[does space expand or are things just moving?]

[Jens]

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

[pzkpfw]

Sorry Jens, but while related, your question is different enough (and quite fundamental in its own right) to require its own thread; lest it become a hijack of the original posters own question. Thread was here: http://www.bautforum.com/showthread....ions-expansion. You can suggest a new title for this thread.

(To be clear: there's no "punishment" or warning here, I've just split the thread.)

[astromark]

The manor and action of the expansion observed does not indicate a uniform expansion rate across the whole universe.
We see that the more distant objects are receding at a greater velocity.
The 'Model' conclusion is of a eccelorating expansion at a greater pace more distant from us.
Building a logic of space expansion not movement through space.
To move away from this model would create a issue of mater movement exceeding c. That is not possible,
so the expanding space idea was born. Interesting questions...

[CaptainToonces]

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

That is the definition of space expanding itself-- objects flying away from one another

[Jens]

That is the definition of space expanding itself-- objects flying away from one another

So when I throw a ball into the air, the space expands between us and then it shrinks again when the ball comes back down?

[astromark]

So when I throw a ball into the air, the space expands between us and then it shrinks again when the ball comes back down?

NO. That is the dark force we call 'Dark Energy'.
or in your case 'Jens' it is the resultant action of a force driven projectile
ie the ball. being forced by velocity imparted to attempt escape velocity
and subsequent failing's and return to Terra fermium..
well sort of and almost... Dark Energy is that what is driving a acceleration we do not understand.

[Cougar]

...how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

"...the academic argument surrounding the expansion of space is not as clear as standard explanations suggest; an interested student and reader of New Scientist may have seen Martin Rees & Steven Weinberg (1993) state:

...how is it possible for space, which is utterly empty, to expand? How can nothing expand? The answer is: space does not expand. Cosmologists sometimes talk about expanding space, but they should know better.

while being told by Harrison (2000) that:

expansion redshifts are produced by the expansion of space between bodies that are stationary in space.

What is a lay-person or proto-cosmologist to make of this apparently contradictory situation?"

So your quandary is understandable. A reading of this article may help: Expanding Space: the Root of all Evil?

[Phylomon]

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

I'm a newby here and this is my first post, but my understanding of the argument goes like this:

- according to the red-shift data, objects that we can detect at a significant distance outside our galaxy are receding from us
- the further away the object is, the faster it is going
- this data appears to suggest that we are at the center of the expansion (a very unique location)
- since the Copernican Principle says that we are not at any unique location, we have to assume that red-shift measurements taken elsewhere would give the same answer
- since ALL objects would appear to be moving away from all others, it must be space itself that is expanding and, therefore, the red-shift data does not support the idea that the motion is THROUGH space. If the detected motion were THROUGH space, we would expect to see different rates of expansion in different directions (unless, of course, we really ARE at the center of the universe)

Not sure that all of that would pass peer review, but that's how I look at it.

Hope this helps.

[Ken G]

The problem here is that we are applying a rather naive treatment of space to make a much more profound issue seem like a dichotomy between two choices, that "space itself expands" or that objects move "through space." Neither of those statements are particularly good physics if taken literally as statements of fact, because both treat space as if it was something real, such that you could tell if it was expanding or if something was moving through it. But space is just a picture we use to help us motivate our own understanding-- the real physics is in the concept of spacetime, and how that concept interfaces with the notions of general relativity. In particular, the cosmos are a strongly gravitational environment on the largest scales, so language that ignores gravity is never going to be applicable-- that's the problem with the "moving away from us" language. Naive notions of space and time are just local approximations, like the way we use "north-south-east-west" directions locally, even though we know they do not have a global meaning (what direction is "east" at the north pole?). So when we say things are "moving through space", we are taking our local meaning of motion through space, and pretending that it extends to some kind of global description of the universe-- it does not, that picture will always break down when applied globally, it's just plain wrong.

