# Dynamics of cosmological perturbations in modified Brans-Dicke cosmology with matter-scalar field interaction

## TL;DR

This paper explores a modified Brans-Dicke cosmology with matter-scalar interaction, demonstrating its potential to explain cosmic acceleration and structure growth, while identifying some theoretical challenges in its gravitational behavior.

## Contribution

It introduces a novel complete Brans-Dicke theory with matter-scalar interaction that fits supernova data without scalar potential and analyzes its impact on large-scale structure.

## Key findings

- Viable accelerating solutions fitting supernovae data.
- Suppressed growth rate compared to ΛCDM.
- Potential issues with effective gravitational constant on small scales.

## Abstract

In this work we focus on a novel completion of the well-known Brans-Dicke theory that introduces an interaction between the dark energy and dark matter sectors, known as complete Brans-Dicke (CBD) theory. We obtain viable cosmological accelerating solutions that fit Supernovae observations with great precision without any scalar potential $V(\phi)$. We use these solutions to explore the impact of the CBD theory on the large scale structure by studying the dynamics of its linear perturbations. We observe a growing behavior of the lensing potential $\Phi_{+}$ at late-times, while the growth rate is actually suppressed relatively to $\Lambda$CDM, which allows the CBD theory to provide a competitive fit to current RSD measurements of $f\sigma_{8}$. However, we also observe that the theory exhibits a pathological change of sign in the effective gravitational constant concerning the perturbations on sub-horizon scales that could pose a challenge to its validity.