Growth of spherical overdensities in scalar-tensor cosmologies

TL;DR

This paper investigates how scalar field perturbations affect the growth of spherical overdensities in non-minimally coupled scalar-tensor cosmologies, highlighting their limited impact on structure formation.

Contribution

It extends previous models by including scalar field perturbations and analyzing their influence on the mass function and growth factor in scalar-tensor cosmologies.

Findings

Scalar field perturbations have limited impact on the growth factor.

Perturbations slightly influence the evolution of the mass function.

The effects are more pronounced for larger coupling constants.

Abstract

The accelerated expansion of the universe is a rather established fact in cosmology and many different models have been proposed as a viable explanation. Many of these models are based on the standard general relativistic framework of non-interacting fluids or more recently of coupled (interacting) dark energy models, where dark energy (the scalar field) is coupled to the dark matter component giving rise to a fifth-force. An interesting alternative is to couple the scalar field directly to the gravity sector via the Ricci scalar. These models are dubbed non-minimally coupled models and give rise to a time-dependent gravitational constant. In this work we study few models falling into this category and describe how observables depend on the strength of the coupling. We extend recent work on the subject by taking into account also the effects of the perturbations of the scalar field and showing their relative importance on the evolution of the mass function. By working in the framework of the spherical collapse model, we show that perturbations of the scalar field have a limited impact on the growth factor (for small coupling constant) and on the mass function with respect to the case where perturbations are neglected.