Perturbations in Horndeski theory above anisotropic cosmological background

TL;DR

This paper develops a framework for analyzing perturbations in Horndeski gravity on anisotropic Bianchi I backgrounds, assessing how anisotropy deviations influence stable solutions in scalar-tensor theories.

Contribution

It constructs the unconstrained perturbation action for Horndeski theory on anisotropic backgrounds, extending previous work to include anisotropic effects.

Findings

Derived the perturbation action for Horndeski theory on Bianchi I backgrounds.

Evaluated the impact of anisotropy deviations on stable solutions.

Provided a foundation for further stability analysis in anisotropic cosmologies.

Abstract

Considering an anisotropic cosmological background is an interesting and simultaneously challenging problem of theoretical physics, since we not only assume a high degree of anisotropy in the early stages of the Universe, but also observe it to a small degree until now. In this paper we have constructed the unconstrained action for the perturbations above Bianchi I type background in the most general scalar-tensor theory of gravity, the Horndeski theory, and evaluate the effect of the deviation from the anisotropic background on the previously established stable solution obtained in previous works.