If you want G(t) or c(t) – i.e. the Newtonian Gravitational ‘constant’ or the Speed of Light as functions of time – then you get their co-varying partner. The two are inextricably linked by General Relativity. Interesting short paper that discusses what that means. One eye-opener is that “dark energy” must be being produced by sources that are increasing in number as the Universe expands. Or it’s the “energy of space-time” thus more space-time means more dark-energy.