In GR gravity is seen as a property of space-time , curvature. Now we know that sometime after the plank's time we had the universe dominated by radiation ( recombination) as such the rate of inflation was in the range of ~ 10^30 m/s , which as you can see is considerably larger than the VOL ( at about 10^-37 to 10^-35 of a second).
I might be waffling here but if the expansion of the universe itself is not constant due to various parameter , however , the way I see things it has no affect on individual photon speeds.On the other hand I am not sure how gravity field ( or quantum graviton would behave).
I find this in conflict with what GR proposes where gravity doesn't stay constant , and acts as a function of the mass's object.
Can someone shed light over this.
Is this one of the reasons why we are finding it hard to unify GR with E-M ?
P.S: I have more questions in mind which I will ask following on to replies.