Less wrong is the claim that space itself expands-- at least that is actually a workable picture of what is happening. But it is only a picture, and it only has meaning in a particularly common and convenient coordinate system, called "comoving frame coordinates." Those are the coordinates that move locally with the average motion of the matter, and are seamlessly united globally only by constantly referring back to the average motion of the local matter. In those coordinates, space itself appears to expand, but that is a result of the coordinates, not a result of space itself-- expressly because our current physics does not support any notion of "space itself." GR only has a spacetime manifold, no "space itself." Of course, that may not always be true, this is merely our current understanding. So I would say, it's fine to say that space itself is expanding, if we also make clear that this is just a picture we are invoking because we have pictorial minds-- it is not a claim on reality independently of our penchant for making pictures and choosing convenient coordinates to motivate our language.

[Phylomon]

Ken,

I'm trying to understand your answer. I get what you mean by saying that "space itself expands" is bad physics. "Space" doesn't exist. Is it better to say the the distances between objects on a large scale are growing larger and the density of the known universe is decreasing, thus leading to the cosmic red-shift? Are you saying that this increase in distance is not "motion" and therefore the distant galaxies are not "moving away from us" as such?

How does your response fit with the concept of inflation? The CMBR is uniform in all directions because inflation occured so quickly, yes? It apparently is not possible for the CMBR to be uniform without the concept of inflation because of the limitation of the speed of light. It seems that we are drawing a distinction of some sort between (here's that phrase again) (a) motion (motion THROUGH space), which is limited to the speed of light, and (b) some increase of distance between objects which is NOT limited by the speed of light.

Would the following statements be correct?
- Distance between two objects sometimes increases because of the relative motion of the two objects
- Distance beween two objects sometimes increases because of the expansion of the universe even though the objects are at rest with respect to each other and no "motion" is involved

Thanks for any light you can shed on this for me.

[Ken G]

I get what you mean by saying that "space itself expands" is bad physics. "Space" doesn't exist. Is it better to say the the distances between objects on a large scale are growing larger and the density of the known universe is decreasing, thus leading to the cosmic red-shift?

Yes, that is better to say, although I would not connect it as the cause of the redshift-- I would say that both the expansion and the redshift come from the same source (the laws of gravity on the largest scales, and the history of the universe as set up in some kind of initial condition). It is awkward to say the expansion causes the redshift, because it isn't clear that expansion is not a function of the coordinates chosen, but it is clear that redshift is not.

Are you saying that this increase in distance is not "motion" and therefore the distant galaxies are not "moving away from us" as such?

Yes, it's certainly not the classical view of motion. Part of the problem is that "distance" is itself not a well defined concept-- there are many operational definitions of measurable distances, and they give different answers. Worse, the whole concept of spatial distance is dependent on the coordinates chosen, because coordinate systems dissect spacetime into spatial and temporal parts, whereas the laws of relativity apply to the entire spacetime manifold at once. The bottom line is, it is important to distinguish what we know from our observations, from what language we choose to talk about what we know, because the latter is often an arbitrary artifact of some particularly convenient coordinate choice. A simple example of that would be the statement that gravity makes things fall "downward", which is really not a statement about gravity it is a statement about how we like to define the concept of "down" to conform to whatever gravity is doing.

How does your response fit with the concept of inflation? The CMBR is uniform in all directions because inflation occured so quickly, yes? It apparently is not possible for the CMBR to be uniform without the concept of inflation because of the limitation of the speed of light. It seems that we are drawing a distinction of some sort between (here's that phrase again) (a) motion (motion THROUGH space), which is limited to the speed of light, and (b) some increase of distance between objects which is NOT limited by the speed of light.

Inflation works within the concept of evolution of the gravitational environment of general relativity, so it requires no concept of motion "through" anything. In that sense, it is similar to the prevailing expansion we are observing now. The idea that motion is limited to the speed of light applies only to the purely local concept of motion, which is the passing of two objects at the same location with some relative speed-- that's also the only precise concept of motion, motion is really only a local relationship-- not a global one. That may be the best way to say all this-- questions about whether or not the universal expansion is really a kind of motion are simply mixing a precise and local concept of motion, with an imprecise, coordinate-dependent, global concept of motion. Ideally, we'd use two different words there!

Would the following statements be correct?
- Distance between two objects sometimes increases because of the relative motion of the two objects

That is precisely correct only when the distance is increasing from zero, or at least small enough to ignore influences by the universal gravity (or the gravity near black holes and so on). When gravity gets involved in a fundamental way, it's not so clear what "relative motion" means when objects are widely separated.

