Speed of light effected by expansion of space?

[rasp]

I once learned (way back when) that nothing can move faster than light in a vacuum. But this now seems simplistic. For if there is an object A whose light we see as it was released 13.7 billion years ago and object A is now 45 billion light years away, then that object A has moved further from its starting point in 13.7 billion years than its own emitted light. Was object A once moving faster than light and now is moving much slower than it? Does this imply that objects are effected by inflation and expansion but light isn't? Thanks

 

[marcus]

I once learned (way back when) that nothing can move faster than light in a vacuum. But this now seems simplistic. For if there is an object A whose light we see as it was released 13.7 billion years ago and object A is now 45 billion light years away, then that object A has moved further from its starting point in 13.7 billion years than its own emitted light. Was object A once moving faster than light and now is moving much slower than it? Does this imply that objects are effected by inflation and expansion but light isn't? Thanks

Rasp, in the case of ordinary motion--the kind we are used to--there is always somewhere you are going. Some destination which at least temporarily you are approaching.

The simple expansion of distances that you get in General Relativity is not like that. Distances between all the widely separated galaxies are increasing without any galaxy going anywhere. Think of it as changing geometry, not motion. GR is a dynamic geometry theory, distances can change. Indeed a lot of distances are changing at a faster rate than c----and this is not breaking any speed rule. It is not ordinary motion of the sort governed by special rel.

Individual galaxies in clusters do orbit their neighbors, they have diddly little motions in their local neighborhoods---typically a few tens of km/s or something on the order of 100 km/s. Those are expanding distance rates. The distances that are expanding are the largescale distances between widely separated galaxies, outside of any gravitationally bound cluster.

Try the uni.edu link in my signature and see if you can make it work for you. You need to prime it by putting in 0.25 for matter, 0.75 for cosmo constant "Lambda", and 74 for Hubble constant.

Or any three numbers approximately like that. Then you put in the redshift of some galaxy, like z = 8, and it will tell you how far away the thing is now and how fast the present distance to it is increasing now.

It will also tell you distance then (when the light was emitted that is now reaching us) and recession rate then.

Try it. If you have any trouble, ask and one of us will talk you thru it.

 

[Rymer]

Rasp, in the case of ordinary motion--the kind we are used to--there is always somewhere you are going. Some destination which at least temporarily you are approaching.

The simple expansion of distances that you get in General Relativity is not like that. Distances between all the widely separated galaxies are increasing without any galaxy going anywhere. Think of it as changing geometry, not motion. GR is a dynamic geometry theory, distances can change. Indeed a lot of distances are changing at a faster rate than c----and this is not breaking any speed rule. It is not ordinary motion of the sort governed by special rel.

Individual galaxies in clusters do orbit their neighbors, they have diddly little motions in their local neighborhoods---typically a few tens of km/s or something on the order of 100 km/s. Those are expanding distance rates. The distances that are expanding are the largescale distances between widely separated galaxies, outside of any gravitationally bound cluster.

Try the uni.edu link in my signature and see if you can make it work for you. You need to prime it by putting in 0.25 for matter, 0.75 for cosmo constant "Lambda", and 74 for Hubble constant.

Or any three numbers approximately like that. Then you put in the redshift of some galaxy, like z = 8, and it will tell you how far away the thing is now and how fast the present distance to it is increasing now.

It will also tell you distance then (when the light was emitted that is now reaching us) and recession rate then.

Try it. If you have any trouble, ask and one of us will talk you thru it.

Rasp, while what marcus is telling you is correct for the currently used mainstream model, it is not necessarily proved fact. There are alternative explanations that produce the same results without such 'faster than the speed of light' behavior.

Much of the current 'mainstream' model has many public attention getting features -- which often seem to be as important as the supporting data.

At this point, it is not clear what model will actually be found to be best. This is not intended to start a heated discussion. But is only a statement to clarify what is really fact from what MIGHT be fiction.

 

[Phrak]

Marcus, I was originally prepared to disagree with you on a finer point with the following:

"In the spacetime of general relativity, it makes sense to say that the velocity of an object is only meaningfully defined in an inertial frame local to that object. We can setup a local system of syncronized clocks and gridwork of distances to define space time coordinates. We can measure or extrapolate velocities with respect to these coordinates. But at any great distance out, the coordinates don't describle space and time."

