where ρ ρ is the density of the gravitational source and G e f f = 2 ω ω 0 + 4 2 ω ω 0 + 3 G Φ Φ 0 (the subscript 0 indicates that the corresponding value is taken at present cosmological time and location). Therefore, the empirical gravitational constant is a function of the present value of the scalar-field background Φ Φ 0 and therefore theoretically depends on time and location. However, no deviation from the constancy of the Newtonian gravitational constant has been measured, implying that the scalar-field background Φ Φ 0 is pretty stable over time. Such a stability is not theoretically generally expected but can be theoretically explained by several mechanisms.