Cosmology of the selfaccelerating third order Galileon

TL;DR

This work identifies a constrained third-order Galileon model with a tadpole term that supports self-accelerating backgrounds while maintaining stable, spherically symmetric solutions. By formulating the Jordan-frame action and deriving the homogeneous, isotropic equations of motion, the authors map the background evolution across several regimes (all coefficients zero, tadpole-only, Brans-Dicke type, and nonzero $c_3$ with a cosmological constant). Numerical analyses reveal that self-acceleration can arise and produce backgrounds close to $\Lambda$CDM, but stability and parameter tuning restrict the viable space, and a potential perturbative instability is noted. The results motivate further study of perturbations and structure formation in this Galileon subclass to sharpen observational constraints and assess viability as an alternative to dark energy.

Abstract

In this paper we start from the original formulation of the galileon model with the original choice for couplings to gravity. Within this framework we find that there is still a subset of possible Lagrangians that give selfaccelerating solutions with stable spherically symmetric solutions. This is a certain constrained subset of the third order galileon which has not been explored before. We develop and explore the background cosmological evolution of this model drawing intuition from other even more restricted galileon models. The numerical results confirm the presence of selfacceleration, but also reveals a possible instability with respect to galileon perturbations.