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AriAstronomer
2010-Jul-12, 10:23 AM
A number of questions have arisen for me based on the large debates that have recently been going on on these topics.

1) Ken G talks about how there is no actual "space" itself, and we just assign co-ordinates to nothing in order to try and make sense of a complicated universe. Then what is expanding? Our hypothetical co-ordinate system? Something must be expanding, carrying along the massive objects, or else we would get FTL problems.

2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?

3) I'm not arguing with the fact, but simply commenting that it's interesting that objects can be receding from each other at a greater speed than c via expanding space without any problem whatsoever, and yet traveling FTL has gigantic consequences (going back in time, blah blah), even though to an untrained eye (e.g. A Gym Teacher!) they could be mistaken for the same thing.

Somehow the object 'knows' what is contributing to its apparent recession from other objects, and what is responsible for the changing EM fields, Gravitational wells, etc. Although, on second thought, since the expansion of the universe is really only noticeable on a global scale, local changes wouldn't take place....

cosmocrazy
2010-Jul-12, 10:43 AM
Often when talking about expanding space people imagine some sort of physical fabric that is stretching with time. This is not the case, space has no physical attributes that we are aware of and doesn't actual stretch/expanding in the classical sense we imagine, basically more "space" is added / created in between. As we well know in GR mass cannot be accelerated to and beyond C but this mass is not being accelerated in anyway, the objects you mention that are speeding away from each other at FTL speeds only appear to do so due to the addition of space. Again they don't actually "speed" apart in the classical way we imagine but in fact appear to do so due to the ever increasing space added / created in between. This is why the appearance of FTL motion between the objects is observed. How this works and what causes this apparent addition of space is not known, but some speculate about dark energy being a possible driving force of the expansion.

Cougar
2010-Jul-12, 01:01 PM
2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?
I'll let others comment on your other points, but as for this one, I think that's about right. Note the grid representation that allows one to visualize the 'nothing' of space is also closely tied to a 'coordinatization'.

Nereid
2010-Jul-12, 01:20 PM
I'm sure Ken G, and others more au fait with GR will jump in shortly ...

A number of questions have arisen for me based on the large debates that have recently been going on on these topics.

1) Ken G talks about how there is no actual "space" itself, and we just assign co-ordinates to nothing in order to try and make sense of a complicated universe. Then what is expanding? Our hypothetical co-ordinate system? Something must be expanding, carrying along the massive objects, or else we would get FTL problems.
When thinking about these sorts of things it is, I think, very important to keep firmly in mind what is observed (or observable) and what is an explanation of the observables.

What is observed is the Hubble redshift-distance relationship (there are actually at least two; they differ by the kind of 'distance' that each uses: the common one uses 'luminosity distance', a less well known one uses 'angular diameter distance'*).

We can interpret this relationship in terms of a theory - GR - applied to the universe as a whole.

The popular account of that explanation involves 'space', which is said to 'expand'.

But what GR actually says, in cosmological models, is a good deal more subtle and hard to grasp.


2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?
Here the explanation is based on the idea that you can use rulers everywhere, to measure everything's locations (and clocks too, but let's stick with rulers).

However, if you actually try to do this, in a well-designed thought experiment, you end up having to address all sorts of curious issues! For example, is it possible to have a rigid ruler? a massless one? how can light be used as a ruler?


3) I'm not arguing with the fact, but simply commenting that it's interesting that objects can be receding from each other at a greater speed than c via expanding space without any problem whatsoever, and yet traveling FTL has gigantic consequences (going back in time, blah blah), even though to an untrained eye (e.g. A Gym Teacher!) they could be mistaken for the same thing.
This is a very good example of where the popular explanations all too easily create confusion.

There are a couple of quite good articles by Lineweaver and Davis on this; I'll see if I can dig them up (unless someone beats me to it).


Somehow the object 'knows' what is contributing to its apparent recession from other objects, and what is responsible for the changing EM fields, Gravitational wells, etc. Although, on second thought, since the expansion of the universe is really only noticeable on a global scale, local changes wouldn't take place....
Stretching the analogies too far, IMHO.

