A scalar-tensor cosmological model with dynamical light velocity

TL;DR

This paper explores a bimetric scalar-tensor gravity model with a dynamic light speed, demonstrating it can produce inflationary solutions without scalar potential and predicts differing speeds for gravitational and matter waves in the early universe.

Contribution

It introduces a novel scalar-tensor cosmological model with a dynamical light speed that achieves inflation without scalar potential dependence.

Findings

The model yields de Sitter inflation solutions with matter.

Inflation occurs without a scalar potential or slow-roll conditions.

Gravitational wave speed differs from matter and photon speeds in early universe.

Abstract

The dynamical consequences of a bimetric scalar-tensor theory of gravity with a dynamical light speed are investigated in a cosmological setting. The model consists of a minimally-coupled self-gravitating scalar field coupled to ordinary matter fields in the standard way through the metric: $\metric_{\mu\nu}+B\partial_\mu\phi\partial_\nu\phi$. We show that in a universe with matter that has a radiation-dominated equation of state, the model allows solutions with a de Sitter phase that provides sufficient inflation to solve the horizon and flatness problems. This behaviour is achieved without the addition of a potential for the scalar field, and is shown to be largely independent of its introduction. We therefore have a model that is fundamentally different than the potential-dominated, slowly-rolling scalar field of the standard models inflationary cosmology. The speed of gravitational wave propagation is predicted to be significantly different from the speed of matter waves and photon propagation in the early universe.