Cosmological constraints on Brans-Dicke theory

TL;DR

This paper uses Planck CMB data to place strong cosmological constraints on Brans-Dicke theory, limiting deviations from General Relativity and informing modifications of gravity related to cosmic acceleration.

Contribution

It provides the most stringent cosmological bounds to date on the Brans-Dicke parameter, applicable to a broad class of scalar-tensor theories including Horndeski models.

Findings

Lower bound on ω > 692 with fixed initial scalar condition

Stronger bound ω > 890 when initial condition is free

Constraints on effective gravitational strength G_eff/G_N between 0.981 and 1.285

Abstract

We report strong cosmological constraints on the Brans-Dicke (BD) theory of gravity using Cosmic Microwave Background data from Planck.We consider two types of models. First, the initial condition of the scalar field is fixed to give the same effective gravitational strength $G_{eff}$ today as the one measured on the Earth, $G_N$. In this case the BD parameter $\omega$ is constrained to $\omega > 692$ at the $99\%$ confidence level, an order of magnitude improvement over previous constraints.In the second type the initial condition for the scalar is a free parameter leading to a somewhat stronger constraint of $\omega > 890$ while $G_{eff}$ is constrained to $0.981 <\frac{G_{eff}}{G_N} <1.285$ at the same confidence level. We argue that these constraints have greater validity than for the BD theory and are valid for any Horndeski theory, the most general second-order scalar-tensor theory, which approximates BD on cosmological scales. In this sense, our constraints place strong limits on possible modifications of gravity that might explain cosmic acceleration.