- Distance beween two objects sometimes increases because of the expansion of the universe even though the objects are at rest with respect to each other and no "motion" is involved

I would say distance never increases "because of" the expansion, rather we say we have expansion because distances are increasing. The reason distances are increasing has to do with how general relativity interacts with the history of the universe, and we look at that interaction and hang the term "expansion" on it, but that term does not cause anything, all the results come together in the same basket. As for whether or not we can say two objects are relatively "at rest", that is only precise if gravity is not involved, or if they are in the same place, because then if we say two objects are mutually at rest across a distance if they can exchange unredshifted signals, we don't know if that's because they are really at rest, or if it is something that gravity is doing. At that point, it becomes a matter of convention what we mean by "at rest", not physics. You can tell what is physics because it comes in the form of an invariant measurable, meaning that it is a testable outcome of an experiment where both the experiment and its reference frame are specified.

[chornedsnorkack]

For example, look at a relativistic jet from a distant quasar.

It is moving at a high speed relative to the source accretion disc and host galaxy.

The host galaxy is receding at a high redshift.

The jet happens to be moving towards us, at such exact speed that its redshift relative to us is zero.

Does the "expansion of space" between us and the jet affect our view of the jet in any way?

[CaptainToonces]

So when I throw a ball into the air, the space expands between us and then it shrinks again when the ball comes back down?

yep. although in your example most people would use the term distance rather than space, probably.

[speedfreek]

For example, look at a relativistic jet from a distant quasar.

It is moving at a high speed relative to the source accretion disc and host galaxy.

The host galaxy is receding at a high redshift.

The jet happens to be moving towards us, at such exact speed that its redshift relative to us is zero.

Does the "expansion of space" between us and the jet affect our view of the jet in any way?

Our view of the jet would be subject to cosmological time dilation.

[tommac]

What is the difference between those two statements?

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

[Bob Angstrom]

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

It is possible that universal expansion is a matter of objects flying away from us in all directions while space remains unchanged but this would involve some unlikely coincidences and conditions. The universe described as "objects flying away" should have an observable center of expansion and an observable outer edge . From our Earthly point of view, the universe appears to be uniform and of equal distance in all directions with no observable edge so we appear to be at the exact center of expansion. This would give the Earth a special but improbable location as the center of a universe where most 'observers' on other galaxies should find themselves closer to an edge than the center. It is an old but now uncomfortable thought that we are the center of a universe made just for us so the possibility you are asking about has been dismissed as too coincidental. Riemann geometry with its four dimensions of curved spacetime makes the observation of being at the exact center of the universe common to all observers no matter where they may be located and it also gives us a clever explanation of gravity. So Riemann geometry has replaced the old Euclidean geometry where 'flying away' is possible. Riemann geometry doesn't permit objects to “fly away” into an infinite space beyond our own because all 'straight line' motions are confined to large curved geodesics where even light can only circle and not escape.

[transreality]

The objects aren't just flying away, they are flying faster the further they are away.

[Luckmeister]

The objects aren't just flying away, they are flying faster the further they are away.

With expansion of space, distance is increasing between objects. One object is not "flying away" from the other. That's an important point Ken G made.

In particular, the cosmos are a strongly gravitational environment on the largest scales, so language that ignores gravity is never going to be applicable-- that's the problem with the "moving away from us" language. Naive notions of space and time are just local approximations, like the way we use "north-south-east-west" directions locally, even though we know they do not have a global meaning (what direction is "east" at the north pole?). So when we say things are "moving through space", we are taking our local meaning of motion through space, and pretending that it extends to some kind of global description of the universe-- it does not, that picture will always break down when applied globally, it's just plain wrong.

[Shaula]

What is the difference between those two statements?

See most of the rest of the posts in this thread...

[WayneFrancis]

Sorry, I'm just curious myself, and maybe this will help the OP as well, but how do we know the expansion is of space itself, and not simply the objects flying away from one another in non-expanding space?

Hi Jens,
If SR is right we can say that the objects aren't just moving because we measure red shift that would require some of them to be travelling through space at a rate > c. That is a simple proof for me until some other data shows SR to be faulty.

[Jeff Root]

With expansion of space, distance is increasing between objects.
One object is not "flying away" from the other. That's an important
point Ken G made.

