Stable cosmological solutions in degenerate theory of gravity

TL;DR

This paper demonstrates the existence of stable, ghost-free cosmological solutions within a degenerate vector-tensor gravity theory, expanding the understanding of viable models in modified gravity.

Contribution

It introduces the first example of healthy, non-trivial degenerate vector-tensor gravity solutions with stable cosmological perturbations.

Findings

Found stable cosmological solutions without ghost or gradient instabilities.

Derived background equations for homogeneous and isotropic universes.

Showed the viability of degenerate vector-tensor theories in cosmology.

Abstract

We study cosmological applications of extended vector-tensor theories, whose Lagrangians contain up to two derivatives with respect to metric and vector field. We derive background equations under the assumption of homogeneous and isotropic universe and study the nature of cosmological perturbations on top of the background. We found an example of healthy cosmological solutions in non-trivial degenerate theory of gravity for the first time, where those perturbations do not suffer from any instabilities, that is, ghost and gradient instabilities.