I was preparing to say, that our local coordinates, mapped over all of spacetime are non-physical, but one must define 'non-physical'. In this context, it would be that v_light ≠ c. There doesn't seem to be any good reason to call these coordinate systems non-physical, but only nonlocal.

 

[Rymer]

Marcus, I was originally prepared to disagree with you on a finer point with the following:

"In the spacetime of general relativity, it makes sense to say that the velocity of an object is only meaningfully defined in an inertial frame local to that object. We can setup a local system of syncronized clocks and gridwork of distances to define space time coordinates. We can measure or extrapolate velocities with respect to these coordinates. But at any great distance out, the coordinates don't describle space and time."

I was preparing to say, that our local coordinates, mapped over all of spacetime are non-physical, but one must define 'non-physical'. In this context, it would be that v_light ≠ c. There doesn't seem to be any good reason to call these coordinate systems non-physics, but only nonlocal.

Good point. That is in essence the difference in what I was saying. In other models this becomes more important -- while often the Standard Model tends to talk in terms of its nonlocal interpretation.

 

[Chalnoth]

Marcus, I was originally prepared to disagree with you on a finer point with the following:

"In the spacetime of general relativity, it makes sense to say that the velocity of an object is only meaningfully defined in an inertial frame local to that object. We can setup a local system of syncronized clocks and gridwork of distances to define space time coordinates. We can measure or extrapolate velocities with respect to these coordinates. But at any great distance out, the coordinates don't describle space and time."

I was preparing to say, that our local coordinates, mapped over all of spacetime are non-physical, but one must define 'non-physical'. In this context, it would be that v_light ≠ c. There doesn't seem to be any good reason to call these coordinate systems non-physical, but only nonlocal.

Well, the correct statement is that differences in velocity are only well-defined locally. When comparing the velocities of two objects far away, General Relativity provides no answer. We can, of course, write down some coordinates and come up with some measure of velocity. But whatever measure we write down will be arbitrary.

This is, fundamentally, why there is no problem with far-away objects moving "faster than light" compared to us: If I compare the velocities between far-away objects, I can come up with any velocity I choose just by picking different coordinates. Obviously there can't be a limit if I can make the velocity anything I please!

The actual speed of light limitation in General Relativity, then, is purely a local limit. It states that any person that is observing the speed of a photon passing by them will always measure it to be moving at the speed of light. This also means that if we look at a far-away object, no matter how it is moving compared to us, it will always be seen to move more slowly than the photons moving past it.

 

[Chalnoth]

Rasp, while what marcus is telling you is correct for the currently used mainstream model, it is not necessarily proved fact.

It's proven about as much as any fact in science.

There are alternative explanations that produce the same results without such 'faster than the speed of light' behavior.

No, there aren't. The other alternative 'explanations' fall flat when compared against the full gamut of cosmological evidence.

 

[Rymer]

It's proven about as much as any fact in science.
No, there aren't. The other alternative 'explanations' fall flat when compared against the full gamut of cosmological evidence.

No they don't. What appears to be falling FLAT is the Standard Model use of General Relativity on the cosmological scale. Note basic physics:

IF the universe is FLAT as current measurements indicate, then

1) Space is Euclidean -- meaning NO 'space/time warp' at the cosmological scale
2) Therefore, General Relativity does NOT apply.
3) Special Relativity DOES apply.
4) The well established and verified relation for redshift is the Special Relativity Doppler Redshift.
5) The Doppler redshift relation NEVER exceeds the speed of light for any value of redshift.

NOTE: General Relativity is still a valid mathematical relation for gravitational fields but ONLY within the universe -- not when applied to its entirety.

 

[Chalnoth]

No they don't. What appears to be falling FLAT is the Standard Model use of General Relativity on the cosmological scale. Note basic physics:

IF the universe is FLAT as current measurements indicate, then

1) Space is Euclidean -- meaning NO 'space/time warp' at the cosmological scale

Our universe is nearly spatially flat. There is space-time curvature. So the rest of your argument fails.

Edit: One other slightly pedantic point is that General Relativity is quite valid in flat space-time. It just reduces to special relativity. This doesn't describe our universe on large scales, however.

