The viability of theories with matter coupled to the Ricci scalar

Understand

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.