A subhorizon framework for probing the relationship between the cosmological matter distribution and metric perturbations

The relationship between the metric and non-relativistic matter distribution depends on the theory of gravity and additional fields, hence providing a possible way of distinguishing competing theories. With the assumption that the geometry and kinematics of the homogeneous Universe have been measured to sufficient accuracy, we present a procedure for understanding and testing the relationship between the cosmological matter distribution and metric perturbations (along with their respective evolution) using the ratio of the physical size of the perturbation to the size of the horizon as our small expansion parameter. We expand around Newtonian gravity on linear, subhorizon scales with coefficient functions in front of the expansion parameter. Our framework relies on an ansatz which ensures that (i) the Poisson equation is recovered on small scales and (ii) the metric variables (and any additional fields) are generated and supported by the non-relativistic matter overdensity. The scales for which our framework is intended are small enough so that cosmic variance does not significantly limit the accuracy of the measurements and large enough to avoid complications due to non-linear effects and baryon cooling. From a theoretical perspective, the coefficient functions provide a general framework for contrasting the consequences of Lambda CDM (cosmological constant + cold dark matter) and its alternatives. We calculate the coefficient functions for general relativity (GR) with a cosmological constant and dark matter, GR with dark matter and quintessence, scalar-tensor theories (STT), f(R) gravity and braneworld models. We identify a possibly unique signature of braneworld models. For observers, constraining the coefficient functions provides a streamlined approach for testing gravity in a scale-dependent manner. We briefly discuss the observations best suited for an application of our framework.