Light speed in relation to the size of the universe

[sinebar]

Can anyone tell me if the speed of light in a vacuum was the same a few seconds after the big bang when the universe was small as it is today? In other words could it be that the speed of light changes as the universe expands?

Also is it actually known why light travels at the speed it does? If not then there is no point in me asking what is so special about that speed at which it travels with no varience.

 

[mathman]

There is no evidence that the speed of light has changed since the big bang.

 

[RossD]

How can there be a doppler effect (red/blue shift) with light when the velocity of light doesn't vary relatively to the motions of source or observer?

 

[Fredrik]

Can anyone tell me if the speed of light in a vacuum was the same a few seconds after the big bang when the universe was small as it is today? In other words could it be that the speed of light changes as the universe expands?

It could, but as Mathman said, there's no evidence. Also, the speed can't vary in general relativity, so GR would have to be modified to allow for that.

Also is it actually known why light travels at the speed it does? If not then there is no point in me asking what is so special about that speed at which it travels with no varience.

If you want to know why light moves at c, the answer is that it's a consequence of Maxwell's equations. (You already got that answer).

So why do Maxwell's equations hold? They are a consequence of the fact that the electromagnetic field (the vector potential) is a massless vector field on Minkowski space. Actually that's more of an equivalent formulation of the theory than a reason why it holds, but if we consider quantum electrodynamics, the concept of renormalizability tells us why there are no other interaction terms in the Lagrangian. (There may be other terms, but non-renormalizable terms do not contribute noticeably to experiments at low energies).

Why is the field massless? That question doesn't make much sense, as it is part of the definition of what we mean by "light".

Why is spacetime Minkowski space? It isn't really. General relativity tells us that there's a relationship (expressed by Einstein's equation) between the geometry of spacetime and its matter content. Minkowski space is the particular solution that corresponds to an empty universe: No matter, no gravity, no nothing. But GR also tells us that a spacetime with matter looks like Minkowski space locally. What that really means is somewhat technical, but you can think about it this way: When you look at a small enough region, its geometry will appear to be flat, just as a small region of the surface of a sphere appears to be flat. This property of spacetimes in GR is sufficient to make the speed limit (locally) the same as in SR.

So why does GR hold? No one has an answer to that yet, and even if they did, it would create another why question. This is a problem you'll eventually run into no matter what question you start with.

 

[MeJennifer]

If we assume that light speed is variable over time then asa a consequence GR is no longer correct. GR requires that the local speed of light remains constant.