Re: Variation of light speed.

Hi Ted,

I asked my theorist friend Amy Lo about this and the answer that came out
of our discussion can be boiled down to this:

Gravity waves propagate at the speed of light by Grand Unification (GUT)
theories.  Therefore, if the speed of light varied, then "effect" of
gravity would be different. So if light has been accelerating over the age
of the universe, then the universal gravitational constant would have been
smaller in the past. This is like saying that the depth of the
gravitational potential well would be spread over a longer time (think of
it as space if you want). 

However, there is no reason (I repeat, no reason)to suspect that the speed
of light has been changing over the age of the universe. This is equal to
saying that the energy of a photon is variable and that gets us into what
is known as the "tired light paradox". Although study that you read may
postulate that the speed of light has been changing, the current trend
among cosmologists is to believe that it has always been equal to its
present-day value.

Thanks for your intriguing question!