Alice: "What is expansion of space?"
Tweedledee: "That means objects are flying away from each other."
Tweedledum: "No it doesn't. It means the distance between objects
is increasing."
Alice: "But if the distance between objects is increasing, they must
be flying away from each other."
Tweedledee: "No, that isn't right either. If objects are flying away
from each other, it means the distance between them is increasing."
Tweedledum: "But objects aren't flying away from each other."
Tweedledee: "Right. So distances between objects aren't increasing."
Tweedledum: "Therefore, space isn't expanding."
Alice: "Thank you. I think."

-- Jeff, in Mirrorapolis

[Jens]

Hi Jens,
If SR is right we can say that the objects aren't just moving because we measure red shift that would require some of them to be travelling through space at a rate > c. That is a simple proof for me until some other data shows SR to be faulty.

That's true as long as the redshift is only caused by expansion. So perhaps the last sentence would be until other data shows SR to be faulty or the cosmological redshift can be shown to be caused by something other than expansion?

[WayneFrancis]

That's true as long as the redshift is only caused by expansion. So perhaps the last sentence would be until other data shows SR to be faulty or the cosmological redshift can be shown to be caused by something other than expansion?

Just thinking through this as I type so bear with me. We believe there are 3 types of redshift. Cosmological, gravitational, and Doppler. If the "Cosmological" redshift is really do to one of the others then...what would we expect?

Doppler seems to be written off right away due to the high z value objects we see.
gravitational ... I can't see how light from distant galaxies could be red shifted in a manner that could be gravitational and us not be in some "unique" part of the universe. It doesn't even follow the profile of a gravitational redshifting when we look at the sky as a whole.
Could it be a combination of the 2? I can't come up with a way to get the maths to work with observed data.

Tommac and I have had the discussion before about cosmological red shift being really from a GR effect and no matter what scale he picked the profile didn't match a GR Redshift. At least nothing I could come up with even trying his ideas out on paper.

[astromark]

Very pleased and disturbed by Jeff's contribution... " Jeff in Mirrorapolis " I wonder... ?

Observed and tested as real is a acceleration seen of the expansion of the whole of the known Universe...
Now you want for a argument of what is expanding... Is it the space between things... or all of it regardless.
Back in post 3, I said that other forces were why the gravity bound or chemically bonded or nuclear locked were not seen to be rushing off... they are not... Does space expand. Only say yes if you understand that space is nothing but time., and it is expanding. Hidden in that understanding is the answer.
Mark in Purgatory.

[Strange]

Alice: "What is expansion of space?"
Tweedledee: "That means objects are flying away from each other."
Tweedledum: "No it doesn't. It means the distance between objects
is increasing."

Surely the point is that if two objects are flying away from one another, then they will be flying towards other objects. Unless everything is flying away from some central point. If that were true then we would see some assymetry in the universe. Which we don't, which would suggest we are near the center of the universe.

So, either everything is moving away from us or everything is moving apart with no center (aka "space is expanding").

[Jeff Root]

We believe there are 3 types of redshift. Cosmological,
gravitational, and Doppler. If the "Cosmological" redshift is
really due to one of the others then...what would we expect?

Doppler seems to be written off right away due to the high z
value objects we see. Gravitational ... I can't see how light
from distant galaxies could be red shifted in a manner that
could be gravitational and us not be in some "unique" part
of the universe. It doesn't even follow the profile of a
gravitational redshifting when we look at the sky as a whole.
Could it be a combination of the 2? I can't come up with a
way to get the maths to work with observed data.

I have long understood (very poorly) that the cosmological
redshift is a combination of Doppler and gravitational. Many
years ago I heard or read somewhere that only a small part --
like maybe 2% -- is due to gravitational redshift. It seems
to me that the gravitational redshift would be due to the
gravitational potential difference in time, not in space, as the
Universe becomes less dense. In that case, I'd expect that
the closer to the Big Bang the light was emitted, the larger
the fraction of redshift due to gravity.

-- Jeff, in Minneapolis

[Jeff Root]

Surely the point is that if two objects are flying away from one
another, then they will be flying towards other objects.

If that is the point, then I disagree with the point.

Unless everything is flying away from some central point. If that
were true then we would see some asymmetry in the universe.
Which we don't, which would suggest we are near the center of
the universe.

I have three alternative arguments, and I'll use all three.

