Was the expansion of the universe always greater than c?

[@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?

 

[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.

 

[@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.

Thanks for your reply by the way

 

[phinds]

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.

Thanks for your reply by the way

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

I suggest that you read the article linked to in my signature. It should help you understand a few things better.

 

[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).

 

[@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

 

[Matterwave]

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

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.

 

[Chalnoth]

"Inflation", if it happened (and it probably did) was FTL for everything.

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.

 

[bcrowell]

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

 

[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.

 

[bapowell]

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.

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.

 

[phinds]

It seems to me nonsense ...

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.

 

[Lino]

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?

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.

 

[bapowell]

Since the inflationary period, only vast distances (that are outside the each others cosmic horizon?) are increasing FTL.

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).

 

[Lino]

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).

Thanks bapowell.

 

[JohnFrancis]

Expansion of Universe

It seems to me nonsense to say...

Please tell me what is wrong with this:-

It is my contention that Einstein’s Special Theory of Relativity can be applied to questions arising from the expansion of the universe. There is a long tradition of using thought experiments to shed light on these matters started by Einstein himself. Let us suppose that we live long enough and have sufficiently powerful devices to communicate with observers, similarly long lived and equipped, in some very distant galaxy A. Let us also assume for simplicity that the space between us is more or less flat (Euclidean) and that we can sensibly take account of our various local movements such as orbits round our local suns and the time it takes to communicate. We also assume that Hubble’s Law is approximately true up to distances where the expansion velocity is subluminal, thus we interpret red-shifts as indicating recessional velocity and that distant galaxies move away from us with greater velocity the greater their distance is from us.
Now suppose that there is a supernova explosion in distant galaxy A and sometime later it’s light is reflected off dust in another galaxy B even further away. Suppose that our friends in galaxy A signal to us that they have seen the supernova and then later signal to us that they have seen the reflection. Given the timings and knowing the velocity of light it is then a simple matter for our friends and for us to calculate the distance between the two galaxies A and B and we do this using exactly the same events. However we must allow for the recession velocities of A and B - galaxy B was moving away while the light was going towards it and galaxy A was moving towards the returning reflected light – and we find our calculation gives a smaller distance than that obtained by our friends at A who do not see such recession velocities. This is the commonly called Lorentz Contraction of √(1 - v²/c²), where c is the velocity of light and v is the recession velocity. It is of course perfectly real and gets greater, further away, as v approaches c.
Clearly we should interpret the red-shift relativistically and deduce that there is no horizon beyond which galaxies recede at greater than the velocity of light - their light is just very red-shifted when it (eventually) reaches us.

 

[phinds]

... galaxy A was moving towards the returning reflected light ...

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).

 

[bobie]

I have concluded that the expansion of the universe must have started at the speed of light or greater

Y objects at the edge of the observable universe are receding at about 3c. .

There IS NO "edge" so your premise is wrong,

What Spuding is trying to say is that he assumed U as the visible U_v. 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 U_H, 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 = V_av (space traveled 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 U_v 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)

 

[phinds]

What Spuding is trying to say is that he assumed U as the visible U_v.

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.

 

[JohnFrancis]

"galaxy A was moving towards the returning reflected light"

Yes it was, relative to us.
phinds has misread.

 

[phinds]

"galaxy A was moving towards the returning reflected light"

Yes it was, relative to us.
phinds has misread.

Yes, I did mess that up didn't I? Thanks for that correction.

Paul