Lessons from the trace of the energy momentum tensor on inflation models

Abstract

In this note, we discuss properties of the trace of the energy-momentum tensor and their implications for inflation models. One property is the decoupling of heavy degrees of freedom from low-energy dynamics. This is intuitively apparent from the effective field theory point of view, but one has to take into account the so-called trace anomaly to explicitly demonstrate the decoupling. It is relevant to scalaron decay in R2 inflation models. Another is a quantum contribution to a non-minimal coupling of a scalar field to gravity. It might modify inflation dynamics. The non-minimal coupling disappears from the action in the flat spacetime, but leaves the so-called improvement term in the trace of energy momentum tensor. We discuss the “natural” value of a non-minimal coupling from the renormalization group point of view.