Cosmology in new gravitational scalar-tensor theories

TL;DR

This paper explores new scalar-tensor gravity theories with higher derivatives, demonstrating their ability to produce cosmological behaviors consistent with observations, including dark energy dynamics and phantom crossing.

Contribution

It introduces and analyzes novel gravitational scalar-tensor theories with 2+2 degrees of freedom, showing their potential to explain cosmic acceleration from higher-derivative modifications.

Findings

Models match observed transition from matter to dark energy era.

Effective dark energy can cross the phantom divide.

Theories arise solely from higher-derivative terms.

Abstract

We investigate the cosmological applications of new gravitational scalar-tensor theories, which are novel modifications of gravity possessing 2+2 propagating degrees of freedom, arising from a Lagrangian that includes the Ricci scalar and its first and second derivatives. Extracting the field equations we obtain an effective dark energy sector that consists of both extra scalar degrees of freedom, and we determine various observables. We analyze two specific models and we obtain a cosmological behavior in agreement with observations, i.e. transition from matter to dark energy era, with the onset of cosmic acceleration. Additionally, for a particular range of the model parameters, the equation-of-state parameter of the effective dark energy sector can exhibit the phantom-divide crossing. These features reveal the capabilities of these theories, since they arise solely from the novel, higher-derivative terms.