NOTE: General Relativity is still a valid mathematical relation for gravitational fields but ONLY within the universe -- not when applied to its entirety.

No deviations from General Relativity have yet been found, though physicists are trying very hard to do so.

 

[Chronos]

In a sense, expansion of space 'speeds up' photons by stretching them out [aka redshift]. In a mythical, absolute, fixed reference frame, photons would appear to travel across space at variable speeds. But, to any observer embedded in the physical universe, all photons appear to travel at exactly 'c'. So even though really distant stuff, like the surface of last scattering, appear to be receding faster than 'c', their photons still reach us.

 

[Rymer]

In a sense, expansion of space 'speeds up' photons by stretching them out [aka redshift]. In a mythical, absolute, fixed reference frame, photons would appear to travel across space at variable speeds. But, to any observer embedded in the physical universe, all photons appear to travel at exactly 'c'. So even though really distant stuff, like the surface of last scattering, appear to be receding faster than 'c', their photons still reach us.

Yes, key word is 'appear'. Note, our current interpretation of gravity MAY only be valid after the time of last scattering -- IF gravity like other forces -- is found to be due to a 'transport particle'. This explanation is appealing in that it would explain the flatness that is found -- assuming the 'particles' cannot travel backward in time.

(from Chalnoth) Our universe is nearly spatially flat. There is space-time curvature. So the rest of your argument fails.

Edit: One other slightly pedantic point is that General Relativity is quite valid in flat space-time. It just reduces to special relativity. This doesn't describe our universe on large scales, however.

Note, if force cannot be transmitted through the time coordinate, then there is no possibility left for a 'warp-age' -- and the argument holds.

Actually, while General Relativity is 'valid' conceptually in a flat space time (reducing to Special Relativity) there is a problem with the assumed metric for a non-zero Lambda. Also, other problems in the structure of the secondary relations. The Standard Model does not address this correctly in a perfectly flat scenario. It wasn't designed to -- it assumes it is warped. This is likely the cause for the apparent 'hump' that has been interpreted as 'dark energy'.

 

[Chalnoth]

Yes, key word is 'appear'. Note, our current interpretation of gravity MAY only be valid after the time of last scattering -- IF gravity like other forces -- is found to be due to a 'transport particle'. This explanation is appealing in that it would explain the flatness that is found -- assuming the 'particles' cannot travel backward in time.

Gravity most certainly didn't change at the surface of last scattering. That was only around 3000K, after all, which is quite a bit cooler than our own sun. And our current knowledge of gravity works quite well for the behavior of stars, as well as in the lab when we perform tests at these and much higher temperatures.

Actually, while General Relativity is 'valid' conceptually in a flat space time (reducing to Special Relativity) there is a problem with the assumed metric for a non-zero Lambda.

What problem?

Also, other problems in the structure of the secondary relations. The Standard Model does not address this correctly in a perfectly flat scenario. It wasn't designed to -- it assumes it is warped. This is likely the cause for the apparent 'hump' that has been interpreted as 'dark energy'.

I can't make heads or tails of what you're trying to say here.

 

[Rymer]

Gravity most certainly didn't change at the surface of last scattering. That was only around 3000K, after all, which is quite a bit cooler than our own sun. And our current knowledge of gravity works quite well for the behavior of stars, as well as in the lab when we perform tests at these and much higher temperatures.

What problem?

I can't make heads or tails of what you're trying to say here.

IF gravity is due to a 'transport particle', then presumably this particle was being scattered as well. It is the cessation of scattering that could cause the loss of repulsion we see as gravity. So in this possible concept, gravity as we now know it only came into existence AFTER the time of last scattering. This is all related to the idea of a 'transport particle'. Agreed, not proved -- but also not falsified. Temperature has nothing to do with this (any relation is an end result not a causative).

Question for you:

Is a General Relativity Lambda term needed in any solution other than a cosmological one?

Also note in a flat (non-warped) universe, the normalization used is likely incorrect. By that I mean, matter will NOT be expanding away from the expansion origin at the speed of light (which is effectively what is assumed with the FLRW metric -- when flat). Expansion is 'two-tiered' -- photons at the speed of light and matter at some velocity less (this is actually what is meant by the surface of last scattering). ALSO, there is no density term involved (no critical density is possible). AND no time dependent scaling factor needed. Likely more -- this is what comes to mind at the moment.

