Instability of anisotropic cosmological solutions supported by vector fields

TL;DR

This paper demonstrates that existing anisotropic cosmological models with vector fields are generally unstable due to ghost or perturbation growth issues, undermining their reliability for explaining inflation or dark energy.

Contribution

It provides a general analysis showing the inherent instabilities in vector field-supported anisotropic cosmologies, challenging their viability for cosmological applications.

Findings

Existing models are plagued by instabilities such as ghosts and perturbation growth.

The instabilities are linked to the longitudinal polarization of vector fields.

Phenomenological results based on these models are unreliable.

Abstract

Models with vector fields acquiring a non-vanishing vacuum expectation value along one spatial direction have been proposed to sustain a prolonged stage of anisotropic accelerated expansion. Such models have been used for realizations of early time inflation, with a possible relation to the large scale CMB anomalies, or of the late time dark energy. We show that, quite generally, the concrete realizations proposed so far are plagued by instabilities (either ghosts, or unstable growth of the linearized perturbations) which can be ultimately related to the longitudinal vector polarization present in them. Phenomenological results based on these models are therefore unreliable.