# Was the expansion of the universe always greater than c?

1. Jun 20, 2014

### @Spuding102

After some thinking, I have concluded that the expansion of the universe must have started at the speed of light or greater than the speed of light but not less than speed of light. I say this because if the expansion of the universe at the instant of the big bang was less than the speed of light, then light would be able to escape the universe. This should not be possible since light "outside" of the universe would imply energy and time which were created at the big bang (if they were created at the big bang than it does not make sense to say that energy and time exist "outside" of the universe). If we take it that there is no light "outside" the universe, then does this mean that the expansion must have always been ≥ c?

2. Jun 20, 2014

### phinds

Your post makes no sense to me. Expansion has occurred, and is occurring, at a continuum of speeds depending on distance. "Inflation", if it happened (and it probably did) was FTL for everything. The current universe is expanding uniformly such that nearby objects are receding at relatively low speeds and objects at the edge of the observable universe are receding at about 3c. and everything in between is every recession "velocity" in between.

None of this is FTL in the normal meaning of that phrase. Google Metric expansion and/or see the link in my signature.

There is no "outside" to the universe, so I REALLY have no idea what you're talking about with that.

3. Jun 20, 2014

### @Spuding102

What I mean is that the "edge" of the universe must have always been expanding faster than the speed of light because if it was expanding slower than the speed of light then a light source at the edge of the universe could "escape" the universe. Essentially, I am asking this. When the universe began at the big bang, its expansion has been accelerating and its velocity has never, at its "edge" been slower than the speed of light.

4. Jun 20, 2014

### phinds

There IS NO "edge" so your premise is wrong, thus your conclusions are meaningless.

5. Jun 20, 2014

### Matterwave

As far as we know, there is no "edge" of the universe, indeed that would be very weird and would start to beg questions like "what is outside of the universe?". The universe in our current models is either infinite, or finite with no boundary.

As there is no edge, there is no definition of how fast this "edge" is expanding.

If the universe is flat, as our models seem to show, the instant after the big bang, the universe was already spatially infinite in size (the observable universe was, of course, much smaller).

6. Jun 21, 2014

### @Spuding102

If the universe is expanding, and there is no "edge" then what is it expanding into? I read your linked article about the balloon very interesting. Also how can the universe be finite with no boundary? Finally, how can the universe be spatially infinite after the big bang? Wouldn't the observable universe and the entire universe be the same at at least one instant after the big bang?

Thanks

7. Jun 21, 2014

### Matterwave

Here you are running into a lot of misconceptions one has when first starting the study of cosmology. Cosmology can be a bit non-intuitive at times.

1) The universe is not expanding "into" anything. It is just expanding. If the universe was "expanding into something", couldn't we then redefine this "something" to be part of the universe? You have to think like in the balloon analogy, except you have to forget that the 3rd dimension (outside the surface of the balloon) even exists. It is not "something".

2) Being finite with no boundary is not as hard to grasp. Imagine the surface of a sphere. This surface is finite, but has no boundary. It means that if you go in one direction for long enough, you end up back where you started.

3) You still seem to be thinking of a bomb going off. This is not the right analogy. But admittedly this point is perhaps the most non-intuitive of the points I mentioned. Right after the big bang, distances between any two points in space, anchored by co-moving particles, are much much closer together than they are now. But this does not mean that there wasn't an infinity of space or an infinity of particles. You can still stretch and expand the universe, even if it is spatially infinite. You are simply stretching all distances between pair-wise points anchored by co-moving particles. You can imagine an infinitely large rubber sheet. Although the rubber sheet is infinitely large, and has always been infinitely large, you can still stretch it to make points on it move farther away from each other.

If this concept is too hard to grasp right now, then, for the time being, perhaps try to think of a finite universe but without boundary. This would be like the balloon growing in size.

8. Jun 21, 2014

### Chalnoth

Nope. The expansion rate was high, but certainly not FTL for everything.

I just don't think it's even remotely useful to compare the expansion to a speed. An expansion is a rate, not a speed. The units just don't match up.

9. Jun 22, 2014

### bcrowell

Staff Emeritus
We have a FAQ that explains why it doesn't make sense to talk about the velocity at which the universe expands: https://www.physicsforums.com/showthread.php?t=508610 [Broken]

Last edited by a moderator: May 6, 2017
10. Jun 25, 2014

### JohnFrancis

Cosmic Expansion

It seems to me nonsense to say that there are distant reaches of the universe beyond a horizon at which the recession speed of the galaxies equals the speed of light in relation to us. No recession speed can equal or exceed the speed of light according to Einstein's Special Theory of Relativity. The distances between the galaxies (to us) along our line of sight will be shorter and shorter the further they are away, because they are moving faster, so that the galaxies will never actually disappear. As time goes by more and more of the universe will come into view as the light reaches us. Of course, the light from the galaxies will be more and more red-shifted the further they are, but that is beside the point. The nice thing about this interpretation is that it is perfectly self-consistent.

11. Jun 25, 2014

### bapowell

This is wrong. Expansion velocities exceed light speed at the Hubble scale. Special relativity does not apply here because two objects separated by the Hubble scale are not in the same inertial frame. The rest of what you say about the observable universe nonetheless growing is correct, as it's true if even these distant reaches are receding at superluminal speeds.

Last edited: Jun 25, 2014
12. Jun 25, 2014

### phinds

Any time you find yourself saying that about something in cosmology or quantum mechanics, you should stop and reconsider. Much of both "seems like nonsense" but is still true and nature doesn't care what "makes sense" to us humans with our incredibly limited range of experience.

Last edited: Jun 25, 2014
13. Jun 26, 2014

### Lino

Spuding, Remember the difference between the universe and the visible universe - during the inflationary period distances within (what is now) the visible universe were increasing FTL and so were distances in the wider universe. Since the inflationary period, only vast distances (that are outside the each others cosmic horizon?) are increasing FTL.

14. Jun 26, 2014

### bapowell

The particle horizon marking the observable universe contains parts that are receding from one another at beyond light speed (i.e. the particle horizon is larger than the Hubble sphere).

15. Jun 26, 2014

### Lino

Thanks bapowell.

16. Jun 27, 2014

### JohnFrancis

Expansion of Universe

17. Jun 27, 2014

### phinds

No, it was not. Galaxy A is stationary, according to it, and B is moving away from it both before and after the supernova sends out light and is still moving away from A when it sends out the reflection back to A.

Everything is always moving away from everything else (on cosmological scales).

18. Jun 27, 2014

### bobie

What Spuding is trying to say is that he assumed U as the visible Uv. And that makes sense, since no man has title to talk of what is/happens beyond its edge.
Since most bodies lie beyond the Hubble radius UH, then it is a fairly acceptable conclusion that most objects in the U(v) have moved at an average speed ≥ C
it's simple arithmetic

v = HD, D≥ C/H, v ≥C

or from another perspective, S/T = Vav (space travelled divided time elapsed gives the unitary speed or average speed if it is not uniform)

T = 13.8 Gy, D≥ 14.4 Gly, 14.4/13.8 : v = 1.04 C

then Spuding may legitimately conclude that the greatest part of bodies in Uv has been moving/receding at an average speed ≥ C,
that is expressed in less precise language saying that expansion of U(v) has taken place at v≥C
But that is quite comprehensible (... to a student)

Last edited: Jun 28, 2014
19. Jun 28, 2014

### phinds

I think you should let Spuding make his own interpretation of what he said. Based on what he HAS said, I disagree w/ your interpretation of it.

20. Jun 28, 2014

### JohnFrancis

"galaxy A was moving towards the returning reflected light"

Yes it was, relative to us.