About Constants
The Einstein field equation has zero divergence. The zero divergence of the stress-energy tensor is the geometrical expression of the conservation law. So it appears constants in the Einstein equation cannot vary, otherwise this postulate would be violated.
However since Einstein's constant had been evaluated by a calculation based on a time-independent metric, this by no mean requires that G and c must be unvarying constants themselves, but that the only absolute constant is their ratio:
Read more about this topic: Einstein's Constant