# Expansion of space

1. Sep 9, 2011

### keepit

I'd like to understand accelerating expansion better. It seems that space is interjected proportionately where there is low density (intercalated is the term in biology). Can this interjection of space be sent back where it came from when that area of space becomes more densely populated?

2. Sep 11, 2011

### Drakkith

Staff Emeritus
To my knowledge the expansion of space is the same everywhere and the density of mass/matter has no effect on it.

3. Sep 11, 2011

### keepit

As i understand it, there is no expansion within galaxies where there is much mass.

4. Sep 11, 2011

### Drakkith

Staff Emeritus
That is incorrect. The force of gravity holds everything together against the expansion. It is only when you get to a scale of galaxy clusters that the effect of expansion starts to overcome gravity. Meaning that cluster A gravitationally attracts cluster B, however the enormous distances between the clusters means that the rate of expansion is starting to overpower gravity, resulting in cluster A and B NOT coming closer together, but getting further apart.

5. Sep 11, 2011

### Ken G

I'm not sure I'd put it that way, it sounds like a rather Newtonian description of gravity. I get the sense the OP is framed in the language of general relativity with a cosmological constant, in which case it's all gravity-- expansion is gravity, acceleration is gravity, bound orbits in galaxies is gravity. So the way I would put it is, gravity has many aspects, and the local density of matter controls which aspect is most important. On the galaxy scale and smaller, what matters is the gravity of the local masses, and you get bound orbits and no expansion of space (indeed, it is useful to imagine that space is contracting in such environments, though of course this really refers to a particular choice of spatial coordinates). On the scale of the universe as a whole, you have the cosmological principle, and that combines the gravity from the average density over the whole universe (most important for about the first half of the universe expansion) and the gravity from vacuum itself (say in a cosmological constant way, most important for the about the latter half of the expansion). Another crucial factor is the initial condition in time-- we have an expanding universe whenever we "start the clock" of our gravity analysis, and that is yet another aspect of the process we must take into account. So in effect we have an initial condition that is responsible for space being added (which was most important early on, say after inflation), we have dark energy that is responsible for space being added (which is more important recently), we have global mass density that is responsible for space being removed (important early), and we have local galaxies that are responsible for space being removed (important locally only).

6. Sep 11, 2011

### Drakkith

Staff Emeritus
Well, all I can say is that your explanation is one that has never been explained to me. I've always seen it explained the way I did.

7. Sep 18, 2011

### Ken G

The point is, we should choose a consistent ontology when we talk about what is happening. If we are going to use GR as our model of gravity to say what is happening on cosmological scales, it's quite awkward to use Newton's gravity to say what is going on in galaxies, which is what you are doing (and yes, it is often done I agree). It's not wrong, because GR works for cosmology and Newton works for galaxies, but it's kind of awful to mix the models and use language like "this is what is happening." I feel if one will do that, they should at least say "using a GR model for cosmological gravity, but Newton's model for galactic gravity"..., then everything would be clear (including the nature of the inconsistency). Otherwise, people tend to take explanations too literally, and cannot see the arbitrary choices that are being made in the language. Maybe that much clarity is itself a cause of some issues, but at least they are educational issues.

8. Sep 18, 2011

### Drakkith

Staff Emeritus
So...are you saying that in GR space is not being created inside of galaxies? I've never gotten into the math and details of the expansion of space, but everything I have read has said that space is expanding. I can easily see that this is a generalization and that it could be that space is not expanding if you have sufficient mass in an area of space.

9. Sep 27, 2011

### Ken G

Sorry I missed this for awhile. GR does not provide any unique or observable way to talk about what space is doing, only the curvature of spacetime. There are coordinates we can choose that suggest various things that "space is doing", but they are only pictorial in GR-- they're not unique descriptions of what is happening. I would say that the most natural types of coordinates I can think of would have space being contracted within galaxies (by the galactic gravity) and expanded between clusters of galaxies (by the cosmological solution). If gravity can do one, it can do the other also, and mixing the gravity-as-spacetime-manifold on the cosmic scale with gravity-as-Newtonian-force on the galactic scale seems like mixing apples and oranges, though it does not give a wrong answer.
Yes, it seems to me that if we talk about "what space is doing", what we really mean is "what observers are doing who are both inertial and in some sense generic." Inside a galaxy, the natural thing for generic inertial observers to do is fall together, into the core of the galaxy. Outside galaxy clusters, the natural thing for generic inertial observers to do is to drift farther away from each other.

10. Sep 27, 2011

### Drakkith

Staff Emeritus
Wait, are you saying that this "expansion" is just something like reverse curvature? So the curvature is a certain way when mass is present, and the cosmological constant causing a reverse curvature or something similar? I've understood the curvature from mass as causing such effects as light curving around a massive object. How does this effect the travel of light on a universal scale if the curvature is it's the same thing but reversed?

11. Sep 27, 2011

### Tanelorn

I have been reading that with the end of the Luminiferous Aether theory that Space itself is truely empty and unlike ponderable matter has no Physical characterisitics at all. Does it therefore make sense to say that space expands to explain how some distant matter is moving away from us at light speed or more? If this expansion at certain distances and attraction locally is all just related to gravity could gravity therefore somehow have a negative sign at large distances?

Last edited: Sep 27, 2011
12. Sep 27, 2011

### bapowell

Drakkith, keep in mind that the FRW solution of an isotropically and uniformly expanding spacetime results from an isotropic and uniform energy density as a source. This is really only true on cosmological scales. Locally, on the scale of a galaxy for example, one should expect different gravitational dynamics on account of the inhomogeneous energy density.

13. Sep 27, 2011

### Drakkith

Staff Emeritus
FRW solution?

14. Sep 27, 2011

### bapowell

Friedmann-Robertson-Walker

15. Oct 1, 2011

### Ken G

That doesn't seem to be quite true-- empty space may have a "cosmological constant", which is essentially a tendency to expand, of if you prefer, a negative gravitational effect on objects placed far enough apart.
Saying that space expands is one way to communicate the idea. All you can really say is that distances increase, the "why" is rather pictorial.
That is indeed the idea behind a cosmological constant, if that is what is responsible for the apparent acceleration of the expansion.