The viability of theories with matter coupled to the Ricci scalar

TL;DR

This paper evaluates a modified gravity theory with matter coupled to the Ricci scalar, demonstrating that certain linear couplings lead to higher derivatives of matter fields without additional gravitational dynamics, thus questioning their viability.

Contribution

It analyzes the implications of matter-Ricci scalar coupling in f(R) gravity, showing such models are not viable due to higher derivatives of matter fields.

Findings

Higher order derivatives of matter fields arise

No additional gravitational dynamics are introduced

Such models are not viable for gravity

Abstract

Recently there has been a proposal for modified gravitational f(R) actions which include a direct coupling between the matter action and the Ricci scalar, R. Of particular interest is the specific case where both the action and the coupling are linear in R. It is shown that such an action leads to a theory of gravity which includes higher order derivatives of the matter fields without introducing more dynamics in the gravity sector and, therefore, cannot be a viable theory for gravitation.