Decaying Gravity
  We consider the possibility of energy being exchanged between the scalar and matter fields in scalar-tensor theories of gravity. Such an exchange provides a new mechanism which can drive variations in the gravitational 'constant' G. We find exact solutions for the evolution of spatially flat Friedman-Roberston-Walker cosmologies in this scenario and discuss their behaviour at both early and late times. We also consider the physical consequences and observational constraints on these models. 
T. Clifton and John D. Barrow