Testing gravity theories with cosmic microwave background in the degenerate higher-order scalar-tensor theory

Understand

We study the cosmic microwave background (CMB) in the framework of the degenerate higher-order scalar-tensor (DHOST) theory to test gravity theories.

• This theoretical framework includes the wide class of dark energy models such as the Horndeski theory and its extensions as certain limits, and the general relativity can be also recovered.
• In this study, to test gravity theories with CMB, we formulate the linear perturbations of gravity and matter in the theory and their effective description parameterised by time-dependent effective field theory (EFT) parameters, $\alpha_i$ $(i=B,K,T,M,H,L)$ and $\beta_i$ $(i=1,2,3)$.
• Based on the resultant DHOST framework, we develop a numerical code to solve Boltzmann equations consistently.