Relativistic scalar fields and the quasi-static approximation in theories of modified gravity

Understand

Relativistic scalar fields are ubiquitous in modified theories of gravity.

• An important tool in understanding their impact on structure formation, especially in the context of N-body simulations, is the quasi-static approximation in which the time evolution of perturbations in the scalar fields is discarded.
• We show that this approximation must be used with some care by studying linearly perturbed scalar field cosmologies and quantifying the errors that arise from taking the quasi-static limit.
• We focus on f(R) and chameleon models and link the accuracy of the quasi-static approximation to the fast/slow-roll behaviour of the background and its proximity to {\Lambda}CDM.