Horndeski in motion

TL;DR

This paper explores anisotropic cosmologies within shift-symmetric Horndeski theories, focusing on their role as moving dark energy models and their observable effects on large-scale cosmic flows and CMB anisotropies.

Contribution

It introduces a class of axisymmetric Bianchi I cosmologies with inhomogeneous scalar fields in shift-symmetric Horndeski theories, linking them to moving dark energy scenarios.

Findings

Universal evolution of momentum density in these models

Realisation of moving dark energy with non-trivial effects

Implications for large-scale dark flows and CMB anisotropies

Abstract

We study a class of homogeneous but anisotropic cosmologies within the family of shift-symmetric Horndeski theories, where the scalar field features an inhomogeneous profile but it preserves a translational symmetry that is realised as a combination of spatial translations and internal shifts. The spatial gradient of the scalar field introduces a preferred direction, so the resulting cosmologies are of the axisymmetric Bianchi I type. The momentum density of these configurations exhibits a universal evolution and an additional component with non-vanishing momentum density is required to have non-trivial effects. We show the relation of these scenarios with cosmologies of non-comoving components and, in particular, we explain how they provide a specific realisation of moving dark energy models. Among the class of shift-symmetric Horndeski theories, we analyse in more detail the case of Kinetic Gravity Braiding with emphasis on its application to moving dark energy models and its effects on large scale dark flows as well as the CMB dipole and quadrupole.