What if the universe expands linearly? A local general relativity to solve the "zero active mass" problem

Abstract: Modern cosmology presents important challenges such as the \textit{Hubble tension}, \textit{El Gordo's collision} or the \textit{impossible galaxies} ($z > 10$). Slight modifications to the standard model propose new parameters (e.g. the early and dynamical dark energy). On the other hand, alternatives such as the coasting universes (e.g. the \textit{hyperconical model} and the spatially flat $R_h = ct$ universe) are statistically compatible with most of observational tests, but still present theoretical problems in matching the observed matter contents since they predict a ``zero active gravitational mass.'' To solve these open issues, we suggest that general relativity might be not valid at cosmic scales, but it would be valid at local scales. This proposal is addressed from two main features of the embedding hyperconical model: 1) the background metric would be independent of the matter content, and 2) the observed cosmic acceleration would be fictitious and because of a distorted stereographic projection of coordinates that produce an apparent radial inhomogeneity from homogeneous manifolds. Finally, to support the discussion, standard observational tests were updated here, showing that the hyperconical model is adequately fitted to Type Ia Supernovae, quasars, galaxy clusters, BAO, and cosmic chronometer data sets.