Is there any evidence to prove (or disprove) that the speed of light in vacuum (or other “universal” constants for that matter) was the same when the universe started out (i know that there is a small time interval from the big bang when no scientific laws are applicable - so neglecting that period) ? Will the speed of light change in the future ?
I remember reading (in New Scientist i think) there are is a theory that the speed of light may be changing very slowly over time. I think it helps explain some puzzling observations to do with very distant galaxies.
Sorry, i know i’m being very vague, but thats all i can remember! Hopefully someone will come along soon who actually knows something about it…
I believe that the path of light is affected by large gravitational forces, and this would suggest that the speed of light could be affected as well, at least that’s how I see it.
As far as I’m aware, there’s no positive proof that fundamental constants are not, in fact, time-dependent. There’s some reason to believe that they’re not, however.
First, of course, is that most of the formulae that we use have these fundamental constants floating around, and they seem to predict observations like redshift pretty well; it doesn’t appear that the spectrum of, say, the hydrogen atom has changed in an unexplained way over time. But that’s not proof.
Secondly, I can’t think off the top of my head of any proposed mechanism which would explain how these constants change. That’s not even evidence.
Third, it would be really quite inconvenient if we had to correct for the change of some constant or another over time. 
Our units of distance and time are matters of definition. The meter is defined in terms of the second and the speed of light. Therefore, the speed of light in meters/sec is necessarily constant by definition.
It is true that gravity affects the velocity of light. The definition of meter assumes an inertial reference frame. When have gravity/acceleration, you no longer have an inertial frame.
This link discusses the theories of Dr. Joao Magueijo concerning variations in the speed of light in the early universe.
In order for the expansion of the universe to be squared with the observed distribution of matter, something has to be variable… the electric charge constant or the rate of expansion, or the speed of light, or something else.
Here’s a related thread: How do we know that physics works the same everywhere?
Apologies- that was a fringe science link 
Try http://theory.ic.ac.uk/~magueijo/vsl.html
his home page- he is a Reader in Theoretical Physics
at Imperial College, London.
There is a story on this in the April issue of Discover that is not yet online.
An earlier piece on how supposed constants might have been different in the early universe appeared in New Scientist along with an interview with Magueijo. (You may have to register for these.)
And Mageuijo has a book out, Faster Than the Speed of Light: The Story of a Scientific Speculation, by Joao Magueijo
A few observations:
The total amount of mass and energy of the entire universe has been constant since the beginning of time.
The universe is expanding.
Thus, the universe as a whole has been getting less dense.
The speed of light is slower in a denser medium than a pure vaccuum.
Is light speeding up or slowing down?
Mass/energy that goes out of the boundary (the boundary due to Hubble’s law) stops existing for the universe. So saying that the mass of the universe is constant is just a matter of convenience and it is certainly not true.
Also the speed of light that is being referred to is a calculated number more rightly called Einstein’s Constant.
Light does slow down in a material, but that’s not particularly interesting. Even without a true, absolute vacuum, you can define what the speed of light would be, if there were such a vacuum.
But it’s probably not meaningful to talk about it changing. Dr. Magueijo’s work seems to very tentatively suggest that the fine structure constant might have been different in the past, and that’s certainly a meaningful thing to discuss, since the fine structure constant is a dimensionless number. But it’s a combination of the speed of light, Planck’s constant, Coulomb’s constant, and the fundamental charge. If the fine structure constant has changed, then at least one of those constants it’s built from must have changed, but there is no conceivable way, in theory or experiment, to even in principle determine which of those is changing. Aesthetically, I’d be much more comfortable saying that the elctrostatic coupling constant (Coulomb’s constant) is what changed, but the Universe doesn’t give a damn about what we think of its aesthetics.
Not to make an ad hominem argument here but the first thing to realize is that not everyone who argues for a changing value of c is exactly unbiased. Specifically, the idea that c was much larger in the past is an idea advocated by many fundamentalist young earth creationists. It gives them a way for light from objects billions of LY away to get to earth in only 6000 years.
That said, the main thing I would want to know is how dependent things like stars and galaxies are on the current value of c. The universe has been much the same for a very long time and if certain observable structures that we know existed in the distant past can only form and sustain themselves over a narrow range of values of c then we can at least put upper and/or lower bounds on the possible values it may have had billions of years ago.
andy_fl Sorry if I’m being dense here, but could you explain this one to me, i.e. how mass/energy could go outside the boundary.
thx.
What andy means, I think, is that we can only observe part of the universe, and the energy contained within the part that we can see is not conserved, because we can’t see the whole. We can’t really make any dogmatic statements about anything outside of the observable universe, because we can’t see it.
I would, however, disagree that saying that the energy of the universe is a constant is “just a matter of convenience” and that it’s “certainly not true.” We have good reasons to think that the energy of the universe as a whole is constant, and while we could be wrong, it’s hardly as if we are certainly wrong.
In other words, I would suggest that there are places that we can’t observe, and that they have energy too, and that the sum of it all is a constant, but I can’t prove that because we can’t see them.
Lastly, I would say that it’s a little silly to suggest that mass that’s not in the observable universe has no effect on it and stops existing for it. Somewhere out there is quite possibly a place from which the earth is part of the observable universe but the moon is not; that doesn’t mean that the moon doesn’t have any effect on the earth or that the moon doesn’t exist, just that at this place, the earth can be seen in principle and the moon can’t.
This may be true, but for the record - Magueijo has the speed of light changing by a small amount only for a brief time very early in the universe’s existence. Accepting his hypothesis would not help the creationists one tiny little bit.
There is an extension to General Relativity known as the Brans-Dicke theory, in which G, the gravitational constant, is a function of space and time. Since the theory allows for G to be constant, it can’t be ruled out, but there was a fair amount of experimental work examing this constant and the speed of light. Last I heard, all the evidence indicated that those two constants are in fact, constant.
Mass that has equaled the speed of light has done so for all observers regardless of the speed of the observers. It cannot have any effect of the observers cos all effect travel at the speed of light - including gravity, etc. So for all practical reasons the mass does’nt exist. To say it exists is a matter of belief just like saying Ghosts/God exist.
This statment is not correct. The horizon of the observable universe is different for every point in space. An observer that is just now disappearing past our horizon would have his own horizon that we are just now leaving. For an observer midway between us, we are both traveling at sublight speed (at least for this moment).
I prefer to think of the observable universe as part of the Universe.