# Distance and space expansion

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Hi, I have a question:

I read many place that space expansion is not proper motion.

That faraway galaxies doesn't move but are caried by space expansion.

So if today a galaxy is mesured to be 1G lightyears away, in 1 millions years that galaxy will still be 1G lightyears a way because she have no proper motion.

If I go back in time, the same will be true. No need for a bigbang...

where am I wrong in my reasonning ?

Thanks

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In one million years, the galaxy will be more than 1G lightyears away, because the amount of space in between them will have increased in the meantime, even if they are not moving relative to one another. That's what the expansion of space means.

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Hi, I have a question:

I read many place that space expansion is not proper motion.

That faraway galaxies doesn't move but are caried by space expansion.

So if today a galaxy is mesured to be 1G lightyears away, in 1 millions years that galaxy will still be 1G lightyears a way because she have no proper motion.

If I go back in time, the same will be true. No need for a bigbang...

where am I wrong in my reasonning ?

Thanks

If we could measure a galaxy to be exactly 1,000,000,000 LY away today. In a million years that galaxy would be measured to be about 1,000,072,000 LY away (if I did my math right). This distance change has nothing to do with proper motion, only motion due to the expansion. A million years is a fairly short time in the life of our universe.

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In one million years, the galaxy will be more than 1G lightyears away, because the amount of space in between them will have increased in the meantime, even if they are not moving relative to one another. That's what the expansion of space means.

I was under the impression that space expansion was meaning that the units of space was expanding. If the amount of space in between increase them galaxies have proper motion...

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I was under the impression that space expansion was meaning that the units of space was expanding. If the amount of space in between increase them galaxies have proper motion...

"Units of space expanding" is another way of saying the same thing. Either way, the distance will be greater at the later time. Light will take more than 1G years to travel the distance.

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I was under the impression that space expansion was meaning that the units of space was expanding. If the amount of space in between increase them galaxies have proper motion...

I always liked the ants on an expanding balloon picture. Proper motion would be analogous to the ants walking around on the surface of the balloon. If the ants stopped walking, the distance between the ants would grow. Not because the ants are moving (proper motion), but because the surface of the balloon itself is expanding (space expansion).

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Let use te balloon analogy

Take 2 points 5cm appart. Trace a line every cm on the surface.

Inflate the ballon. If I use the line traced on the surface the distance will be 5cm. But for someone outside of the surface world the ruler didn`t expand and he will mesure more than 5cm.

Since we are in the universe our ruler will also expand. When we use the redshift, the wavelength of light is our ruler, so our ruler is expanding...

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Welcome to the club.

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Since we are in the universe our ruler will also expand.

No. The forces holding atoms in place and together are enormously more massive than those involved in the expansion of space. We are able to recognize and measure the expansion of space only because of the huge distances involved. At a human level, even at a solar system level, the EM force and gravity are far far bigger than any background expansion. Its the space that is expanding and by an incredibly tiny amount; to such a small amount that it does not affect lumpen creatures like us at all, but on the mind-boggling scales of inter-galatic space (with no competing forces) it becomes important.

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Would it be correct to tell space is created between gravitationnaly unbonded systems ?

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If we could measure a galaxy to be exactly 1,000,000,000 LY away today. In a million years that galaxy would be measured to be about 1,000,072,000 LY away (if I did my math right). This distance change has nothing to do with proper motion, only motion due to the expansion. A million years is a fairly short time in the life of our universe.

From the following ?

H = 75 km/s / Mpc = 0.25e-3 c / Mpc

1 Gly = 300 Mpc

Thus, at 300 Mpc, the recession velocity is 0.75e-1 c, or ~75 Kly / Myr.

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Would it be correct to tell space is created between gravitationnaly unbonded systems ?

I wonder if you are serious or sarcastic.

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I wonder if you are serious or sarcastic.

I am not completely satisfied by the answers, but I thanks for the effort people did to answer.

Noting new all answer by the book, but it forced me to think more deeply about space expansion and photon travelling throught that space.

Let follow a photon emitted in a faraway galaxy.

In that galaxy space is not expanding, so it is not redshifted before he leave the galaxies cluster and enter the great inter cluster void.

