# How is expansion manifested physically?

1. Aug 3, 2015

### ash73

Hi, newbie reading the forum with interest, and trying to understand how expansion manifests itself physically.

Four possibilities;

1. New space is created between existing space, pushing galaxies apart
2. Existing space is stretched, pushing galaxies apart
3. Empty space is nothing, and galaxies are just thrown apart
4. Galaxies aren't physically moving apart (other than peculiar motion)

#1 & 2 allow for FTL separation (if redshift sufficient) by manipulating space, rather than moving in space, but they imply empty space has some sort of physical structure, which I don't understand. #3 requires no structure for empty space, but implies FTL motion in space, which I don't understand. I'm assuming #4 is not compatible with accepted theory, therefore moot.

What is this space "stuff" that is expanding? Reminds me of the aether. I'm told in GR curved spacetime is a mathematical treatment, not a physical reality; is expansion is the same?

TIA.

2. Aug 3, 2015

### phinds

#3 is the one. It does not imply FTL motion but rather FTL recession, which is not the same thing. Hard to get your head around at first but that's the way it is. Google "metric expansion" for more details.

Last edited: Aug 3, 2015
3. Aug 3, 2015

### Staff: Mentor

GR is a theory about geometry. Variable geometry to be more accurate, which is geometry that is capable of changing. The geometry of spacetime changes in such a way that distances between unbound objects (like galaxy clusters) increases over time. That's really all expansion is saying.

4. Aug 3, 2015

### phinds

Just as a slight clarification on Drakkith's post, since I find the wording slightly confusing, he isn't saying that galactic clusters are unbound objects themselves, he means that two galactic clusters would not be bound to each other and thus would be subject to receding from each other due to expansion. If you already know that that's what he means, then that's the way it reads, but if you don't already know that, his wording is subject to confusion.

5. Aug 4, 2015

### bahamagreen

Would #4 be most correct if it was changed to read thus?..:

4. Galaxies* are moving apart but aren't physically being moved** apart.

*Unbound large scale cluster structures
**Not being pushed, pulled, forced, pressed, or thrown apart; none are subject to proper accelerations of their centers of mass, all are at rest with respect to an inertial frame of reference.

6. Aug 4, 2015

### ash73

Interesting comments thus far. I've read the wiki page on metric expansion and I don't understand it. For example, I don't understand how the gravitational attraction between galaxies* decelerated the expansion of the early universe, if expansion is just a feature of the metric scale between them; how does one interact with the other? And how can this interaction not be subject to the cosmic speed limit?

*shorthand

7. Aug 4, 2015

### bapowell

The form of the metric, including the expansion rate of the universe, is determined by the matter content of the universe via Einstein's equations. The mutual gravitational attraction of elements of a uniformly distributed density (think many, many galaxies across cosmic scales) is manifested as decelerated expansion.

As for the "cosmic speed limit", there is no such thing. Einstein's special theory of relativity, which famously demonstrates that the speed of light is the ultimate speed limit of the universe, applies to so-called local, inertial frames. In other words, it applies to situations where there is no gravity or acceleration. If we look at two galaxies that are receding from each other due to expansion of the universe, they do not occupy the same local inertial frame and so there is no applicable limit on their rate of recession. Of course, they must *locally* obey the speed limit v < c, i.e. their peculiar velocities (their motion relative to the expansion) must be subluminal.

8. Aug 4, 2015

### ash73

Thanks, it makes more sense with that explanation. I think the problem is I've been taking layman's guides too literally; balloon analogies and talk of new space being created is actually getting in the way of my understanding.

9. Aug 4, 2015

### bapowell

Sure, that can happen. I rather like the balloon analogy for illustrating homogeneous and isotropic expansion. Just don't take the presence of the rubber (or any "substance" for that matter) seriously.

10. Aug 4, 2015

### phinds

The balloon analogy is excellent *IF* you understand exactly what it is saying. I recommend the explanation linked to in my signature which had input from several people here.

11. Aug 4, 2015

### ash73

I like the "Misconceptions about the Big Bang" article written by Prof Tamara Davis included as a footnote in your balloon analogy page, I watched her lecture on cosmic horizons on YouTube yesterday, it was excellent.

bapowell's point about inertial frames made the penny drop. At the risk of introducing another confusing analogy, it sounds rather like the inverse of length contraction one would encounter in a rocket hurtling towards a star at relativistic speeds, i.e. everything seems normal in your own frame but the distance to the star really does contract; whereas with expansion everything obeys the rules locally, but the distance (in empty space) to other galaxies really does increase FTL. I understand they are quite different, but if I can accept one I can accept the other.

12. Aug 4, 2015

### bcrowell

Staff Emeritus
These all read to me as different verbal descriptions of the same thing. None looks any better or worse to me than any other. A statement like "Empty space is nothing" could mean anything, and could be either true or false, depending on what you mean by "nothing."