 

[vin300]

In a sense, expansion of space 'speeds up' photons by stretching them out [aka redshift]. .

No. The stretched photons have lesser frequencies

 

[Chalnoth]

IF gravity is due to a 'transport particle', then presumably this particle was being scattered as well.

The surface of last scattering is an electromagnetic phenomenon. There is no reason whatsoever to believe that it would have similar behavior upon other forces. In fact, there are very strong reasons to believe this is not the case.

The biggest reason, in this case, is that our models of structure formation make use of gravitational dynamics before the surface of last scattering. And they manage to predict very accurately what the surface of last scattering looks like. This interpretation is confirmed by comparing the distribution of the CMB to the distribution of closer galaxies (through Baryon Acoustic Oscillation observations).

Another, more theoretical reason is that the unification scale of gravity with the other forces is expected to be at the Planck scale, which is about 10^19 GeV, or very roughly 10^33K, around 10^30 times higher in temperature than the surface of last scattering. There is some possibility that this might be brought down all the way to the TeV scale (about 10^26K), but experiment completely rules out anything below this.

Question for you:

Is a General Relativity Lambda term needed in any solution other than a cosmological one?

If it turns out that the cosmological constant is the explanation for the observed acceleration, then its value is far, far too small to be detectable in any other context at this time.

Also note in a flat (non-warped) universe, the normalization used is likely incorrect. By that I mean, matter will NOT be expanding away from the expansion origin at the speed of light (which is effectively what is assumed with the FLRW metric -- when flat). Expansion is 'two-tiered' -- photons at the speed of light and matter at some velocity less (this is actually what is meant by the surface of last scattering). ALSO, there is no density term involved (no critical density is possible). AND no time dependent scaling factor needed. Likely more -- this is what comes to mind at the moment.

Once again you're making no sense. This paragraph smacks of a fundamental lack of understanding of general relativity as well as the FLRW metric and its implications.

 

[Chalnoth]

No. The stretched photons have lesser frequencies

Well, that too. But the effect Chronos described is a valid one, from a certain point of view.

First, to talk about this sort of thing, we have to define some sort of global reference frame. This choice is somewhat arbitrary, but a particularly nice choice is a reference frame in which the universe looks approximately homogeneous to all observers, and observers at different spots in the universe see the same average density at the same time.

One might think of this as defining this global reference frame by the cosmological constant: "now" is comprised of all observers who see the CMB at the temperature we see it today. As our universe expands, the CMB will cool, and then the global "now" will be comprised of all observers who see it at that temperature.

Now, if we define this global reference frame (which is arbitrary, but convenient), then we can talk about what a far-off object is doing right now. If we take a photon that I send off into space, for instance, we can say how much space it has traveled in a given time. In this global reference frame, the photon will always be traveling at speed c with respect to the local matter. But the universe is expanding, so its total velocity in this reference frame will be:

v = c + Hd

So we will see this photon as receding from us faster than the speed of light. It will be sort of "carried away" by the expansion.

Please bear in mind, as I said in my earlier post, that this is an artifact of the fact that we can define velocities however we choose when comparing objects that are separated by some distance. This choice is just a convenient one.

 

[vin300]

Let me ask you this:Where is the origin of this universal reference frame?
No answer, and hence no such thing.

No observer can measure the speed of light different from c (in vacuum, of couse)
You fail to prove that the speed of light varies because you cannot define this unique frame
All the real frames are based on an illusion of still space, that's why you say v=Hd in the first place

 

[Chalnoth]

Let me ask you this:Where is the origin of this universal reference frame?

The physics of the production of our region of the universe acted in such a way that for some observers, the universe appears nearly homogeneous and isotropic. We can take advantage of this fact to define a universal reference frame.

No answer, and hence no such thing.

Arbitrary does not equate to "no such thing".

No observer can measure the speed of light different from c (in vacuum, of couse)
You fail to prove that the speed of light varies because you cannot define this unique frame
All the real frames are based on an illusion of still space, that's why you say v=Hd in the first place

No observer can measure the speed of light locally to be different from c. But it is perfectly possible to look at light (or even non-relativistic objects) traveling between far-away points at speeds that appear to exceed that of light.

