Exact solutions of Brans-Dicke cosmology and the cosmic coincidence problem

TL;DR

This paper derives exact cosmological solutions within Brans-Dicke theory that address the cosmological constant problem by proposing a decaying vacuum energy density and a constant matter-vacuum energy ratio, aligning with observational data.

Contribution

It provides new exact solutions in Brans-Dicke cosmology that explain the small vacuum energy and constant matter-vacuum ratio, offering insights into the cosmic coincidence problem.

Findings

Solutions match observed matter density and universe age

Vacuum energy density decays over time

Constant ratio between vacuum and matter energy densities

Abstract

We present some cosmological solutions of Brans-Dicke theory, characterized by a decaying vacuum energy density and by a constant relative matter density. With these features, they shed light on the cosmological constant problems, leading to a presently small vacuum term, and to a constant ratio between the vacuum and matter energy densities. By fixing the only free parameter of our solutions, we obtain cosmological parameters in accordance with observations of the relative matter density, the universe age and redshift-distance relations.