If the mass of the universe were cut in half, would it affect the speed of light?
Would it be twice as fast?
Would it stay the same?
Do we have instruments that are sensitive enough to measure the speed of light at different positions relative to high-mass objects to empirically answer this question?
The speed of light is (something of) a universal constant, but is it really dependent on the universe or on something intrinsic to photons?
Since gravity is a relationship between one atom and every other atom in the entire universe, and it takes all the energy in the universe to travel at the speed of light, is there something about the energy/gravity/mass of the universe that "slows" light from going a faster speed?