Massless Fileds in Scalar-Tensor Cosmologies

TL;DR

This paper derives exact cosmological solutions in scalar-tensor theories, illustrating early universe behavior dominated by massless scalar fields and exploring the evolution of scalar fields and parameters in various universe geometries.

Contribution

It provides new exact solutions in scalar-tensor cosmologies for stiff matter and radiation, offering insights into early universe dynamics and scalar field evolution.

Findings

Exact solutions for scalar-tensor cosmologies with stiff matter and radiation.

Insights into early universe behavior dominated by massless scalar fields.

Evolution patterns of scalar fields and parameters at late times.

Abstract

We derive exact Friedmann--Robertson--Walker cosmological solutions in general scalar--tensor gravity theories, including Brans--Dicke gravity, for stiff matter or radiation. These correspond to the long or short wavelength modes respectively of massless scalar fields. If present, the long wavelength modes of such fields would be expected to dominate the energy density of the universe at early times and thus these models provide an insight into the classical behaviour of these scalar--tensor cosmologies near an initial singularity, or bounce. The particularly simple exact solutions also provide a useful example of the possible evolution of the Brans--Dicke (or dilaton) field, $\phi$, and the Brans--Dicke parameter, $\omega(\phi)$, at late times in spatially curved as well as flat universes. We also discuss the corresponding solutions in the conformally related Einstein metric.