General relativity doesn't say whether or not space itself is expanding. You can describe it verbally in either way, and it's neither right nor wrong.

General relativity doesn't have any unambiguous way to describe the relative motion of objects that are far apart. Therefore we can't define whether a distant galaxy's motion relative to us is or is not FTL.

13. Aug 5, 2015

### slatts

The problem with numbers 1 and 2, in Ash73's list of possibilities, is that "created" and "stretched" are conjugations of transitive verbs (verbs that require a subject and an object), and consequently imply that their action is being accomplished by something or Someone outside space. Numbers 3 and 4 rely on the intransitive verb "to be", and don't have that problem.

14. Aug 5, 2015

### ash73

That wasn't my intended meaning, I was trying to understand whether the mathematics could be translated into an understanding of what is actually happening in physical reality. It seems as impossible as imagining what an atom looks like; the maths helps us predict stuff but that's all.

1 and 2 were intended to imply space itself is a 'thing' with structure that can be created, or manipulated. That is to say, the energy of space, or quantum foam, or GR curvature, or whatever, actually physically exists and changes during expansion. If that was true you could explain expansion by saying new space is created between existing space, thus pushing objects apart, and the expansion will obviously be geometric. But how could nothing have structure?

Whereas 3 implies empty space is just that, an empty void, and expansion is understood via a mathematical treatment using a repulsive force (dark energy) which enables us to calculate what happens. So with 3, one could say the galaxies are actually, physically, moving apart faster than light in empty space, or at least their proper distance is bahaving that way, except it seems GR cannot pin down relative velocity at that distance; so even this interpretation is too literal.

Compare it to GR, which says objects follow geodesics or curved space. If I've understood it correctly (probably not!) Einstein provided some geometry and clever maths which enables us to calculate the path objects will take, but it doesn't describe reality i.e. space isn't really physically curved; that would require it to be something 'real' with a physical structure... like old fashioned ether.

The answer from bapoweell has satisfied my curiosity, I think.

15. Aug 5, 2015

### bcrowell

Staff Emeritus
No, it's nonsense to try to apply this kind of grammatical reasoning in this situation. Equations don't have subjects and objects. The implied subjects of these verbs could be processes, principles, or relationships rather than concrete objects. Even if the subjects were interpreted as being concrete objects (which is possible, since one side of the Einstein field equations is the stress-energy tensor), there is no implication that the object is outside space. When you attempt to reason about mathematics in verbal terms, it's important to have a clear picture of the mathematics underlying the words. To the extent that your picture is fuzzy or is one that you don't fully understand, you need to maintain a proper attitude of self-skepticism and uncertainty rather than trying to make authoritative pronouncements.

16. Aug 5, 2015

### bcrowell

Staff Emeritus
This isn't meaningful unless you define what you think qualifies as a "thing." If you did come up with a definition of "thing," it wouldn't automatically be useful or interesting.

Same problem, now with "nothing" rather than "thing."

This is a common misconception. Cosmological expansion doesn't need to be explained by a force or by dark energy. GR doesn't describe gravity as a force, and cosmological expansion exists in essentially all cosmological models based on GR, including those that don't have dark energy.

17. Aug 5, 2015

### phinds

No, "proper distance" is NOT what is changing due to expansion. There is no definition of proper distance except in a single inertial frame of reference and it is not possible to have an inertial frame of reference for things that are far enough apart from each other to be affected by expansion.

18. Aug 5, 2015

### bcrowell

Staff Emeritus
Hmm, not sure I agree with this. I would certainly agree with it in a general context in GR. But in cosmological models that are isotropic and homogeneous, there is a preferred local frame in which the local region displays the isotropy. This is the rest frame of the Hubble flow or CMB, for example. The existence of such a frame can be extended globally to give a preferred time-slicing. E.g., everywhere on such a time-slice, the temperature is the same. So it does make sense to define a proper distance between cosmologically distant objects, which is the metric distance along a spacelike geodesic connecting the points where these two objects' world-lines intersect the time-slice. Cosmologists certainly do use this definition of proper distance, although I can't remember if that's the term that is actually used in the literature.

What I think gives rise to a lot of confusion at the pop-sci level is that popularizations don't make it clear that this is only one very special way of defining the distance between two objects, and if the definition is preferred over others, it's only because of the special symmetry properties of cosmological models. Laypeople get the impression that this is how far apart the objects "really" are, which implies to them that we have an unambiguous notion of how fast the objects are "really" moving away from each other.

19. Aug 5, 2015

### ash73

20. Aug 5, 2015

### ash73

Yes, and what I'm asking for is a clarification of how far apart they "really" are. I think I'm hearing it's indeterminate (: the proper distance is "real" in the sense you really would have to travel that far to get there, but it's complicated by GR frames of reference etc), but now I'm not sure.

Last edited: Aug 5, 2015