 

[vin300]

But it is perfectly possible to look at light (or even non-relativistic objects) traveling between far-away points at speeds that appear to exceed that of light.

That's because the velocity of a "nothing" between two photons reaching a point from two different points is >c

 

[Chalnoth]

That's because the velocity of a "nothing" between two photons reaching a point from two different points is >c

Well, that's one situation. We're talking about a different one, though, due to the expansion of the universe.

 

[Rymer]

The surface of last scattering is an electromagnetic phenomenon. There is no reason whatsoever to believe that it would have similar behavior upon other forces. In fact, there are very strong reasons to believe this is not the case.

The biggest reason, in this case, is that our models of structure formation make use of gravitational dynamics before the surface of last scattering. And they manage to predict very accurately what the surface of last scattering looks like. This interpretation is confirmed by comparing the distribution of the CMB to the distribution of closer galaxies (through Baryon Acoustic Oscillation observations).

Another, more theoretical reason is that the unification scale of gravity with the other forces is expected to be at the Planck scale, which is about 10^19 GeV, or very roughly 10^33K, around 10^30 times higher in temperature than the surface of last scattering. There is some possibility that this might be brought down all the way to the TeV scale (about 10^26K), but experiment completely rules out anything below this.If it turns out that the cosmological constant is the explanation for the observed acceleration, then its value is far, far too small to be detectable in any other context at this time.Once again you're making no sense. This paragraph smacks of a fundamental lack of understanding of general relativity as well as the FLRW metric and its implications.

Yes, the surface of last scattering is a electromagnetic phenomenon -- or better termed a 'photon' phenomenon -- and so may be the gravitational one.

I'm not referring to our models before and during the surface of last scattering -- have no disagreement with them as far as I know.

Your statement "unification scale of gravity with the other forces is expected to be at the Planck scale" is simply an opinion -- not justified.

I see we agree on the cosmological constant.

The problem is with the FLRW metric and its application to cosmology -- agreed it makes no sense.

 

[Chalnoth]

Yes, the surface of last scattering is a electromagnetic phenomenon -- or better termed a 'photon' phenomenon -- and so may be the gravitational one.

Nope. Gravity is a different force.

Your statement "unification scale of gravity with the other forces is expected to be at the Planck scale" is simply an opinion -- not justified.

What is not opinion, however, is that the unification scale is no lower than the TeV scale. If it were, we would have produced microscopic black holes in particle accelerators by now. We haven't, so it isn't.

However, the theories where the unification scale is as low as the TeV scale are extremely speculative, requiring the "true" strength of gravity to be many orders of magnitude larger than what we measure, because it is being diluted into extra dimensions. If the true strength of gravity is given by the G we measure, then the unification scale basically has to be somewhere near the Planck scale.

The problem is with the FLWR metric and its application to cosmology -- agreed it makes no sense.

No, the problem is with your understanding of it. There are no self consistency issues with the FLRW metric.

 

[rasp]

"The simple expansion of distances that you get in General Relativity is not like that. Distances between all the widely separated galaxies are increasing without any galaxy going anywhere. Think of it as changing geometry, not motion. GR is a dynamic geometry theory, distances can change. Indeed a lot of distances are changing at a faster rate than c----and this is not breaking any speed rule. It is not ordinary motion of the sort governed by special rel."

Marcus, thanks. I can understand that the galaxies are not moving per se at velocities greater than c, but only in relation to each other as "the "backdrop of dynamic geometry" expands. Can we call this backdrop the balloon? Well, then what of the balloon itself? Is it moving? If so, what is moving? Is the space of the balloon a membrane of sorts, carrying within it's structure the objects of galaxies, photons and such? Or are you equating the balloon to empty space; a pure vacuum which only separates and defines distance between objects?
I know you are sparing me the technical details (and I appreciate that!) but to call the expansion of the Universe "a changing geometry not motion" seems like we are substituting the mathematical model for the reality. Thanks!

 

[Rymer]

Nope. Gravity is a different force.

Your opinion.

What is not opinion, however, is that the unification scale is no lower than the TeV scale. If it were, we would have produced microscopic black holes in particle accelerators by now. We haven't, so it isn't.

Nothing to do with the subject.

