Imperfect Dark Energy from Kinetic Gravity Braiding

TL;DR

This paper introduces a new class of scalar-tensor models with kinetic braiding, leading to unique cosmological behaviors and providing a novel approach to modeling Dark Energy without instabilities.

Contribution

The paper presents a broad class of kinetic braiding scalar-tensor models that exhibit rich cosmological phenomenology and can cross the phantom divide safely.

Findings

Models can exhibit phantom behavior without instabilities

Cosmologies tend toward a de Sitter state

Potential observational signatures include early dark energy

Abstract

We introduce a large class of scalar-tensor models with interactions containing the second derivatives of the scalar field but not leading to additional degrees of freedom. These models exhibit peculiar features, such as an essential mixing of scalar and tensor kinetic terms, which we have named kinetic braiding. This braiding causes the scalar stress tensor to deviate from the perfect-fluid form. Cosmology in these models possesses a rich phenomenology, even in the limit where the scalar is an exact Goldstone boson. Generically, there are attractor solutions where the scalar monitors the behaviour of external matter. Because of the kinetic braiding, the position of the attractor depends both on the form of the Lagrangian and on the external energy density. The late-time asymptotic of these cosmologies is a de Sitter state. The scalar can exhibit phantom behaviour and is able to cross the phantom divide with neither ghosts nor gradient instabilities. These features provide a new class of models for Dark Energy. As an example, we study in detail a simple one-parameter model. The possible observational signatures of this model include a sizeable Early Dark Energy and a specific equation of state evolving into the final de-Sitter state from a healthy phantom regime.