# Universal speed of light

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1. Jun 30, 2014

### castrius

Sorry if I mess up some terms, im not a physics guru in any way.

To my understanding, the universe inflates like a balloon being filled with air. So..

Q1) Which is correct to say, that everything is being pulled away from the each other at different speeds from 0 to C, the further away 2 objects are the faster they move apart, or is it that the space between everything is growing, and objects are stationary?

Q2) If 2 opposite edges (assuming there is such a thing) of the universe are expanding at the speed C, then the distance across the universe grows at a rate of 2c, correct?

Q3) Would an object at the 'center'(assuming there is such a thing) of the universe stand still while everything moves away from it?, and would an object half way between the center and an edge of the universe be moving at 1/2C away from the center, while the edge moves at 1/2C away from it?

!--This would seem impossible, since the visible view of the universe from the object would reach 1/2 way across past the 'center' in one direction, where objects would be moving away from it at near C speed, while only moving away from objects near the universes edge on its side at 1/2C.

If objects closer to the universes edges do move faster than would time dilation change anything? If the closer to the edge you are, the faster you are moving, then the slower your time would move due to dilation. since the slower time moves, the faster everything else moves, (since speed = distance / time).

So then is the speed of light constant, and time variable? or the opposite?

2. Jun 30, 2014

### Bandersnatch

Hi castrius, welcome to PF!

You've made a bunch of false assumptions in your reasoning, so it's bound to give you odd results.

First of all, the universe doesn't expand like a balloon. It expands like the surface of a balloon. The third dimension of the balloon is completely irrelevant to the analogy, and is in no way a required physical attribute of the universe it describes.

Reading the analogy otherwise leads to misconceptions like the edge, and the centre. There is no centre or edge to the surface of the balloon, as I'm sure you can readily see.

Next assumption you've made is that the universal speed limit of c applies to the expansion of space - it doesn't. Objects about 4.5 billion light-years apart in the universe are already receeding from each other at c, and the farther you look the faster they go. For most intents and purposes the expansion of space leaves objects stationary, while distances between them increase according to Hubble's law.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

3. Jun 30, 2014

### phinds

As Bandersnatch has said, you completely misunderstand cosmology.

I think the article linked to in my signaure, one page long, would be a big help to you.

4. Jun 30, 2014

### Staff: Mentor

Since the other respondents answered 2 and 3, I thought that I would answer 1. In general relativity, the underlying theory of cosmology, there is no unique way to compare the speed of two distant objects. Relative speed only has a unique meaning for nearby objects. So, when you are talking about the speed of distant objects you have to specify some convention.

In the case of cosmology the convention is to measure the Doppler shift and use that to calculate the speed. This measure of speed interprets the receiver as stationary and does not distinguish between the speed due to expanding space and the speed due to relative velocity.

I hope this helps

5. Jun 30, 2014

### PAllen

I believe the most common formula relating recession velocity to red shift in FLRW cosmology gives a separation speed in coordinates such the the CMB is everywhere istotropic. It is then unspecified (and indeterminate) how much of this coordinate separation speed is 'relative velocity' and how much is 'expanding space'.

If you apply the regular SR Doppler formula (rather than one derived from FLRW metric), you would get a relative speed < c because SR Doppler maps shifts up to infinite as corresponding to relative speed < c. This direct application of SR Doppler actually gives a relative speed per one particular (not unique, not preferred) way of comparing velocity at a distance: parallel transport the distant galaxy's motion (4-velocity) along the light path (null geodesic) to the receiver, and the relative velocity between this transported 4-velocity and the receiver corresponds to direct application of SR Doppler to the red shift.

6. Jun 30, 2014

### castrius

Bandersnatch seems to say objects are stationary, DaleSpam that we cant tell by the way we measure, am I reading that correct?

Does this mean that objects are stationary, and space between them increases, or just that the way we measure far away objects by their red shift "does not distinguish between the speed due to expanding space and the speed due to relative velocity" ?

Thanks

7. Jun 30, 2014

### Mordred

objects are stationary and the space between them increases

8. Jun 30, 2014

### phinds

There is a relatively tiny "speed due to relative velocity" among cosmological objects, but that's pretty much irrelevant given that their recession "speed" is ENORMOUSLY larger, particularly as you look at ones farther away.

9. Jun 30, 2014

### Staff: Mentor

It is more than just an inability to measure. The key point is that there is no unique answer, even in principle with perfect knowledge. Therefore we adopt a completely arbitrary human-made convention.

This lack of unique answer is due to parallel transport. It is a topic which is central to curved spacetime.

10. Jun 30, 2014

### PAllen

In one coordinate system ....