On validity of the quasi-static approximation in scalar-tensor theories

TL;DR

This paper evaluates the validity of the quasi-static approximation in scalar-tensor theories, focusing on the symmetron model, and provides criteria for when this approximation holds in cosmological contexts.

Contribution

It compares exact solutions with the QSA in the symmetron model to establish parameter ranges where the approximation is valid, aiding cosmological analyses.

Findings

QSA validity depends on background scalar field evolution

Derived a criterion for symmetron model applicability

Applicable to other scalar-tensor theories of Brans-Dicke type

Abstract

The discovery of cosmic acceleration motivated extensive studies of dynamical dark energy and modified gravity models. Of particular interest are the scalar-tensor theories, with a scalar field dark energy non-minimally coupled to matter. Cosmological constraints on these models often employ the quasi-static approximation (QSA), in which the dynamics of the scalar field perturbations is proportional to the perturbation in the matter density. Using the QSA simplifies the physical interpretation of the phenomenology of scalar-tensor theories, and results in substantial savings of computing time when deriving parameter constraints. Focusing on the symmetron model, which is a well-motivated scalar-tensor theory with a screening mechanism, we compare the exact solution of the linearly perturbed field equations to those obtained under the QSA and identify the range of the model parameters for which the QSA is valid. We find that the evolution of background scalar field is most important, namely, whether it is dominated by the Hubble friction or the scalar field potential. This helps us derive a criterion for the symmetron model, but same argument can be applied to other scalar-tensor theories of generalized Brans-Dicke type. We consider two scenarios, one where the scalar field is only coupled to dark matter and where it couples to all of the matter.