First, I don't agree that everything would have to be flying away
from a central point in order for everything to be flying away
from everything else. That just isn't necessary.

Second, even if everything were flying away from a central
point, we would not necessarily see any asymmetry. I have
two alternative reasons for that, too. First, the expansion could
be uniform even if it is directional. Second, the Universe is so
fantabulously humongous that we apparently see only a very
tiny fraction of it. So even if the expansion is not uniform, the
little bit we can see could be so close to uniform that we can't
detect any difference.

Third, there's no reason we couldn't be near the center if there
is one. We wouldn't have to be right at the exact center.

-- Jeff, in Minneapolis

[Strange]

First, I don't agree that everything would have to be flying away
from a central point in order for everything to be flying away
from everything else. That just isn't necessary.

OK. But isn't that the "space is expanding" case? I guess there is a slight difference depending whether you think there is (infinite?) empty space that things are flying away into, or whether the universe is also expanding as things fly away from one another. As I understand it, only the latter makes sense in the context of GR. (But ... depth, out of. )

Second, even if everything were flying away from a central
point, we would not necessarily see any asymmetry. I have
two alternative reasons for that, too. First, the expansion could
be uniform even if it is directional.

That doesn't work. If we were not near the center the actual expansion would have to be highly asymmetrical in order to appear symmetrical.

Second, the Universe is so
fantabulously humongous that we apparently see only a very
tiny fraction of it. So even if the expansion is not uniform, the
little bit we can see could be so close to uniform that we can't
detect any difference.

I don't think that works either. However large, we would still see a dominant motion in one direction (unless we are at the center).

Third, there's no reason we couldn't be near the center if there
is one. We wouldn't have to be right at the exact center.

That is possible. It just seems very, very unlikely.

[Jeff Root]

First, I don't agree that everything would have to be flying away
from a central point in order for everything to be flying away
from everything else. That just isn't necessary.

OK. But isn't that the "space is expanding" case? I guess there is
a slight difference depending whether you think there is (infinite?)
empty space that things are flying away into, or whether the
universe is also expanding as things fly away from one another.
As I understand it, only the latter makes sense in the context of
GR. (But ... depth, out of. )

I don't know what the "space is expanding" case is. I just want
to explain the observed cosmic redshift as increasing separation
between widely-separated clusters of galaxies.

Second, even if everything were flying away from a central
point, we would not necessarily see any asymmetry. I have
two alternative reasons for that, too. First, the expansion could
be uniform even if it is directional.

That doesn't work. If we were not near the center the actual
expansion would have to be highly asymmetrical in order to
appear symmetrical.

I doubt that. Can you describe the asymmetry and explain
why it would need to be that way?

Everyone in this uniformly-expanding 2-D Universe who can't
see the edge would see the same thing:

http://www.freemars.org/jeff2/expand5a.htm

Second, the Universe is so
fantabulously humongous that we apparently see only a very
tiny fraction of it. So even if the expansion is not uniform, the
little bit we can see could be so close to uniform that we can't
detect any difference.

I don't think that works either. However large, we would still see
a dominant motion in one direction (unless we are at the center).

Why? What direction?

Third, there's no reason we couldn't be near the center if there
is one. We wouldn't have to be right at the exact center.

That is possible. It just seems very, very unlikely.

Its probability is inversly proportional to the asymmetry and
directly proportional to the size of the Universe. The stronger
the asymmetry, the nearer we'd need to be to the center. The
larger the Universe, the farther we could be from the center.

-- Jeff, in Minneapolis

[Staticman]

Alice: "What is expansion of space?"
Tweedledee: "That means objects are flying away from each other."
Tweedledum: "No it doesn't. It means the distance between objects
is increasing."
Alice: "But if the distance between objects is increasing, they must
be flying away from each other."
Tweedledee: "No, that isn't right either. If objects are flying away
from each other, it means the distance between them is increasing."
Tweedledum: "But objects aren't flying away from each other."
Tweedledee: "Right. So distances between objects aren't increasing."
Tweedledum: "Therefore, space isn't expanding."
Alice: "Thank you. I think."
-- Jeff, in Mirrorapolis

Ha Ha,thanks. Really brilliant and hilarious too. It accurately describes the absurdity that reigns in part of modern cosmology.