Timeline for Why space expansion affects matter?
Current License: CC BY-SA 2.5
18 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Dec 13, 2010 at 15:21 | comment | added | Marek | @Jerry Schirmer: well said indeed. | |
| Dec 12, 2010 at 21:16 | comment | added | David Z | @Jerry: well said. I agree with that too (that the real results come from the calculations), I guess it's just a little too easy to get carried away arguing about the interpretations. | |
| Dec 12, 2010 at 19:59 | comment | added | Zo the Relativist | @David @Marek: That's why my attitude about these things is generally "calculate the relevant physical property using the rules of differential geometry and GR" and only then, provide a plain english set of words to interpret the result. Otherwise, you just end up talking in circles about things that might be equivalent in different coordinates. Proper distances between galaxies increase in a FRLW spacetime. <b>Why</b> this happens depends on what coordinate system/interpretation scheme you're using, and different schemes clarify some things and obscure others. | |
| Dec 11, 2010 at 11:55 | comment | added | Marek | @David: fair enough. Regarding movement: precisely. There are two independent definitions and none of them is more correct. But I think @serg had a pretty good picture of this in mind (that I also mentioned) already. Namely, that you can imagine the universe as a balloon. As you blow in it it gets bigger and distances increase but the actual rubber molecules aren't really getting anywhere (in the second sense of movement). Just intermolecular distances increase (which corresponds to the expanding space and decreasing density of the galactic fluid). | |
| Dec 11, 2010 at 4:12 | comment | added | David Z | @Marek: well, in the answers I suppose we were trying to explain to laymen, but now we are in the middle of a technical discussion ;-) Anyway, I think this whole discussion results from opposing definitions. I'm defining "movement" to be a change in measured relative position, whereas if I now understand you correctly, you're defining it as something more like a change of comoving coordinate - which is fine, of course; sorry for making a fuss about it! I just think that the latter is likely to be much more confusing to someone who isn't familiar with GR at the mathematical level. | |
| Dec 11, 2010 at 2:01 | comment | added | Marek | @David: regarding movement... do you actually know how to derive FRLW? You start with stationary observers (which you can do thanks to homogeneity assumption) that are stationary w.r.t. the fluid that fills the space. So yes, you can define absolute movement for these kinds (of very special) space-times. | |
| Dec 11, 2010 at 1:57 | comment | added | Marek | @David: I am not trying to hold a technical discussion. I thought we are trying to explain something to a layman ;-) | |
| Dec 11, 2010 at 1:51 | comment | added | David Z | @Marek: well, there you have a precise definition of what it means to create space: increase the circumference (and thus the volume) of the universe. That's perfectly fine. What I really have a problem with is trying to hold a technical discussion using words whose meanings are unclear. Also re: 2 comments up, how do you define the movement of the objects themselves, except by the change in their relative position? | |
| Dec 11, 2010 at 1:00 | comment | added | Marek | @David: why do you have problems with "space is being created"? Sure, it's just popular view but it's totally true. Take again the 3-sphere. It has some volume. You can measure that. This volume is increasing in time. So indeed, space must've been created. | |
| Dec 11, 2010 at 0:58 | comment | added | Marek | @David: it depends. Both statements "they are moving apart" and "they are not moving apart" are correct depending on how you interpret them. It's true that there is an effective motion arising from the expansion but this doesn't come from the movement of the objects themselves. | |
| Dec 11, 2010 at 0:53 | comment | added | Marek | @David: ah, all right then. But if I remember FRLW metric correctly, $a(\tau)$ has a very clear connection to the actual size of the universe (at least for the compact case). | |
| Dec 11, 2010 at 0:53 | comment | added | Zo the Relativist | Describing these things in terms of words is tricky. Take the flat Roberston-Walker metric, and make the coordinate transformation $R=a(\tau)r$. Then the metric becomes $ds^{2}=-\left(1-\left(\frac{\dot a R}{a}\right)^{2}\right)d\tau^{2} + 2d\tau dR \left(\frac{dot a R}{a}\right)+dR^{2}+R^{2}d\Omega^{2}$. Now, it literally appears that there is a frame dragging effect (the $g_{\tau R}$ term) describing the expansion of space, and that distances are fixed. Which formulation is right? Both of them. It's a matter of interpretation. | |
| Dec 11, 2010 at 0:34 | comment | added | David Z | @serg: It seems like you have this idea that space is some entity separate from matter and energy, i.e. that it can expand while the objects in it stay still or it can stay static while the objects in it move apart. Do your best to forget that. The whole "fabric of spacetime" analogy is horribly misleading if you push it beyond what it was meant to explain, as I think you're doing here. | |
| Dec 11, 2010 at 0:26 | comment | added | David Z | (cont) And I have to disagree with your comment in one point: I would say things are moving apart, and the idea that "space is being created" is nonsense - unless you develop a precise definition for what it means to create space. | |
| Dec 11, 2010 at 0:23 | comment | added | David Z | @Marek: I think you misinterpreted my answer. I wasn't saying that it's wrong to think about expansion of spacetime in terms of $a(\tau)$, but rather that it's misleading to think about $a(\tau)$ as characterizing space itself. I simply meant that it's better to associate it with measurable distances, not some abstract notion of "space." If the universe were a 3-sphere, its circumference would be one of those measurable distances; there's certainly no reason you couldn't associate it with $a(\tau)$. | |
| Dec 10, 2010 at 23:40 | comment | added | Marek | @serg: It's just as you say. "Space is being created", or rather, "space-time expands" and the fact that things move apart is just an illusion. So I don't agree with David that it's wrong to think about expansion of space-time in terms of $a(\tau)$. According to GR that is precisely what happens. E.g. if our universe were a 3-sphere then you could (in principle) actually measure it's diameter along main circle and this would indeed be increasing with time. | |
| Dec 10, 2010 at 23:20 | comment | added | serg | So you mean objects are just flying apart under unknown force in a static space, not because space itself is expanding and pulling objects apart? I always was under impression that fabric of space is being created, so objects move apart because more "emptiness" being created between objects, while objects don't actually move. | |
| Dec 10, 2010 at 22:56 | history | answered | David Z | CC BY-SA 2.5 |