Anisotropic Mimetic Cosmology

TL;DR

This paper explores a mimetic cosmological model where a scalar couples to a vector field, resulting in stable isocurvature perturbations and an anisotropic universe without ghost or gradient instabilities.

Contribution

It introduces a novel mimetic setup with vector fields that yields stable perturbations and anisotropic backgrounds, expanding the understanding of mimetic cosmology.

Findings

Time-like vector component does not affect background evolution.

Space-like vector components lead to an anisotropic universe.

Stable perturbations with healthy UV behavior are achieved.

Abstract

We consider a mimetic set up in which the mimetic scalar is coupled to a vector field. It is shown that such a field with a time-like component does not contribute to the background equations and yet produces healthy isocurvature perturbations with respect to ghost and gradient instabilities in spite of the absence of any propagating curvature perturbations at the level of the quadratic action. We then consider a vector field with space-like components which leads to an anisotropic Bianchi universe and show that the ghost and gradient instabilities are absent in the limit of high momenta and that the propagating curvature perturbations have healthy UV behavior.