seems to me that space could expand and the galaxies could remain right where they wer? I know its just an analogy but when I see a documentary they always give some analogy that the galaxies are like dots on a balloon or blueberries in a muffin but all those assume some friction to drag the object along with the substrate on which it rests. I hope that makes my question a little clearer and I promise to keep the jokes to a minimum!
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Old Aug 29th 2017, 12:00 AM   #5
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Was this inspired by my old post "the big bang or the big fizz"?

I think in that post I was trying to wrap my head around whether the inflation field was conservative or non-conservative but somehow had difficulty asking the right questions. I guess if the the inflationary field drags matter with it then it would have to be non-conservative (guessing).

For example if my swimming pool were to suddenly expand some of the mass in it would be dragged along by virtue of the friction of the water but if there were no water in it merely expanding its space would have no effect on any mass within it.
Is the water a kind of ether?
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Old Aug 29th 2017, 02:42 AM   #6
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Originally Posted by jlr01 View Post
not at all . I should explain myself better. If the galaxies are receding away from us I understood that it was because space was expanding. Doesnt this require something like friction ?
Not at all. Consider the sphere (i.e. balloon) analogy. Let's assume the universe is closed. Let the surface of a sphere represent space itself. Place dots on the surface randomly located to represent galaxies. Now imagine the sphere expanding (blowing up the balloon).
Then the dots would be getting further and further away from each other. Notice how they remain at rest though. Its only by observing other dots that observers confined to the surface can tell that the surface is expanding. You can repeat this with a plane. Notice that there's no friction to speak of. And there's no friction in space either. The notion doesn't even make sense since friction is an effect between two bodies with solid surfaces

Originally Posted by jlr01 View Post
It seems to me that space could expand and the galaxies could remain right where they wer?
They actually do remain where they are. It's the space that's expanding. The galaxies aren't moving. Its only that the space between the galaxies is getting larger.

Originally Posted by jlr01 View Post
I know its just an analogy but when I see a documentary they always give some analogy that the galaxies are like dots on a balloon or blueberries in a muffin but all those assume some friction to drag the object along with the substrate on which it rests.
Actually there's no friction whatsoever at work in those analogies. If you were an ant on the surface of an expanding balloon you would not be moving with respect to the surface. You have to keep in mind that when you use analogies that there's always a flaw in it because its an analogy and not the real thing. When you picture the expanding balloon you're thinking of the latex surface, which is made of matter. There is no equivalent matter in the universe.
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Old Aug 29th 2017, 01:07 PM   #7
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So is space still expanding or has it stopped now?
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Old Aug 29th 2017, 02:19 PM   #8
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Originally Posted by Pmb View Post
They actually do remain where they are. It's the space that's expanding. The galaxies aren't moving. Its only that the space between the galaxies is getting larger.
I wouldn't describe it that way. The distance between galaxies is indeed increasing, at an average rate of about 72 kilometers per second per megaparsec (a rate that depends on distance, known as Hubble's Constant). Thus from our perspective sitting here in the Milky Way we see that distant galaxies are receding from us at a rate that averages out to Hubble's Constant.

Originally Posted by kiwiheretic
So is space still expanding or has it stopped now?
Still expanding.
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Old Aug 29th 2017, 03:41 PM   #9
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Then why is the expansion considered to be isotropic? Surely with all the particles moving with constant velocity it would hollow out in the middle (see attached diagram).
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expanding space-primordial-egg.png  
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Old Aug 29th 2017, 08:06 PM   #10
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Originally Posted by Pmb View Post

They actually do remain where they are. .
I guess what I cant get my head around is how the galaxies can remain where they are while at the same time receding.
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