There, space is expanding so the photon is also expanding. On it's way space expand and so does the photon until it approach our local cluster.

In our local cluster space didn't expand so the space here is contracted compared to the inter cluster space. So why doesn't the photon also recontract ?

I read many thing on the subject, and I accepted the cosmoloogical redshift without questioning. Now that I try to go deeper I have many question with no answer.

You can give numbers or equations, but what I need is comprehension.

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I am not completely satisfied by the answers, but I thanks for the effort people did to answer.

Noting new all answer by the book, but it forced me to think more deeply about space expansion and photon travelling throught that space.

Let follow a photon emitted in a faraway galaxy.

In that galaxy space is not expanding, so it is not redshifted before he leave the galaxies cluster and enter the great inter cluster void.

There, space is expanding so the photon is also expanding. On it's way space expand and so does the photon until it approach our local cluster.

In our local cluster space didn't expand so the space here is contracted compared to the inter cluster space. So why doesn't the photon also recontract ?

I read many thing on the subject, and I accepted the cosmoloogical redshift without questioning. Now that I try to go deeper I have many question with no answer.

You can give numbers or equations, but what I need is comprehension.

I'm pretty sure space expands everywhere, actually. It's just that gravity holds structures together anyway.

But in any case, if it were true that there was expansion only between structures, the photon still wouldn't contract unless the space it was traveling through was contracting. If it was just not expanding, then nothing would happen.

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I'm pretty sure space expands everywhere, actually. It's just that gravity holds structures together anyway.

That is what I think also I was under the impression that because of gravity space was not expanding.

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In one million years, the galaxy will be more than 1G lightyears away, because the amount of space in between them will have increased in the meantime, even if they are not moving relative to one another. That's what the expansion of space means.

I wonder. Instead of talking about "space expansion" which makes people believe that "something" is expanding, why don't we introduce a second kind of motion, a "motion-that-is-not-motion", call that s-motion, that simply does not correspond to standard laws of motion? But corresponds to some other universal law.

Edited by michel123456
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I wonder. Instead of talking about "space expansion" which makes people believe that "something" is expanding, why don't we introduce a second kind of motion, a "motion-that-is-not-motion", call that s-motion, that simply does not correspond to standard laws of motion? But corresponds to some other universal law.

If you were floating at the bottom of an enormous swimming pool in the shape of an upside-down cone and that cone was slowly filling up from the bottom, your position on the surface of the water would be growing more distant from other objects on the surface. This distance-growth would correspond to an increasing number of water molecules between you and your surrounding objects but you wouldn't have to swim through the water to increase the number of water molecules between you and your beach ball. Of course, you could swim either toward or away from the beach ball, and by doing so you would increase or decrease the number of surface molecules between you and it, but the rising water level in the cone would also be contributing to your distance from it. If this is not a flawed analogy, there's no reason to differentiate between distance-increase from expansion and distance gained by object-motion.

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My proposal was intended to avoid the molecules analogy.

Einstein struggeld so hard to erase the concept of aether, but those molecular anologies make it come back, I am afraid. Although the space expansion concept does not support necessarily the aether theory, all analogies are based upon some material background (a balloon, a cake, water molecules,...) that make people believe that "something" is expanding or "created".

Edited by michel123456
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The fabric of spacetime is, itself, a 'substance'. Could not 'wave-like' distortions, of that fabric, account for the 'Aether' ? Please ponder (and explain ??), that EM waves travel at precisely the same speed, as gravity waves, as if they both were 'similar sorts' of disturbances, of the same underlying spacetime fabric.

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I wonder. Instead of talking about "space expansion" which makes people believe that "something" is expanding, why don't we introduce a second kind of motion, a "motion-that-is-not-motion", call that s-motion, that simply does not correspond to standard laws of motion? But corresponds to some other universal law.

I propose scalar-motion, it can be expressed with a simple number the Hubble constant H0 and has units of inverse time. It doesn't have any special direction unlike vectorial motion. It can also be negative and that is gravity.

Also I have taught experiment:

Take a perfect mirror box and put some photon of all the same wave lenght. After a billion year the photons would have travelled 1 billion light year in the box.

Will it be redshifted ?

