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Does the expansion of space mean anything for the movement of matter in space?


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Earth is not moving in some direction due to expansion. It is moving in some direction (relative to say the cosmological frame) because net forces on Earth or the materials from which it is composed have cumulated in its present velocity/momentum.  Our velocity (relative to the frame of your choice) has changed by over 400 km/sec since the Spinosaurus was around, so those external forces are not insignificant.

In other words, if I put two objects in space separated by say a megaparsec, that are stationary relative to each other and not subject to external forces like gravity from nearby objects, then under just expansion alone, they will remain forever the same distance apart.  Dark energy is probably the only actual force (that is not expansion itself) that will push the two objects apart over time. This is not expansion, but the acceleration of expansion, and yes, that does require force and energy and would affect said mutually stationary things.

Edited by Halc
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7 hours ago, Alex Mercer said:

For example, is earth moving in some sort of direction due to expansion, are the galaxies and other planets movies in a direction at some pace. 

 

Thank you in advance!

There are two things to consider here. The observed space expansion is over large distances, and a good analogy here are the dots on a balloon being blown up. The dots are not really moving, but are getting further apart due to the expanding balloon material [space] expanding.

Over smaller distances,(like our solar system, our galaxy, our local group of galaxies, and even further afield to our galactic wall) the expansion is negated by the gravitational attraction of objects. Best example of this of course is M31 (Andromeda) and our own Milky Way which in a distant time will be merging.

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10 hours ago, Halc said:

Earth is not moving in some direction due to expansion. It is moving in some direction (relative to say the cosmological frame) because net forces on Earth or the materials from which it is composed have cumulated in its present velocity/momentum.  Our velocity (relative to the frame of your choice) has changed by over 400 km/sec since the Spinosaurus was around, so those external forces are not insignificant.

In other words, if I put two objects in space separated by say a megaparsec, that are stationary relative to each other and not subject to external forces like gravity from nearby objects, then under just expansion alone, they will remain forever the same distance apart.  Dark energy is probably the only actual force (that is not expansion itself) that will push the two objects apart over time. This is not expansion, but the acceleration of expansion, and yes, that does require force and energy and would affect said mutually stationary things.

How do they not grow further apart?

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8 hours ago, J.C.MacSwell said:

How do they not grow further apart?

The question is, why do you think they do? 'Space' isn't something that can apply a friction-like force and begin accelerating an object.

Assume we have two pebbles X and Y that are a megaparsec apart, and they're way out in the void between galaxies.  Now a megaparsec is a long way. You can fit at least a couple dozen galaxies like ours side by side on one. Nearby galaxies are going to exert tidal forces on our pebbles drawing them apart, so we're going to ignore gravity and also dark energy and just concentrate on the expansion.

Pebble X is stationary relative to the cosmological frame, that is, the CMB appears isotropic from X's PoV. It is said to have a peculiar velocity of zero.  Y is stationary relative to X, that is, the proper separation between X and Y is constant at 1 megaparsec.  Since 'space' is expanding at a rate of 70 m/sec/mpc, Y must have a peculiar velocity of 70 m/sec towards X which exactly cancels the space expansion.  There's no reason to think it will lose this velocity per Newtons first law. If expansion exerted a force on it, that would be akin to friction with some kind of aether, and that would drag any peculiar motion down to zero over time, which would have all orbiting things (going around a galaxy especially) lose their orbital motion and spiral into the centers. Angular momentum would not be preserved.

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10 hours ago, J.C.MacSwell said:

How do they not grow further apart?

Quote from jcMcSwell

 Dark energy is probably the only actual force (that is not expansion itself) that will push the two objects apart over time ". 

 

Excuse me ,  dark energy is not an ACTUAL  force .

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5 minutes ago, Prof Reza Sanaye said:

Quote from jcMcSwell

 Dark energy is probably the only actual force (that is not expansion itself) that will push the two objects apart over time ". 

 

Excuse me ,  dark energy is not an ACTUAL  force .

