Scalar-tensor extension of the $\Lambda$CDM model

TL;DR

This paper introduces a scalar-tensor extension of the ΛCDM model where a scalar field modifies the Hubble rate, enabling clear observational tests of deviations from general relativity using cosmological data.

Contribution

It presents a simple scalar-tensor model with a single parameter to quantify deviations from ΛCDM, facilitating direct observational constraints.

Findings

Supernovae, galaxy ages, and BAO data strongly limit deviations from ΛCDM.

The model provides a transparent way to compare scalar-tensor theories with GR.

Scalar field effects are effectively captured by a single parameter.

Abstract

We construct a cosmological scalar-tensor-theory model in which the Brans-Dicke type scalar $\Phi$ enters the effective (Jordan-frame) Hubble rate as a simple modification of the Hubble rate of the $\Lambda$CDM model. This allows us to quantify differences between the background dynamics of scalar-tensor theories and general relativity (GR) in a transparent and observationally testable manner in terms of one single parameter. Problems of the mapping of the scalar-field degrees of freedom on an effective fluid description in a GR context are discused. Data from supernovae, the differential age of old galaxies and baryon acoustic oscillations are shown to strongly limit potential deviations from the standard model.