# Can space curve faster than C?

1. Oct 16, 2014

### ktx49

Hi,

We all know everyone's favorite "exception" to the speed of light limit....that space itself can expand faster than C.

My question is whether space can curve faster than C? I understand that gravity travels at the speed of light...but can space itself move faster than this limit in other ways besides expansion?

Thanks

2. Oct 16, 2014

### phinds

Space does NOT "move faster than light". Things get farther apart at a recession rate that is faster than light but that's not the same thing at all.

Since the curvature of space is a function of gravity and gravitational changes propagate at c, the curvature will not happen faster than c.

3. Oct 17, 2014

### ktx49

thanks for response.
i know my question was poorly formulated....but am I incorrect to say that space expanded faster than C in the early stages of inflation after the big bang?

could the curvature of space ever create an effect similar to that of expansion; in which things moved apart at a speed faster than light?

4. Oct 17, 2014

### A.T.

The distance between the things can increase faster than speed of light due to metric expansion. But that is different from moving away faster than light. For example a light signal sent by one thing can still reach the other thing, even if the distance between them increases faster than speed of light due to metric expansion.

5. Oct 17, 2014

### phinds

No. Again, things moved farther apart at a rate bigger than c (staggeringly bigger in fact) but again, that is NOT "space expanding". "Space" is just a framework in which things exist. If a person went from 2' tall to 5' all in a matter of several years you would not say that "length itself expanded" you'd just say the kid got taller.

No, the "curvature of space" is an emergent property based on the strength of gravity. Your statement would be equivalent to saying that rulers could make things get taller.

6. Oct 17, 2014

### harrylin

I suspect that mainstream thinking is a bit more subtle, see for example: https://en.wikipedia.org/wiki/Metric_expansion_of_space

7. Oct 17, 2014

### A.T.

8. Oct 17, 2014

### pervect

Staff Emeritus
This isn't incorrect, per se, but it is often misinterpreted. IF you want some reading on the topic of expanding space, I'd suggest http://arxiv.org/abs/0707.0380 and http://arxiv.org/abs/astro-ph/0310808

I'm not particularly fond of the idea of expanding space, but it's taught a lot, and if you want to do it (mostly) right, you might try the first paper, i.e. http://arxiv.org/abs/0707.0380, as that's the goal of the paper.

Personally, I"m not a fan of expanding space, an so I might not be the best to answer questions on it - but you can read the papers of people who are fans of the idea and see what they have to say about applying it correctly.

My own views are that metrics are a result of a choice of coordinate system. In most circumstances you want to use cosmological coordinates, which do expand. But if you happen to be interested in a local area, you might well want to use Fermi Normal coordinates, in which space doesn't expand. Furthermore, expanding space is taught and percieved in too many cases as if it were something physical, rather than something that ultimately depends on one' choice of coordinates, which is my biggest complaint.

Last edited: Oct 17, 2014
9. Oct 17, 2014

### PAllen

One point about expanding space is that it is dependent on the foliation chosen. While this doesn't mean it has no value, it is worth noting the completely flat Minkowski space can be foliated (Milne foliation) as to show expanding space exactly analogous to Friedman universes.. Yet, it is obviously completely static. To me, this means that what really distinguishes plausible cosmological models from static, flat, spacetime is something other than expanding space.