Yes I agree, good point. +1

 

There is a lot of wooly thinking in this thread because we actually need to admit that we really don't know the answers here.

But we should also state that Science is still investigating and has no definitive opinion as yet.

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40 minutes ago, Halc said:

The question is, why do you think they do? 'Space' isn't something that can apply a friction-like force and begin accelerating an object.

Assume we have two pebbles X and Y that are a megaparsec apart, and they're way out in the void between galaxies.  Now a megaparsec is a long way. You can fit at least a couple dozen galaxies like ours side by side on one. Nearby galaxies are going to exert tidal forces on our pebbles drawing them apart, so we're going to ignore gravity and also dark energy and just concentrate on the expansion.

Pebble X is stationary relative to the cosmological frame, that is, the CMB appears isotropic from X's PoV. It is said to have a peculiar velocity of zero.  Y is stationary relative to X, that is, the proper separation between X and Y is constant at 1 megaparsec.  Since 'space' is expanding at a rate of 70 m/sec/mpc, Y must have a peculiar velocity of 70 m/sec towards X which exactly cancels the space expansion.  There's no reason to think it will lose this velocity per Newtons first law. If expansion exerted a force on it, that would be akin to friction with some kind of aether, and that would drag any peculiar motion down to zero over time, which would have all orbiting things (going around a galaxy especially) lose their orbital motion and spiral into the centers. Angular momentum would not be preserved.

Very beautiful line of reasoning. Real enjoyed it.  I only venture to add that spatial status of coordinances makes separation velocity repetitively differential to any likely acceleration. They are more relative than that, even regarding astronomists'  scrutinizing red shift. The placement of coodinances relative to each other and deemed fixed to their adjacent "towards-ness" , cannot be non-relative with respect to any other moving object's drag.

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50 minutes ago, Halc said:

The question is, why do you think they do? 'Space' isn't something that can apply a friction-like force and begin accelerating an object.

Assume we have two pebbles X and Y that are a megaparsec apart, and they're way out in the void between galaxies.  Now a megaparsec is a long way. You can fit at least a couple dozen galaxies like ours side by side on one. Nearby galaxies are going to exert tidal forces on our pebbles drawing them apart, so we're going to ignore gravity and also dark energy and just concentrate on the expansion.

Pebble X is stationary relative to the cosmological frame, that is, the CMB appears isotropic from X's PoV. It is said to have a peculiar velocity of zero.  Y is stationary relative to X, that is, the proper separation between X and Y is constant at 1 megaparsec.  Since 'space' is expanding at a rate of 70 m/sec/mpc, Y must have a peculiar velocity of 70 m/sec towards X which exactly cancels the space expansion.  There's no reason to think it will lose this velocity per Newtons first law. If expansion exerted a force on it, that would be akin to friction with some kind of aether, and that would drag any peculiar motion down to zero over time, which would have all orbiting things (going around a galaxy especially) lose their orbital motion and spiral into the centers. Angular momentum would not be preserved.

Okay. So you are saying one is at rest wrt the CMBR frame, which is an inertial frame only locally, and the other is not?

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11 minutes ago, Prof Reza Sanaye said:

Excuse me ,  dark energy is not an ACTUAL  force .

 

4 minutes ago, studiot said:

Yes I agree, good point. +1

I agree too. @Prof Reza Sanaye has an interesting way of saying things that are absolutely spot-on, and then trying to clarify by sending what to me looks like impenetrable clouds of philosophical fog. :D 

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1 minute ago, Prof Reza Sanaye said:

You think dark energy is an Actual Force  ??   !!!

It's a mystery how you reach this conclusion from such a straightforward statement

P RS: X

J: I agree

P RS: You think X is not true?

 

 

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Despite the wording above, I agree that dark energy isn't an actual force, but dark energy will begin to increase the proper separation between two relatively stationary objects, and that translates to a sort of kinetic energy which could hypothetically be harnessed to do work (imagine a seriously long thin string between them).  Gravity will also separate the pair of objects when placed in the deep void * (regions of negative mass density relative to the mean), and gravity isn't an actual force either, at least not under relativity. Like gravity, I think dark energy can be expressed as a force only when considered in Newtonian terms, and it doesn't make much sense there since Newtonian physics has no concept of expanding space.

