Constraining the scalar-tensor gravity theories with and without screening mechanisms by combined observations

Screened modified gravity (SMG) and Brans-Dicke (BD) gravity are typical examples of scalar-tensor theories with and without screening mechanisms, which can suppress the scalar field in dense regions. In this paper, we investigate the tests of time-varying gravitational constant G, gravitational dipole radiation, and Nordtvedt effect in BD and SMG theories, respectively. We place new constraints on these theories by combining Cassini experiment, lunar laser ranging (LLR) measurements, and pulsar observations from PSRs J1738 + 0333 and J0348 + 0432. We find that the screening mechanism has important influence on theoretical constraints. The strongest, second, and weakest constraints on BD are from Cassini, pulsar, and LLR tests, respectively. The most stringent constraint on SMG comes from LLR measurements and improves the previous best constraint by more than seven orders of magnitude. We derive the bounds on the cosmological evolution of the scalar background in these theories using the time variation of G. The results of all tests agree well with general relativity (GR) and give more stringent constraints on the deviations from GR. Finally, as an example, we consider the chameleon model and derive the constraints on the model parameters.