Perturbative emergent modified gravity on cosmological backgrounds: Kinematics

Abstract: Emergent modified gravity has shown that the canonical formulation of general relativity gives rise to a larger class of covariant modifications than action-based approaches, so far in symmetry-reduced models. This outcome is made possible by distinguishing between the space-time metric on a given solution, and the basic field degrees of freedom in which equations of motion are formulated. In this general treatment, the metric is no longer fundamental but emerges after field equations and covariance conditions are solved. Here, the results are extended to perturbative inhomogeneity on a spatially flat cosmological background, showing that new modifications are possible while maintaining the classical derivative order and setting the stage for dynamical equations suitable for detailed studies of early-universe models.