If you stick to asking 'what would an observer, on Earth, see (assuming {GR-based cosmology model X})?' you are unlikely to get too confused; once you start to take a 'God's eye' view of the universe, you'll easily get lost (remember that such a perspective does not exist, in GR).

* there's also a very interesting one, that comes from observations of time delays in gravitational lenses; it is the only direct observation of 'distance' - the rest rely upon a multi-tiered ladder - and its interpretation is ... interesting

Ken G
2010-Jul-12, 02:49 PM
1) Ken G talks about how there is no actual "space" itself, and we just assign co-ordinates to nothing in order to try and make sense of a complicated universe. Your questions here cut so deeply to the heart of the theory of general relativity as it applies to cosmology that it is the kind of thing that even GR experts (those who do research on it, which I do not) are likely to give you somewhat or even completely different answers about! You'll have to sift through all the excellent answers you are getting here to chart your own course, but I can give you my take, with apologies to those whose points I am repeating.

Then what is expanding? Our hypothetical co-ordinate system?Yes, what is expanding is our hypothetical spatial coordinate system. However, sometimes one particular coordinate system seems so extremely "natural" that we get a sense there is some physical truth to it that transcends the coordinates, but we must keep an eye to the implicit assumptions that give those coordinates their special status (like how we associate "up" and "down" coordinates so intimately with the effects of gravity, yet when we look at the whole planet, we find that our concepts of "up" and "down" were only locally meaningful).

That happens in cosmology because of the "cosmological principle"-- the idea that if we choose for our coordinates the local time registered on hypothetical clocks moving with the average of the material around them (Earth clocks come close enough to this for most purposes), and interpret that time as "age of the universe" for that local region, then all the different local regions are pretty much the same everywhere but change with age. (Here by "local" I am still referring to very large scales, say many millions of light years). When you see, in some coordinates, a universe that is just aging all together like one thing, you are sorely tempted to see physical significance in those coordinates, and when you look at the space that appears in those coordinates, you find that large distances increase with age. That's what "the universe is expanding" means.

2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?In addition to being a matter of coordinatization, that depends on what you mean by "1 unit". In the "comoving-frame" coordinates I just mentioned, 1 unit of space (not space-time, remember we are choosing an explicit way to tease out of space from time) is the same as it has always been, if by "1 unit" you mean a unit as perceived by the local comoving observers. However, the "locations", if you will, of the galaxy clusters has not changed-- what has changed is the coordinate distance between them. So do you mean by 1 unit of space a coordinate distance, or some kind of coordinate independent measure of space? Not so easy, is it? That's where you will likely start to get into different opinions (even among experts, I suspect).

Thus although Cougar's answer sounded different from mine, he was simply taking the perspective that "1 unit" is a particular void between galaxies, shall we say, and we are coordinatizing the distance across it in a way that increases with age, in response to the fact that the various things we like to use as distance measures (there are quite a handful, maybe someone can link to Ned Wright's list) will be giving increasing results with increasing proper time of the local comoving observer (that connects with what Nereid said).


3) I'm not arguing with the fact, but simply commenting that it's interesting that objects can be receding from each other at a greater speed than c via expanding space without any problem whatsoever, and yet traveling FTL has gigantic consequences (going back in time, blah blah), even though to an untrained eye (e.g. A Gym Teacher!) they could be mistaken for the same thing.
All I can say is your comment is very perceptive. It might help you to recognize the following two things:
1) there is an extremely important difference between local physics, like the FTL and going back in time issues you get when one object can zoom past another, locally, faster than c, and the global coordinatizations we use to try and cobble together all that local physics into some unified whole "universe" of happenings, and
2) GR experts often do not think in terms of a space and time that are having a "trial separation", if you will, but instead think of a mathematical entity called the spacetime manifold. If you think that way, expansion is not something that unfolds with time, because time is already there, embedded into the manifold. Instead, it is some mathematical property of the manifold itself, a property which I would not even care to try and state with any mathematical rigor. Instead, I would just revert to my description above of what the term "expansion" means physically, within the context of the cosmological principle and the "most natural" global coordinates it suggests.