However, the theories where the unification scale is as low as the TeV scale are extremely speculative, requiring the "true" strength of gravity to be many orders of magnitude larger than what we measure, because it is being diluted into extra dimensions. If the true strength of gravity is given by the G we measure, then the unification scale basically has to be somewhere near the Planck scale.

Again nothing to do with the subject.

No, the problem is with your understanding of it. There are no self consistency issues with the FLRW metric.

Again, the point was FLRW does NOT apply -- so any 'self consistency' issues are moot.

 

[Chalnoth]

Rymer, you've got some very strange, and very wrong ideas. If you want anybody to take you seriously, you're going to have to follow this checklist:
http://blogs.discovermagazine.com/c...alternative-science-respectability-checklist/

Because so far, you've failed miserably.

 

[vin300]

We can take advantage of this fact to define a universal reference frame.

Define it then.

No observer can measure the speed of light locally to be different from c.

What do you mean by "the speed of light in vacuum is a universal constant" ?

 

[Chalnoth]

Define it then.

Defined here:
https://www.physicsforums.com/showpost.php?p=2324712&postcount=16

What do you mean by "the speed of light in vacuum is a universal constant" ?

I don't know how you think this answers what I said. In General Relativity, differences in speed are only well-defined at a single point. Therefore, GR only says that people will measure the speed of light to be c if they measure a photon moving past them. It doesn't say they'll measure far-away photons to be moving at c.

 

[rasp]

Marcus wrote: "The simple expansion of distances that you get in General Relativity is not like that. Distances between all the widely separated galaxies are increasing without any galaxy going anywhere. Think of it as changing geometry, not motion. GR is a dynamic geometry theory, distances can change. Indeed a lot of distances are changing at a faster rate than c----and this is not breaking any speed rule. It is not ordinary motion of the sort governed by special rel."

This thread has evolved far beyond me, can I interject back to my original question? I understand that the galaxies are not moving per se at velocities greater than c, but are only in relation to each other as "the "backdrop of dynamic geometry" expands. Can we call this backdrop the balloon?
Well, then what of the balloon itself? Is it moving? If so, what is moving? Is the space of the balloon a membrane of sorts, carrying within it's structure the objects of galaxies, photons and such? Or are you equating the balloon to empty space; a pure vacuum which only separates and defines distance between objects?
I know you are sparing me the technical details (and I appreciate that!) but to call the expansion of the Universe "a changing geometry not motion" seems like we are substituting the mathematical model for the reality. Thanks!

 

[vin300]

Defined here:
https://www.physicsforums.com/showpost.php?p=2324712&postcount=16

I don't see it there. I already pinpointed the mistake.Maybe you didn't understand. If you refer distances to the expanding space, you will have a constant v as the distance is the same.That's not an accelerating universe.But it is. So v=Hd is wrong.But it isn't. So where is the mistake?In that you say the both simultaneously.

I don't know how you think this answers what I said. In General Relativity, differences in speed are only well-defined at a single point. Therefore, GR only says that people will measure the speed of light to be c if they measure a photon moving past them. It doesn't say they'll measure far-away photons to be moving at c.

Oh is it? Why is th light cone the way it is and not distorted? We have to redifine spacetime interval, space like events and time like events

 

[Chalnoth]

Well, then what of the balloon itself? Is it moving?

Stretching would be a more accurate term.

If so, what is moving?

Space-time is expanding. In General Relativity, space-time is seen as an entity in and of itself, with its shape determined by the matter that inhabits it. The existence of matter in the universe induces curvature in space-time, which, when we look at just the space components, looks like space is stretching or expanding.

 

[Chalnoth]

I don't see it there. I already pinpointed the mistake.Maybe you didn't understand. If you refer distances to the expanding space, you will have a constant v as the distance is the same.That's not an accelerating universe.But it is. So v=Hd is wrong.But it isn't. So where is the mistake?In that you say the both simultaneously.

v = Hd is correct. It's just that H changes with time. At any given time, the recession velocity between two points, with "at the same time" laid out by the two points seeing the same temperature CMB, will be v = Hd, where d is the distance between those points. This picture works whether you're talking about an accelerated expansion or not. The only part that is relevant is that the universe is, on large enough scales, homogeneous and isotropic to some observers.

