B How Did the Big Bang Happen If the Speed of Light Is a Limit?

[Gondur]

Hello,
If the speed of light is the maximum speed limit in our universe, how was the big bang event possible because surely the expansion would have been constrained by the speed of light?

 

[phinds]

Hello,
If the speed of light is the maximum speed limit in our universe, how was the big bang event possible because surely the expansion would have been constrained by the speed of light?

No, the speed of light constrains how fast things can move IN space. It does not constrain how fast space can move. This is discussed in hundreds of threads here on PF. I suggest a forum search, or just do some basic reading in cosmology.

 

[PeterDonis]

Moderator's note: Thread moved to the Cosmology forum.

 

[kimbyd]

Hello,
If the speed of light is the maximum speed limit in our universe, how was the big bang event possible because surely the expansion would have been constrained by the speed of light?

Short answer: in General Relativity, the speed of light limit is a local limit. It means that nothing can outrun a light ray.

Basically, the curvature of space-time is complicated and mucks up the notion of far-away speed enough that there's no such thing as a speed limit for a far-away object, except that that object can't outrun a light ray in its vicinity.

 

[phinds]

No, the speed of light constrains how fast things can move IN space. It does not constrain how fast space can move. This is discussed in hundreds of threads here on PF. I suggest a forum search, or just do some basic reading in cosmology.

And by the way @Gondur I wrote that reply rather hurriedly. It's really not correct to say "how fast space can move" since space doesn't really move. Space is just geometry. A more appropriate way to say it is that recession velocities due to changing geometry of spacetime are not constrained to c.

 

[sweet springs]

In inflation of universe (distance between us and a galaxy far far away) / (Universal time) is proportional to distance that seems to have no upper limit, so this "speed" does not have an upper limit. It seems like a natural "warp" in sci.-fi. to me.

Multiplying Hubble time = 4.55E17 second by c, such receding speed reaches c at distance D=1.37E26 m, "warp 1".

 

[Dale]

Hello,
If the speed of light is the maximum speed limit in our universe, how was the big bang event possible because surely the expansion would have been constrained by the speed of light?

The expansion of the universe cannot be constrained by c anyway. The expansion of the universe has dimensions of 1/T and c has dimensions of L/T. You cannot compare the two so there is no way to say that the universe is expanding faster or slower than c.

 

[kimbyd]

In inflation of universe (distance between us and a galaxy far far away) / (Universal time) is proportional to distance that seems to have no upper limit, so this "speed" does not have an upper limit. It seems like a natural "warp" in sci.-fi. to me.

Kind of a pointless warp drive since you can't use it to actually go anywhere.

In a region of the universe which currently has a recession velocity of $2c$, everything in that region has approximately the same recession velocity. It doesn't help a civilization there get from one galaxy to another.

As I stated above, in curved space-time, the speed of light limit is local only. The curvature of space-time itself mucks up the very definition of speed for far-away objects. It's like describing how quickly a car far away on the Earth is moving relative to you: the velocity you pick depends upon a choice of path between yourself and the car. That path is arbitrary: no one path is any better a choice than another. Different path choices will be useful in different contexts. In the human context, we usually use the path the car will actually drive, meaning that a car that is to the East of you might be considered to be moving towards you even if it is also facing East (e.g. if it's traveling towards a freeway on-ramp).

Recession velocities are like that. There are lots of ways of defining how quickly a far-away galaxy is moving compared to us. That very fact makes it impossible for General Relativity to impose a speed limit for far-away objects. The limit is only local.

 

[sweet springs]

The limit is only local.

Thanks. Well, there is no clear border between local and universal. I would like to know how much or small in quantity does this receding universe affect to our daily life physics represented by special relativity or maximum "speed" of c, I assume.

 

[Dale]

Thanks. Well, there is no clear border between local and universal. I would like to know how much or small in quantity does this receding universe affect to our daily life physics represented by special relativity or maximum "speed" of c, I assume.

The expansion of the universe is not a speed so it cannot be limited by c.

 

[kimbyd]

Thanks. Well, there is no clear border between local and universal. I would like to know how much or small in quantity does this receding universe affect to our daily life physics represented by special relativity or maximum "speed" of c, I assume.

No effect at all.

If you want to be pedantic about it, relative speed is only well-defined for objects at the exact same location. You can extend that out some distance by approximating space-time as flat. In a flat space-time, the speed of light limit is universal, so to the extent you can approximate the local space-time to be flat, you can define relative velocities between objects at different locations.

Once that flat space-time approximation starts to break down, so does the ability to uniquely define relative velocity at a distance. How far away you have to be for it to break down depends critically upon the specific geometry of space-time.

 

[sweet springs]

Thanks a lot.

Another point view I had is that expanding universe effect is much much smaller than nearby gravity from the Earth, the Sun, the stars and the galaxy and so on. We cannot pick such a small and background effect up from these big noise disturbance.

 

[kimbyd]

Thanks a lot.

Another point view I had is that expanding universe effect is much much smaller than nearby gravity from the Earth, the Sun, the stars and the galaxy and so on. We cannot pick such a small and background effect up from these big noise disturbance.

As discussed elsewhere, the effect of the expansion on gravitationally-bound systems is literally zero. There is a small effect from dark energy, but it's too small to be detectable.

 

[PeterDonis]

Thread closed since the OP question has been answered and no further value is being added.