Bi-galileon theory II: phenomenology

TL;DR

This work extends Galileon gravity to a two-field bi-galileon framework, exploring asymptotically self-accelerating and self-tuning vacua. By constructing a two-scalar Lagrangian with Galilean invariance and analyzing stability via an action polynomial and Hessians, the authors identify ghost-free self-accelerating solutions that exhibit Vainshtein screening and avoid common pathologies, while also examining self-tuning scenarios that evade Weinberg's no-go by breaking Poincaré invariance. The study develops a detailed perturbative analysis around spherically symmetric backgrounds to assess ghosts, tachyons, and superluminality, and discusses the limitations of backreaction and strong coupling, particularly for self-tuning vacua. An erratum subsequently argues that achieving a completely pathology-free bi-galileon theory with self-acceleration may be untenable, prompting proposed constraints to the theory's higher-order terms and highlighting the need for covariant completions and further scrutiny of consistency. Overall, the paper provides a nuanced assessment of the phenomenology and viability of bi-galileon gravity as an alternative to dark energy, while outlining both promising directions and inherent challenges.

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 Poincaré 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.