Bi-galileon theory II: phenomenology

TL;DR

This paper explores the phenomenology of bi-galileon theories with two coupled scalar fields, presenting models that are free from common theoretical issues and compatible with solar system constraints, while evading Weinberg's no-go theorem.

Contribution

It introduces self-accelerating and self-tuning bi-galileon models that overcome previous limitations and evade theoretical constraints such as ghost and superluminality problems.

Findings

Self-accelerating models free from ghosts, tachyons, and superluminality.

Models pass solar system constraints via Vainshtein screening.

Self-tuning models evade Weinberg's no-go theorem by breaking Poincaré invariance.

Abstract

We continue to introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This paper is the second of two, and focuses on the phenomenology of the theory. We are particularly interesting in models that admit solutions that are asymptotically self accelerating or asymptotically self tuning. In contrast to the single galileon theories, we find examples of self accelerating models that are simultaneously free from ghosts, tachyons and tadpoles, able to pass solar system constraints through Vainshtein screening, and do not suffer from problems with superluminality, Cerenkov emission or strong coupling. We also find self tuning models and discuss how Weinberg's no go theorem is evaded by breaking Poincar\'e invariance in the scalar sector. Whereas the galileon description is valid all the way down to solar system scales for the self-accelerating models, unfortunately the same cannot be said for self tuning models owing to the scalars backreacting strongly on to the geometry.