Somehow the object 'knows' what is contributing to its apparent recession from other objects, and what is responsible for the changing EM fields, Gravitational wells, etc. And ask yourself if the recession is reponsible for those changes, or if the changes are the reason we choose to interpret the existence of the recession. Is there really such a clear one-causes-the-other, or is this all just a complex whole we are trying to tease out any way we can get insight from?

Although, on second thought, since the expansion of the universe is really only noticeable on a global scale, local changes wouldn't take place....Exactly, that's part of the most important lesson at the heart of your perfectly natural confusion: the difference between local physics and global interpretation.

forrest noble
2010-Jul-13, 12:30 AM
AriAstronomer,


1) Ken G talks about how there is no actual "space" itself, and we just assign co-ordinates to nothing in order to try and make sense of a complicated universe. Then what is expanding? Our hypothetical co-ordinate system? Something must be expanding, carrying along the massive objects, or else we would get FTL problems.


In the Newtonian/ Descartes model of space, space could be defined as the volume which matter occupies as well as an extension of matter. As to GR as stated by Ken G, there are varying opinions concerning the nature of space.


2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?

According to the expanding space theory via the Big Bang, space within the entire universe is accordingly expanding in volume. Since according to GR space also has a physical fourth dimension (it closes on itself having no center), it is not possible to visualize the space of the universe as a whole but some visual analogs may be better than others such as expanding grid squares for the microcosm.



3) I'm not arguing with the fact, but simply commenting that it's interesting that objects can be receding from each other at a greater speed than c via expanding space without any problem whatsoever, and yet traveling FTL has gigantic consequences (going back in time, blah blah), even though to an untrained eye (e.g. A Gym Teacher!) they could be mistaken for the same thing.

Somehow the object 'knows' what is contributing to its apparent recession from other objects, and what is responsible for the changing EM fields, Gravitational wells, etc. Although, on second thought, since the expansion of the universe is really only noticeable on a global scale, local changes wouldn't take place....

The expanding space theory involves no uniform motion of galaxies. Relative motion of galaxies in this theory is generally random in all directions, therefore there accordingly would be no general recession of galaxies or FTL problems.


........hypothetical co-ordinate system
The Cartesian coordinate system is not theoretical or hypothetical as you may know. The system is a quintessential mathematical tool and analog of three dimensional reality. By adding the additional coordinate/dimension of time one can mathematically explain the relative position of something in space at a particular point in time which is an underpinning concerning our explanation and definition of space-time.

WayneFrancis
2010-Jul-13, 01:54 AM
A number of questions have arisen for me based on the large debates that have recently been going on on these topics.

1) Ken G talks about how there is no actual "space" itself, and we just assign co-ordinates to nothing in order to try and make sense of a complicated universe. Then what is expanding? Our hypothetical co-ordinate system? Something must be expanding, carrying along the massive objects, or else we would get FTL problems.


I don't think I've ever heard Ken G say that there is no actual space. What I hear him say, and agree with, is that the metric you choose within space is arbitrary any most often dictated by the "most convenient" one.



2) Spacetime is usually conveyed as a grid in every movie or documentary I've watched. In an expanding universe, are the grid squares simply getting larger, and '1 unit of space-time' is bigger today than it was yesterday?

No, if that was the case we wouldn't be able to detect it. Just like any other analogy, if you take it beyond the limits of the analogy then you'll get the wrong idea.



3) I'm not arguing with the fact, but simply commenting that it's interesting that objects can be receding from each other at a greater speed than c via expanding space without any problem whatsoever, and yet traveling FTL has gigantic consequences (going back in time, blah blah), even though to an untrained eye (e.g. A Gym Teacher!) they could be mistaken for the same thing.

No offence to Gym teachers but this is why a Gym Teacher shouldn't teach Physics. Just like said Gym teacher shouldn't remove a melanoma even though it doesn't look much harder then cutting a bit of crackling off a pork roast.



Somehow the object 'knows' what is contributing to its apparent recession from other objects, and what is responsible for the changing EM fields, Gravitational wells, etc. Although, on second thought, since the expansion of the universe is really only noticeable on a global scale, local changes wouldn't take place....

Again you are picturing it wrong. The beauty of expansion being a property of space is nothing else has to know or coordinate anything to produce what we observe. Just because an effect is to small for us to measure doesn't mean it doesn't happen.