Stable cosmological solutions in degenerate theory of gravity

Understand

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.