Perturbative emergent modified gravity on cosmological backgrounds: Kinematics

TL;DR

This paper extends emergent modified gravity to include perturbations on cosmological backgrounds, revealing new modifications that preserve classical derivative order and enabling detailed early-universe studies.

Contribution

It introduces a perturbative framework for emergent modified gravity on cosmological backgrounds, expanding the class of covariant modifications beyond symmetry-reduced models.

Findings

New modifications compatible with classical derivative order are identified.

Framework suitable for detailed early-universe model analysis is developed.

Perturbative inhomogeneity on flat backgrounds is successfully incorporated.

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.