Oh is it? Why is th light cone the way it is and not distorted? We have to redifine spacetime interval, space like events and time like events

Light cones are very distorted by the expansion of space. As for redefining the space-time interval, I hope you are aware that as compared to special relativity, general relativity does do this, as it takes into account the curvature of space-time.

 

[rasp]

Stretching would be a more accurate term.


Space-time is expanding. In General Relativity, space-time is seen as an entity in and of itself, with its shape determined by the matter that inhabits it. The existence of matter in the universe induces curvature in space-time, which, when we look at just the space components, looks like space is stretching or expanding.

Space has what type of characteristics that stretches or expands with time? How are you defining space, that it might stretch as if it were actually the same as its balloon analogy?

 

[Chalnoth]

Space has what type of characteristics that stretches or expands with time? How are you defining space, that it might stretch as if it were actually the same as its balloon analogy?

Well, technically, space-time has curvature that looks to observers like ourselves as if space is expanding.

 

[rasp]

Well, technically, space-time has curvature that looks to observers like ourselves as if space is expanding.

Space has no characteristic that I am aware of. It doesn't "look" like anything to observers. It has no primary meaning but is defined only secondarily as distance in relationship to the objects it separates.

 

[marcus]

Space has no characteristic that I am aware of. It doesn't "look" like anything to observers. It has no primary meaning but is defined only secondarily as distance in relationship to the objects it separates.

I agree. At least in cosmology we can assume that space has no physical material existence, it is not a substance like rubber.
The balloon analogy is not a mechanical model but a device to help people visualize. Think only of the surface of the balloon---all existence concentrated on the 2D surface. No rubber.

Space has what type of characteristics that stretches or expands with time? How are you defining space, that it might stretch as if it were actually the same as its balloon analogy?

You answered your own question. There is no physically existing space. There is, however, geometry. Once can make measurements. One can measure the angles of a triangle. there is an idea of a straight line. Of distance. As you indicated, geometry is primary. Space is "only secondary", as you said.

Can we call this backdrop the balloon? Well, then what of the balloon itself? Is it moving? If so, what is moving?...

A common perspective on the universe is to consider the backdrop to be the CMB---the microwave background. Light left over from ancient matter which was approximately uniformly distributed (before it began to curdle and clump) and of an approximately uniform temperature.

One can tell one is moving relative Background if there is a doppler hotspot ahead of one, and one can measure the speed by the dopplershift. One has an idea of rest with respect to Background.

It's convenient. They use it to analyze data. It is convenient to think of the background as not moving. then each of the galaxies has some proper motion relative background which can in many cases be estimated. These proper motions are normally rather slow. Not part of the expansion pattern.

This thread has evolved far beyond me, can I interject back to my original question? I understand that the galaxies are not moving per se at velocities greater than c, but are only in relation to each other as "the "backdrop of dynamic geometry" expands. Can we call this backdrop the balloon?
Well, then what of the balloon itself? Is it moving? If so, what is moving?...

It is convenient to think of the backdrop as not moving. It is a handy anchor point, a reference for all other motions. I would prefer not to call it "the balloon" because the balloon is just a visualization aid.

I guess a communication problem might be that I believe that geometry exists (not as a substance but) as a pragmatic reality. And you might not believe this. It would make it hard to communicate if that is true. Let me explain how I think that geometry exists.

GR equates the gravitational field to the geometry. The gravitational field is nothing else but the geometry. Geometry is as real as gravity. And it as real as the pragmatic fact that I can measure angles and distances. Moreover geometry is not naturally Euclidean. It evolves. At any given place we have no right to expect it to be static or flat. GR explains why in certain circumstances it will be approximately flat. Why in others it will be expanding or contracting according to some pattern.

Some other branch of physics may explain why the gravitational field (i.e. geometry) exists. Here we just take as given that it exists and that it evolves according to the main GR equation.

So distances between stationary observers (at rest w.r.t. background) increase according to some pattern (Hubble law) and this is not surprising.

And space is not a substance, neither is geometry a substance.

You might enjoy reading Rovelli's parable of the whale. I will see if I can find a link.
This links gets an early draft the whole book,
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Look at section 1.1.3 "The physical meaning of general relativity," page 7.

And also more elaboration, with quotes from Einstein and others, in section 2.3.2 "The disappearance of spacetime". On pages 52 and 53.