Cosmological closure of metric and area-metric ideal fluids

To shed some light on the construction of physically viable gravity theories, I employ the gravitational closure framework to investigate a cosmology that is not a priori based on a metric but assumes an area-metric spacetime geometry that arises from the most general linear theory of electrodynamics. To this end, I develop a symmetry reduction procedure to find, for the first time, an exact non-metric solution of the gravitational closure framework under cosmological symmetries. I investigate cosmological sources of gravity and construct an area-metric description of ideal fluids. I conclude by illustrating that, even though we cannot rely on metric concepts such as a metric line-element or a Levi-Civita connection to begin with, we can recover all required notions, such as light rays and observers, that allow us to conduct cosmological observations.