Cosmological constraint on Brans-Dicke Model

We combine new Cosmic Microwave Background (CMB) data from Planck with Baryon Acoustic Oscillation (BAO) data to constrain the Brans-Dicke (BD) theory, in which the gravitational constant $G$ evolves with time. Observations of type Ia supernovae (SNeIa) provide another important set of cosmological data, as they may be regarded as standard candles after some empirical corrections. However, in theories that include modified gravity like the BD theory, there is some risk and complication when using the SNIa data because their luminosity may depend on $G$. In this paper, we assume a power law relation between the SNIa luminosity and $G$, but treat the power index as a free parameter. We then test whether the difference in distances measured with SNIa data and BAO data can be reduced in such a model. We also constrain the BD theory and cosmological parameters by making a global fit with the CMB, BAO and SNIa data set. For the CMB+BAO+SNIa data set, we find $0.08\times10^{-2} < \zeta <0.33\times10^{-2}$ at the 68\% confidence level (CL) and $-0.01\times10^{-2} <\zeta <0.43\times 10^{-2}$ at the 95\% CL, where $\zeta$ is related to the {BD} parameter $\omega$ by $\zeta=\ln(1+1/\omega)$.