Non-minimal fluid Lagrangian couplings

TL;DR

This paper derives gravitational and fluid equations for non-minimal matter-curvature coupling models, clarifying thermodynamic effects and comparing different fluid Lagrangian formulations.

Contribution

It provides a detailed derivation of field and fluid equations for non-minimal couplings, highlighting differences between Schutz and Brown formulations.

Findings

Established the complete set of fluid equations with non-minimal couplings.

Demonstrated the non-equivalence of Schutz and Brown Lagrangian formulations.

Connected thermodynamic quantities to non-minimal coupling effects.

Abstract

Gravitational models with non-minimal couplings involving functions of the matter Lagrangian and curvature have become popular in recent decades. By coupling the matter Lagrangian directly to the gravitational Lagrangian, one hopes to construct theories that can explain dark energy or dark matter without introducing additional sources. When this matter Lagrangian describes a perfect fluid, some technicalities are involved in its variational formulation. We present a careful derivation of the gravitational field equations together with the complete set of fluid equations. The fluid's equations allow us to define thermodynamic quantities such as temperature, chemical potential and number density and thus allow us to understand the effects of the non-minimal couplings on these quantities. We demonstrate the non-equivalence of the Lagrangian formulations of Schutz and Brown for these types of models and provide a detailed interpretation of our results.