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I propose scalar-motion, it can be expressed with a simple number the Hubble constant H0 and has units of inverse time. It doesn't have any special direction unlike vectorial motion. It can also be negative and that is gravity.

But expansion increases with distance so its inverse should be related to the reciprocal of distance; however gravity is related to the reciprocal of the square of distance.

edit

Just noticed you said negative not inverse - but same argument applies.

Edited by imatfaal
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The fabric of spacetime is, itself, a 'substance'. (...)

See what I mean?

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If this is not a flawed analogy, there's no reason to differentiate between distance-increase from expansion and distance gained by object-motion.

Every analogy is flawed.

And I think we were mistaking 'proper motion' with 'peculiar motion' I am pretty sure it is peculiar motion that we are talking about in this thread.

Here's my understanding of 'peculiar motion'... We have assigned a geometry to space. The balloon analogy is always the easiest to work with, so we decorate the balloon with a drawn-on grid system, which represents the geometry of space (since the surface of the balloon is representing space itself). On our decorated balloon, we place a grain of sand in the top left corner of one quadrant (or square) of it's geometry. we place another grain in the top right corner of a quadrant 3 quadrants over from the first. We now define 'peculiar motion' as any motion relative to the geometry (or gridlines, or quadrants) of space (the surface of the balloon). Take a picture of the position of the grains of sand in their quadrants of 'space'. Measure the distance between the grains of sand.

Now blow the balloon up some more. Take a picture of the position of the grains of sand in their quadrants of 'space'... They haven't moved. One is in the top left corner of it's quadrant, and the other is still in the top right corner of it's quadrant, 3 quadrants over. By our definition of 'peculiar motion', they have not moved 'through space'. Now measure the distance between the 2 grains... the distance has grown. Without moving 'through space', the distance between the 2 grains has increased.

The law 'nothing can go faster than the speed of light' only applies to objects moving 'through space'.

Their is a clear difference between objects moving 'through space' and objects moving 'with the expansion of space itself'

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My proposal was intended to avoid the molecules analogy.

Einstein struggeld so hard to erase the concept of aether, but those molecular anologies make it come back, I am afraid. Although the space expansion concept does not support necessarily the aether theory, all analogies are based upon some material background (a balloon, a cake, water molecules,...) that make people believe that "something" is expanding or "created".

I agree with the aether/substance problem and that was not the point of my analogy. I was only using molecules to show that distance from expansion and distance from motion can be the same kind of distance albeit generated in different ways. I believe the post I was responding to suggested differentiating distance into distance from expansion and distance from motion. This is unnecessary, imo, if you can understand that distance resulting from the two causes can still add up to "net distance" by virtue of time (duration of expansion) AND motion across an expanding distance.

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Every analogy is flawed.

And I think we were mistaking 'proper motion' with 'peculiar motion' I am pretty sure it is peculiar motion that we are talking about in this thread.

Here's my understanding of 'peculiar motion'... We have assigned a geometry to space. The balloon analogy is always the easiest to work with, so we decorate the balloon with a drawn-on grid system, which represents the geometry of space (since the surface of the balloon is representing space itself). On our decorated balloon, we place a grain of sand in the top left corner of one quadrant (or square) of it's geometry. we place another grain in the top right corner of a quadrant 3 quadrants over from the first. We now define 'peculiar motion' as any motion relative to the geometry (or gridlines, or quadrants) of space (the surface of the balloon). Take a picture of the position of the grains of sand in their quadrants of 'space'. Measure the distance between the grains of sand.

Now blow the balloon up some more. Take a picture of the position of the grains of sand in their quadrants of 'space'... They haven't moved. One is in the top left corner of it's quadrant, and the other is still in the top right corner of it's quadrant, 3 quadrants over. By our definition of 'peculiar motion', they have not moved 'through space'. Now measure the distance between the 2 grains... the distance has grown. Without moving 'through space', the distance between the 2 grains has increased.

The law 'nothing can go faster than the speed of light' only applies to objects moving 'through space'.

Their is a clear difference between objects moving 'through space' and objects moving 'with the expansion of space itself'

Right

http://en.wikipedia.org/wiki/Peculiar_velocity

http://en.wikipedia.org/wiki/Proper_motion

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