From a geometric standpoint, spacetime with expansion but without gravity and without dark energy is effectively indistinguishable from Minkowskian spacetime. There would be no event horizon since light would eventually reach us from galaxies of any distance and any recession speed. That horizon is only there because of the effects of dark energy.

 

* One of the nearest such regions is the Dipole Repeller which seems to expel all the galaxies in its vicinity. This shows that in an otherwise uniform distribution of matter, the absence of matter in an isolated pocket of that distribution will have a repulsive gravitational effect on all the nearby surrounding masses.

12 minutes ago, J.C.MacSwell said:

Okay. So you are saying one is at rest wrt the CMBR frame, which is an inertial frame only locally, and the other is not?

Right. The CMB would not appear isotropic from Y's PoV, and from this Y could measure his peculiar velocity of 70 km/sec towards X.

 

16 minutes ago, Prof Reza Sanaye said:

The placement of coodinances relative to each other and deemed fixed to their adjacent "towards-ness" , cannot be non-relative with respect to any other moving object's drag.

I couldn't quite parse that, but by stationary relative to each other, I explicitly qualified that to mean 'constant proper separation', meaning a string between the two would not break or stretch.

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6 minutes ago, swansont said:

It's a mystery how you reach this conclusion from such a straightforward statement

P RS: X

J: I agree

P RS: You think X is not true?

 

 

Well ,   He says  Professor Reza Sanaye sends  impenetrable clouds of philosophical fog . .. .

 

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Just now, Prof Reza Sanaye said:

Well ,   He says  Professor Reza Sanaye sends  impenetrable clouds of philosophical fog . .. .

 

Yes, that is also true. Can you see that these are distinct sentiments?

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37 minutes ago, Prof Reza Sanaye said:

You think dark energy is an Actual Force  ??   !!!

 

I think this was a genuine mistake or misreading so I am going to say +1 to cancel the -1.

Perhaps the "impenetrable clouds of philosophical fog . .. ." are obscuring my vision but I like to give the benefit of the doubt.

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1 hour ago, Halc said:

Despite the wording above, I agree that dark energy isn't an actual force, but dark energy will begin to increase the proper separation between two relatively stationary objects, and that translates to a sort of kinetic energy which could hypothetically be harnessed to do work (imagine a seriously long thin string between them).  Gravity will also separate the pair of objects when placed in the deep void * (regions of negative mass density relative to the mean), and gravity isn't an actual force either, at least not under relativity. Like gravity, I think dark energy can be expressed as a force only when considered in Newtonian terms, and it doesn't make much sense there since Newtonian physics has no concept of expanding space.

From a geometric standpoint, spacetime with expansion but without gravity and without dark energy is effectively indistinguishable from Minkowskian spacetime. There would be no event horizon since light would eventually reach us from galaxies of any distance and any recession speed. That horizon is only there because of the effects of dark energy.

 

* One of the nearest such regions is the Dipole Repeller which seems to expel all the galaxies in its vicinity. This shows that in an otherwise uniform distribution of matter, the absence of matter in an isolated pocket of that distribution will have a repulsive gravitational effect on all the nearby surrounding masses.

Right. The CMB would not appear isotropic from Y's PoV, and from this Y could measure his peculiar velocity of 70 km/sec towards X.

 

I couldn't quite parse that, but by stationary relative to each other, I explicitly qualified that to mean 'constant proper separation', meaning a string between the two would not break or stretch.

But remember that neither Newton nor Einstein has  solved the Problematic of separate  frames ( of coordinanace ) in anisotropic space with accelerating velocity . . . . Newton did not know about Relativity ; And , Einstein who knew about that , neglected  the fact that all accelerated objects feel a frame-variant 3-vector force F right  in the direction of their acceleration. Information is , thus , partially lost.

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