Cosmological Averaging in Nonminimally Coupled Gravity

Abstract

We address the challenge, commonly referred to as the cosmological averaging problem, of relating the large-scale evolution of an inhomogeneous universe to that predicted by a homogeneous matter distribution in theories of gravity with nonminimal matter-gravity couplings. To this end, we focus on the class of $f(R,T)$ models given by $f(R,T) = R + F(T)$, where $R$ denotes the Ricci scalar and $T$ the trace of the energy-momentum tensor. This framework provides a simple yet theoretically consistent realization of nonminimal coupled gravity and can be recast as General Relativity minimally coupled to a modified matter Lagrangian. Using global K-monopoles as an illustrative toy model, we show that, when $F$ is a nonlinear function of $T$, the ratio between the spatial average of $F$ and $F$ evaluated at the spatial average of $T$ can deviate significantly from unity and depends on the particle number density. We demonstrate that the common assumption that this ratio is equal to unity generally leads to an inaccurate description of cosmological dynamics. We further show that dust in these theories generally exhibits a non-vanishing proper pressure. Our results highlight the importance of properly accounting for spatial averaging in cosmological models with nonminimal matter-gravity couplings.